94 need help

wow this is a long one.
first no big issues, had a dragging starter and bad a/c compressor. changed both of them out. didn't hook up a/c ele cause i still need to vacume it. drove trk for about an hour turned it off, 20 min tried to start and no crank just clicks dead battery. i jump it off starts up dash volt meter says 14+ so i drive 15 min home and turn it off. next day try to start it dead again, jump it off starts runs for about a min while i'm getting the multi meter then dies. i put the jumper cables back on try to start but will not start just cranks, i have to take a pissed off breather 5 min later i try to start it again and it starts. i left the jumper cables on 15 to 20 sec later it dies with the cables still on. i check fuel line in back of throtle body pull it off turn ign. to run and good pressior for about 3 to 4 sec. put it back on. i figured the battery was abused and it was so i changed out battery with new one, took to Adv Auto. tested alternator it was bad put new one on. then i started it again good to go for about 15 sec it dies again. i check #1 plug good spark, so i check fuel again nothing pumping thru ele inj. throtle body. pull fuel line still no fuel pumping this time. i hear relay clicking but no sound in tank.
is there somethng sensor like o2 or pressior that would stop fuel pump?
i don't think this truck has a manual fuel pump does it?
the starting up then dieing the waiting like it seems to reset then starts again. i am so confused please any suggestions would be cool. thanks billy

wow this is a long one.
first no big issues, had a dragging starter and bad a/c compressor. changed both of them out. didn't hook up a/c ele cause i still need to vacume it. drove trk for about an hour turned it off, 20 min tried to start and no crank just clicks dead battery. i jump it off starts up dash volt meter says 14+ so i drive 15 min home and turn it off. next day try to start it dead again, jump it off starts runs for about a min while i'm getting the multi meter then dies. i put the jumper cables back on try to start but will not start just cranks, i have to take a pissed off breather 5 min later i try to start it again and it starts. i left the jumper cables on 15 to 20 sec later it dies with the cables still on. i check fuel line in back of throtle body pull it off turn ign. to run and good pressior for about 3 to 4 sec. put it back on. i figured the battery was abused and it was so i changed out battery with new one, took to Adv Auto. tested alternator it was bad put new one on. then i started it again good to go for about 15 sec it dies again. i check #1 plug good spark, so i check fuel again nothing pumping thru ele inj. throtle body. pull fuel line still no fuel pumping this time. i hear relay clicking but no sound in tank.
is there somethng sensor like o2 or pressior that would stop fuel pump?
i don't think this truck has a manual fuel pump does it?
the starting up then dieing the waiting like it seems to reset then starts again. i am so confused please any suggestions would be cool. thanks billy

Is fuse box getting good 12 volts from battery?
Fuel pump is electric in tank.
Is fuel pump getting good 12 volts to it?

Does it still help to jump start it?

a faulty oil sending unit will cause that. The ecm will cutoff power to the fuel pump if it detects less then 10 psi of oil pressure. How ever first you may want to beat on the bottom of the fueltank with a broomstick or something to jar the fuel pump while someone cranks the engine. If the engine starts when you do that then you have a bad fuel pump.

a faulty oil sending unit will cause that. The ecm will cutoff power to the fuel pump if it detects less then 10 psi of oil pressure. How ever first you may want to beat on the bottom of the fueltank with a broomstick or something to jar the fuel pump while someone cranks the engine. If the engine starts when you do that then you have a bad fuel pump.
Oil pressure is not a fuel pump shut off switch but is on there to aid in case of relay fail.
Oil pressure switch does not shut off fuel pump relay.

Oil pressure switch is a saftey switch to keep fuel pump running in case fuel pump relay fails.

Oil pressure is not a fuel pump shut off switch but is on there to aid in case of relay fail.
Oil pressure switch does not shut off fuel pump relay.

Oil pressure switch is a saftey switch to keep fuel pump running in case fuel pump relay fails.



Oh really.... I think you need to recheck your data there chief. Since the 80's GM has been using that feature to avoid catastrophic failures. The oil pressure sensor does not turn the fuel relay off. The ECM turns "fuel relay request" off at detection of low oil pressure. A faulty oil pressure switch will give the ecm false info and cause it to shutdown the fuelpump. Atleast that is what the GM instructers teach. Have you attended such a class ??

Oh really.... I think you need to recheck your data there chief. Since the 80's GM has been using that feature to avoid catastrophic failures. The oil pressure sensor does not turn the fuel relay off. The ECM turns "fuel relay request" off at detection of low oil pressure. A faulty oil pressure switch will give the ecm false info and cause it to shutdown the fuelpump relay. Atleast that is what the GM instructers teach. Have you attended such a class ??

