I have a 1995 Chevy Caprice with a 4.3L V8 engine that starts and runs fine when cold. When it warms up, the engine dies and won't start, although it cranks.The B terminal, on the ignition contyrol module, shows an AC voltage of around 2.5 volts AC when cold, however when hot this voltage disappears. I put a noid light on one of the fuel injectors, and of course the noid light flashes when running and when you crank the car when cold. After the engine warms up and the car dies, and wont start, the flashing noid light disappears while cranking the engine.I have replaced the coil wire, the coil, the ignition control module, the wire going from the computer to the B terminal on the ignition control module, the PCM(computer), the optispark distributoe, the fuel pressure regulator, the ignition switch, the fuel filter, the fuel pump, and the ignition switch.The engine has no crank position sensor.The engine shows no diagnostic trouble codes. What is wrong?

Thanks,
Don
astra1@earthlink.net

I would look around the distributor pickup wiggle the wire and see what happens. I have found wires broke in the plug that come loose touch then don't. I think it is caused by another mechanic pulling on the wire causing a separation. Look closely where other mechanics have removed insulation or the wiring looks like a pin cushion.
I have found many wiring problems. www.alldata.com sells a years supply of data for your vehicle it used to be $15.00 for a year.
You may have to get a wiring diagram and ohm wires where you aren't getting voltage.
I would check voltage on your ignition switch.
Look around o2 sensors for cracks in the manifold or exhaust pipe.
Spray around the intake manifold with carb cleaner a bad air leak won't allow the engine to idle when hot. Also spray around the injectors base.
Pinch off the hose to the vacuum booster with vise grips if this corrects the problem get a rebuilt unit.
Is your battery voltage around 14v when running a bad cell and 8v will give you many problems.
Check for loose grounds left cylinder head,alternator bracket. Also check the fuel pump ground at the frame.

The computer is shutting down when hot. Does anyone know what sensors, or relays can actually shut down the computer?

How do you know the computer is shutting down. Unless you have a short in the wiring harness causing power loss. Your symptons shutting off as soon as the computer switches loops. When starting the system is running very rich when the engine gets hot it switches loops. This is like opening a closed choke. A big vacuum leak intake manifold gasket, or brake booster admits a large amount of excess air The computer tries to richen the mixture but can't as large amounts of air overwhelm it so it shuts down stalls.
Check basics before going into the harness.
On old cars if they wanted a tune up and it was idleing at 2000 rpm it generally was a vacuum leak as all parts had been replaced.
I had a 1984 t-b that had a short to the pickup in the dist. Plus wiring coming apart. I unplugged at the computer and took the harness out. I laid it out on a ping pong table. Purchased the best looking harness I could find at the junk yard, I used terminal tools and changed all deteriated wires. I ohmed the wires. Then I taped it up and covered it with the plastic looms.
I only had to make two splices on a couple of large wires. I did this on a saturday and the car ran fine when I finished by suppertime. I did have the junkyard harness before I started.

Electronic wise, this system is 100 times more complicated then what you were working on. Fixing problems on the cars you are referring to is a breeze.

Don

As I stated before, the pulsed signal to the injectors from the computer disappears, as well as the ac reference signal voltage from the computer that goes to the B terminal on the ignition module. This is why I know the cpmputer is shutting down. All the answers, or sugestions, or proposed solutions are so far below what I have allready done, it is not even worth mentioning. I need an expert on 95 Chevrolet Caprices.

I know you said the distributor was replaced. Was the optical sensor mechanism replaced? I'm not saying replace anything.

My info says that if the low resolution signal is lost, the vehicle will not start, or if it is running and the signal is lost, the vehicle will die. No spark and no injector pulse.

When the problem is ongoing, crank the engine and check for signal on the red wire with black tracer. That wire comes from the distributor to the pcm.

I didn't read where you checked that circuit, but seeing as how you're way ahead of the rest of us, maybe you did. goodluck

I just wanted to add, if you think a sensor, is shutting everthing down, when the problem occurs, check for reference voltage at one of your sensors. When the problem is ongoing, does the cel still lite-up?