I have 60 years of on hands training.
You better get out the good book and read it.
Oil pressure switch is only along for the ride unless the fuel pump relay quits.

To prove it unplug your oil pressure switch and see if engine still runs.

thanks i will check the oil pressure sending unit, all devices are good with a full 12 volts. i will also check the broom thumping on the tank while cranking. i'm pritty sure i hear the fuel pump relay clicking on/ off like 3/4 sec long. fuse is good too. just to make sure to reset the ecm just disconnect the battery for a few min right? and one last thing the fuel pump only pumps for 3 to 4 sec then centrifical or throtle body vacume pulls the "running" fuel from the tank? correct?

I have 60 years of on hands training.
You better get out the good book and read it.
Oil pressure switch is only along for the ride unless the fuel pump relay quits.

To prove it unplug your oil pressure switch and see if engine still runs.


If what you say is true then perhaps you can explain to me the purpose of the third wire in the oil pressure switch.. If it was a 2 wire switch you would be correct. However, unless I am mistaken, that switch has 3 wires.
The third wire leads back to the ecm. Most wiring diagrams are simplified and only show 2 wires. Now if you notice the fuel pump relay gets it's request straight from the ECM, which turns that request off when the signal from that 3rd wire tells it that there is oil pressure. At that point the only power going to the fuel pump routes through the oil pressure switch.
You are right that the switch and relay are wired parrallel but the side that goes to the relay does nothing until the ecm decides it is needed such as on start up and priming the fuel rail. People get confused when looking at diagrams because they do not show how the ECM controls them.

Even though it is a 94 GM OBD1 system and is incapable of throwing a PO520 code the circuitry was allready inplace and functioning in preparation for the conversion to the OBD2 system that they started in 95.

Even though it is a 94 GM OBD1 system and is incapable of throwing a PO520 code the circuitry was allready inplace and functioning in preparation for the conversion to the OBD2 system that they started in 95.

All repleys to a post should be keep to there year and engine.

But looking at 94 and 96 and 98 5.7 1500 engines and 2000 -2003 C-1500 trucks that have the 3 wire oil pressure switch the 2 wires feed power to fuel pump with oil pressure up.
The 3 th wire only goes to insturment cluster for gauge.
Not to pcm or body computer.
None have a code 520 oil pressure gage problem on engine spefic codes.

No way oil pressure switch is connected to computer to shut of fuel pump.

On gm trucks V6 -V8
Factory repair manuals and all data and mitchell on demand all tell us the 3 wire oil pressure switch is not a saftey switch to turn off fuel pump but a aid to keep it fuel pump running while engine has oil pressure in case the fuel pump relay fails relay fails.

A few older gm early 80's V6 engines did run fuel pump off of oil pressure switch.
After key on prime up to start it.

What year make and modle and engine and engine code have you run into where the oil pressure switch is a fuel pump cut off switch tied threw the computer to shut off fuel pump?

All repleys to a post should be keep to there year and engine.

But looking at 94 and 96 and 98 5.7 1500 engines and 2000 -2003 C-1500 trucks that have the 3 wire oil pressure switch the 2 wires feed power to fuel pump with oil pressure up.
The 3 th wire only goes to insturment cluster for gauge.
Not to pcm or body computer.
None have a code 520 oil pressure gage problem on engine spefic codes.

No way oil pressure switch is connected to computer to shut of fuel pump.

On gm trucks V6 -V8
Factory repair manuals and all data and mitchell on demand all tell us the 3 wire oil pressure switch is not a saftey switch to turn off fuel pump but a aid to keep it fuel pump running while engine has oil pressure in case the fuel pump relay fails relay fails.

A few older gm early 80's V6 engines did run fuel pump off of oil pressure switch.
After key on prime up to start it.

What year make and modle and engine and engine code have you run into where the oil pressure switch is a fuel pump cut off switch tied threw the computer to shut off fuel pump?


Look, I am not going to argue with you. Just answer me this... Have you had GM factory training? I don't think so. btw... a PO520 is not an oil pressure guage problem, it is a circuit malfunction of the oil pressure switch circuit. The DTC can be present when the guage or light works fine. The last time I ran accross one it was on a 97 Camaro with 3.8L K engine it also had a start and stall problem. I replaced the oil pressure switch and it drove away. But maybe I was just lucky... After all, what kinda fool would listen to uneducated GM factory reps when Mitchell-On-Demand and AllData both say differently....

Did you know that every electronicly controlled GM car ever sold is governed back by it's ECM so that it cannot exceed the speed rating of the tires it was sold with? Why?... So they can't be sued for highspeed blowouts. You won't find that in Mitchell or AllData either.

You know what MT-2500.... I just noticed that you are a AF Advisor. Meaning that you give advice and assistance to alot of people which is very commendable. It is also because of that fact I must take it upon myself to educate you... I called a buddy of mine and we went down to the local dealership (yes, on Saturday night) and we researched the matter in the GM database. We found a wiring diagram of the Fuel Injection and Fuel Pump Control System that illustrated our point. We then went to my shop and got on my AllData Online computer to compare the data. To my amazement the diagrams are exactly the same. Which serves my purpose well so you can see it too. But first let me say that I did check in Alldata under description and operation of the Oil Pressure Switch and it does say that it serves as a backup incase of fuel relay failure... Now we can use their own wiring diagram to prove that to be total nonsense... Please open your allData program and look at this diagram located under Powertrain Management/ Electrical Diagrams/ Fuel Injection and Fuel Pump Control for a 1994 c-1500 w/ 5.7L... First look at the relationship of the fuel Pump Relay and the Oil Pressure switch. Just as you had stated they are wired in parrallel with eachother utilizing the same 12v power source as well does the ECM. Now notice circuit 465 going from the request terminal of the FP Relay to ECM pin# F6 Labeled FP Relay Control. That is the circuit the ECM uses to energize the relay to close the switch to power the fuel pump. Now lets look at the Oil Pressure switch. Notice that it is similair to a relay/crossed with a variable resistor. Notice the location of pin A that leads to the Inst Cluster. When the sensor detects oil pressure it changes the resistance which is monitored by the guage or light on one side of the switch. That half energizes the switching of the other half ( So that switch can fail and store a PO520 and the guage or light still work properly on later models utilizing the OBD2 diagnostics systems). Now at this point it seems that the ECM powers up the fp relay and then oil pressure opens the OP switch and both power sources run at the same time powering up circuit 120 through the fp fuse to the fuel pump. That is where the mistake is made... Now look at the ECM pin# B12. Notice it is labeled Oil Pressure Input and not FP Input. That is where the ECM monitors for adequate oil pressure. Now notice the wire attached to that pin is a gray wire of circuit 120 (the very same circuit that powers the fuel pump). Now if you trace that wire it leads to splice#127 where it monitors the voltage supplied to the fuel pump.Now use some common sense... How can oil pressure be monitored on a wire that is powered up by the fp relay and the OP switch at the sametime? It can't... This is how it works... The ecm powers up the fp relay providing power to the fuel pump to start the engine. Then when oil pressure comes up and the OP switch closes the additional 12V power source causes a spike in the current flowing to the fuel pump that is monitored at pin# B12 of the ECM. The ECM then cutsoff fp relay request at pin# F6 and disables the fp relay. At that point all power flows through the OP switch to fuel pump allowing pin# B12 to monitor oil pressure and not fp relay voltage. So you should be able to see now that if the OP switch failed the engine would die from the fuel pump being shut off. Also if the engine was to suddenly stop ( as in a crash) the fuel pump is immediately killed very similairly to Ford's fuel inertia switch system.. IT IS A SAFETY ISSUE TO PREVENT FIRE AFTER A COLLISION.
So right there is the proof on one of your own wiring diagrams. It further proves that Mitchell and AllData has the information but they only understand about half of it. You can now consider yourself educated with some GM knowledge on this matter anyway.
I am sorry if I appear to be trying to make you look bad, I honestly am not. I simply find it imperative to help you. You do some fine work and help alot of people and I just want to insure that the info you graciously provide is accurate.

thanks i will check the oil pressure sending unit, all devices are good with a full 12 volts. i will also check the broom thumping on the tank while cranking. i'm pritty sure i hear the fuel pump relay clicking on/ off like 3/4 sec long. fuse is good too. just to make sure to reset the ecm just disconnect the battery for a few min right? and one last thing the fuel pump only pumps for 3 to 4 sec then centrifical or throtle body vacume pulls the "running" fuel from the tank? correct?