If there is no reference voltage, you might unplug the sensors, one at a time, and see if the ref voltage returns? When you unplug the shorted sensor, if there is one. The ref voltage should return.

I thought somebody else already mentioned this, but I didn't see it, just now.

If finding wires touching each other or with breaks that are touching sometimes is a breeze. I had a 1993 mazda 929 that had a wire broke but touching going to the dist every time I tested it the voltage was correct start the engine and it would hit and miss. I used shielded cable and run new wires which fixed the problem. I had a 1998 GMC with a broken terminal in 2003. The mazda was fixed in 1999. I'm telling you you probably have a bad wire. You can replace every computer part on the car and it won't fix a broken wire. Normally wires or terminals break on older vehicles. A vacuum leak on a 2006 will shut down or stall an engine. Where you are losing voltage I would almost bet you money its a bad wire.
Weather its a 1950 or a 2007 it needs fuel,spark,air. Ohm your wire from the computer to where you are losing voltage. If you have a bad wire on a 1984 ford or a 1995 chevrolet from the dist pickup the engine won't run. I used a paper clip in the terminal and had somebody touch the ohm meter to the gem clip and a copper wire. I checked from the pickup to the copper wire. If you don't have continuity replace the wire. Any shielded wires have to be replaced with shielded wire or they won't run right. Any always check wires before buying a new computer. I went over to a friend who replaced the harness on a volvo he swore he checked every wire but two wires were missing so it didn't run. I found a toyoto supra where somebody took a soldering iron between the wires to the pickup touched each other and shut down the engine. If you don't check wires buy a new $900.00 computer and then do it.

I am going to answer all the e-mails as possible with this e-mail. I didn't mean to sound like a smart ass in some of the earlier e-mails, sorry if I did. Yes the distributor was replaced, and the optical sensor is in the distributor. I am glad you came up with the low resolution signal idea, and yes I have checked it when hot and cold,at the distributor. The AC signal is there when cold, and disappears when the engine dies. The low resolution signal is the AC voltage measured on terminal A ot the four pin distributor connector. When the connector is pulled out of the distributor, terminal A is the First terminal from the right, Looking at the pins on the connector, with the tab on top. When the engine was hot, after it had died and no resolution signal was available, I checked for a low resolution signal, of which there was none. I then rang out the wire from terrminal A to connector B(black connector) , pin 2 and got 0 ohms when hot(good wire and pin). According to Mitchell, this means the PCM (computer) is bad. I would normally agree with this, but the PCM has been replaced, with the same result.The problem with removing the connectors, on the sensors, is that this may render the same result, as sometimes the computer is lookin for a varing signal, and if an open or short is detected,sees this as an error, and may not run. If anyone knows the sensors, and relays that can shut down the computer when hot, please let me know. Rember, not only does the car die when running and hot, but then will not start, because of the loss of the low resolution signal,although it cranks fine.

Thanks,
Don

Since you ohmed the wires, Unplug the connector at the computer. Touch one end of the wire to a ground and see if you find any bleed thru. Then at the three wires to the pickup connect the leads to each one I think these are in a shielded cable see if you get a resistance reading. Check all wires around where you are having problems for bleed through.
Check grounds right front of engine under ignition module was this replaced. Side of engine between ac and starter. 20 amp fuse under hood fuse.A 15amp and10 amp fuse relay center. Fuse 6 20amp relay center right side i/p shroud.
Fuel pump ground left kick pad.34 heated oxygen sensor ground right kick pad.
If you have a logic probe check the black wire from ignition module and see if you lose ground. A cheap unit looks like a 12v test light with led lights red for hot and green for ground. Hooks to both terminals on the battery $20.00 on ebay. A loose ground as the car warms up and vibration will trip you off. All of these go to the computer.
On my 1994 T-B I had both shutdowns and random codes which went away when cleared and come back a few days later also the harness was getting bad on the ends. This is why I repaired it.
If the grounds are not in the correct area I need a vin number and body type. And engine liter size.
Some fuel pump grounds are under the back seat. I have a product reasearch manual with grounds and computer wiring diagram.
I think your problem is around the dist and module. Did you use new parts or junk yard parts I have found problems in both. Advance or auto zone will check your module for free. Take a hair dryer with you and warm it up if a drop of water will start to bubble stop or you will ruin it also ohm the ground under the module. Using your logic probe use a connection make one so you can determine if the light blinks brightly when starting. When going bad they will have a dim light.
Hopefully something will work. If my ground location don't work mitchell has a good wiring diagram showing all grounds. As I'm now retired I don't keep up but remember how to check out a system and I help out some people.