Actually Billy, the fuel pump runs by a constant electrical current provided as long as the ECM can see a pulse signal from the distr (for injector firing) and current flowing through the oil pressure switch to the fuel pump. It does not rely on vacuum or centrifical force to flow. Also, it is not a good idea to unhook the battery to reset an ECM. It is best to clear the codes with a OBD1 scanner. Think of it this way... over time as you drive the ECM tweaks adjustments on ign timing, injector pulse width, and air/fuel mixture, fine tuning it to peak efficiency based on your driving habits and the fact that every engine will behave slightly differently than even another identical one (no 2 engines will ever run exactly the same). When you unhook the battery it's memory is erased and it then has to start that process of tweaking all over again. Futhermore if you have to unhook the battery always unhook the ground cable first and connect it back last. Disconnecting or reconnecting the positive cable while the ground cable is still connected can cause a 12V spike that can fry an ECM's circuit board.

Actually Billy, the fuel pump runs by a constant electrical current provided as long as the ECM can see a pulse signal from the distr (for injector firing) and current flowing through the oil pressure switch to the fuel pump. It does not rely on vacuum or centrifical force to flow. Also, it is not a good idea to unhook the battery to reset an ECM. It is best to clear the codes with a OBD1 scanner. Think of it this way... over time as you drive the ECM tweaks adjustments on ign timing, injector pulse width, and air/fuel mixture, fine tuning it to peak efficiency based on your driving habits and the fact that every engine will behave slightly differently than even another identical one (no 2 engines will ever run exactly the same). When you unhook the battery it's memory is erased and it then has to start that process of tweaking all over again. Futhermore if you have to unhook the battery always unhook the ground cable first and connect it back last. Disconnecting or reconnecting the positive cable while the ground cable is still connected can cause a 12V spike that can fry an ECM's circuit board.

As far as I have found no 2005 or older gm computer system is set up so it can shut down the fuel pump from the pcm from no oil pressure.


What year make and modle and engine and engine code are you refering to here in this quote?

Quote
We found a wiring diagram of the Fuel Injection and Fuel Pump Control System that illustrated our point. We then went to my shop and got on my AllData Online computer to compare the data. To my amazement the diagrams are exactly the same. Which serves my purpose well so you can see it too.

I need to remind you this is the gm CK forum and the origional post is about a 94 CK truck.

All data and mitchell repair info is the same as and is copyed from the gm factory info repair info.

My Gm training has nothing to do with this post or my answers.
And should not be used for a pissing match as far as who has the most GM training.

The 97 Camero 3.8 V6 K code you reffered to is a poor example for a 94 gm truck post/problem.

It may have 2 or 3 oil pressure switch systems on it.

How ever on it if it had the wide blade 3 prong oil pressure switch it may run the fuel pump after enginee prime up start and would kill engine running if switch did not turn on.
The large wide 3 blade oil pressure switch is a carry over from early 80 GM V6 systems that did run the fuel pump from oil pump from oil pressure after start up.

MT-2500 said....


What year make and modle and engine and engine code are you refering to here in this quote?

Quote
We found a wiring diagram of the Fuel Injection and Fuel Pump Control System that illustrated our point. We then went to my shop and got on my AllData Online computer to compare the data. To my amazement the diagrams are exactly the same. Which serves my purpose well so you can see it too.

I need to remind you this is the gm CK forum and the origional post is about a 94 CK truck.

All data and mitchell repair info is the same as and is copyed from the gm factory info repair info.

My Gm training has nothing to do with this post or my answers.
And should not be used for a pissing match as far as who has the most GM training.
__________________________________________________ ______________

My reply:
If you would simply take your time and carefully read my entire posting that you partially quoted. You would see that it is all about the 94 CK truck. Futhermore if you would simply look at that wiring diagram from AllData where I said you could find it, and follow it through with me step by step, it should become obvious to you that I was not attacking you. I was providing you with everything you asked for and you won't even take the time to read it ?? That is sorry.


If you weren't too busy to learn this valuable info that is ONLY available from GM that directly pertains to the truck in question and many others (also the disspelling of a widely accepted inaccurate myth), you would see that what I am offering is a valuable (high tech from the inside) GIFT to you and the forum readers. I didnot recruit the help of a friend on a Saturday night to attack you personally but to help enrich your (and the forum readers) knowledge. However before you can recieve the value of this GIFT, you must first drop your obvious idea that you all ready know everything there is to know. Now this is an attack on you... If you are not open to additional knowledge for constant learning, then what is it exactly that makes you think that you have the competant knowledge to teach others ??? HAVE A NICE DAY.

MT-2500 said....


What year make and modle and engine and engine code are you refering to here in this quote?

Quote
We found a wiring diagram of the Fuel Injection and Fuel Pump Control System that illustrated our point. We then went to my shop and got on my AllData Online computer to compare the data. To my amazement the diagrams are exactly the same. Which serves my purpose well so you can see it too.

I need to remind you this is the gm CK forum and the origional post is about a 94 CK truck.

All data and mitchell repair info is the same as and is copyed from the gm factory info repair info.

My Gm training has nothing to do with this post or my answers.
And should not be used for a pissing match as far as who has the most GM training.
__________________________________________________ ______________

My reply:
If you would simply take your time and carefully read my entire posting that you partially quoted. You would see that it is all about the 94 CK truck. Futhermore if you would simply look at that wiring diagram from AllData where I said you could find it, and follow it through with me step by step, it should become obvious to you that I am not attacking you.
If you weren't too busy to learn this valuable info that is ONLY available from GM that directly pertains to the truck in question and many others (also the disspelling of a widely accepted inaccurate myth), you would see that what I am offering is a valuable (high tech from the inside) GIFT to you and the forum readers. I didnot recruit the help of a friend on a Saturday night to attack you personally but to help enrich your (and the forum readers) knowledge. However before you can recieve the value of this GIFT, you must first drop your obvious idea that you all ready know everything there is to know. Now this is an attack on you... If you are not open to additional knowledge for constant learning, then what is it exactly that makes you think that you have the competant knowledge to teach others ??? HAVE A NICE DAY.

Well slanders and slams will not break my bones.
You are only digging you hole deeper.
I do not see any proof of what you are trying to tell us.
B12 is ony tied to fuel pump power to tell the the pcm the fuel pump is getting power.
A 94 C-1500 5.7 has no crash sense built int it to turn off fuel pump.
The oil pressure switch is no way tied to pcm to shut off fuel pump.
It is not a saftey switch to kill engine only a aid to help fuel pump keep running in case of a fuel pump relay fail.
Low oil pressure can not turn off fuel pump if the fuel pump relay is working right.

Well slanders and slams will not break my bones.
You are only digging you hole deeper.
I do not see any proof of what you are trying to tell us.
B12 is ony tied to fuel pump power to tell the the pcm the fuel pump is getting power.
A 94 C-1500 5.7 has no crash sense built int it to turn off fuel pump.
The oil pressure switch is no way tied to pcm to shut off fuel pump.
It is not a saftey switch to kill engine only a aid to help fuel pump keep running in case of a fuel pump relay fail.
Low oil pressure can not turn off fuel pump if the fuel pump relay is working right.


Okay... I'll except that, now look closer at that diagram and see if you can explain to me why GM would labeled ECM pin# B12 as "Oil Pressure Input"?
How more straight forward can it be ? Didn't you say , "in no way does any wire from the oil pressure switch go to the ECM" ? Let me trace it again for you Circuit 120( gray wire) from pin C of the OP switch. it goes to splice # S149 , to the fuel pump fuse and through it to splice # S127, where it branches off to ECM pin # B12 that is even labeled as "Oil Pressure Input".. How can you argue with that ??

well fellas, thanks for the small arrows of direction. they did indeed help point the right direction. i eventually quit getting any power to the fuel pump, or lets say it wasn't pumping anything. so i started to dorp the tank, to check the fuel pump closer. well to my supprise the power wire comming from the FP RELAY/ ECM was almost cut in half. the houseing was the only thing holding it together i would assume that the wire occasionaly seperate the internal wire just enough to stop power then after vibration quit would lay back together. anyway i would chalk it up to MULTI-SYMPTOMS/ INTERMITTENT ISSUES with the starter, battery, generator, a/c compressor and accumulator, and a partial seperation of a fuel pump wire.
i guess a rock or something bounced between the tank and the bed?
anyway in all my short 18-20 years of being a mechanic i have never had something like all of these things happen all at once, or in such a manner. but it is fixed and running like a champ. well except for the tail pipe hanging, and the a/c still needing to be vacumed. by the way you two were very ammusing to listen to. thanks again, you two really shouldn't ever have any hard feeling tward eachother.
later
billy

Good job Billy,
and in all honesty give some credit to MT. He did say check for 12V at the fuel pump... Should of led you straight to it. Good job to both of you.

No hard feelings MT.. I started a new thread about the OP switch usage issue ... I really think I need more opinions on the matter. Of course your input would not only be welcomed it will also be appreciated. Let there be no doubt that I appreciate the work you are doing in this forum. Please join me on the other thread so we can finish this as a FRIENDLY debate. I know I'm right and you know you're right... Let us continue until we reach an agreement.