The distributor was replaced, and the optical sensor is in the distributor. I am glad you came up with the low resolution signal idea, and yes I have checked it when hot and cold,at the distributor. The AC signal is there when cold, and disappears when the engine dies. The low resolution signal is the AC voltage measured on terminal A ot the four pin distributor connector. When the connector is pulled out of the distributor, terminal A is the First terminal from the right, Looking at the pins on the connector, with the tab on top. When the engine was hot, after it had died and no resolution signal was available, I checked for a low resolution signal, of which there was none. I then rang out the wire from terrminal A to connector B(black connector) , pin 2 and got 0 ohms when hot(good wire and pin). According to Mitchell, this means the PCM (computer) is bad. I would normally agree with this, but the PCM has been replaced, with the same result.The problem with removing the connectors, on the sensors, is that this may render the same result, as sometimes the computer is lookin for a varing signal, and if an open or short is detected,sees this as an error, and may not run. If anyone knows the sensors, and relays that can shut down the computer when hot, please let me know. Rember, not only does the car die when running and hot, but then will not start, because of the loss of the low resolution signal,although it cranks fine.

Thanks,
Don

If any one knows where I can get a hand held diagnostic computer please tell me. I need more than a code reader, such as the EZ2000 by Auto X Ray, as no codes are being trigered. System is a mixture between OBD 1and OBD2, from the factory, but reades using an OBD1 connector. I need one that will look at the engine while running, and I would like to be able to activate relays and sensors, or at least check them out. I don't have a lot of money.

Thanks,
Don

The engine code is W, it is a 4.3L V8 police Caprice (Is that A body?).The fuel pump goes on for about 2 seconds, when the key is turned on, even when hot.There is supposedly a valve,in the fuel tank, that feeds the gas back into the tank. I forgot to tell you, the fuel pump pressure regulator has also been changed. Is there a sensor in the fuel rail, that sends a shut down signal, or interupts ground to the computer, that could cause it to shut down? If so, where is it located at? The reason I ask this, is that the fuel pressure seems a little low. I guess I will buy the approiate ful pressure testor for this car and test it. There is a half tank of gasoline, the fuel pamp has been replaced with a Delco, new strainer, and fuel filter.

For a hand held computer go to ebay.com type in otc computer checker look for a good used one in your price range. An otc 4000e can only be updated to 02 duece cartridge. The gensis uses software you update yearly.

You are ohming wires back to the computer. The computer is unplugged. You have 3 wires to the dist pickup in a shielded wire. If a fine wire pierces one of the three wires from vibration you are shut down. Put one lead on the shield woven wire. Then touch the lead to each wire if you get over 4 ohms it is considered a ground. If your meter reads 0 when the leads are touched together. If any two of the three have a bare spot as you touch any two of over 4 ohms you are losing signal. When you are touching leads with with an ohm meter if one end is a ground you should get a reading betweena wire with a bare spot touching another wire will shut down.
Either make up some temporary leads or use a couple of bed of nails. This is an otc lead for a voltmeter with a lot of fine pins. Check both leads for voltage when running. If you have 12 volt with the ignition on running you would have 6 volt as when running as it goes from positive negative faster than the meter will read. This is how I found the mazda problem. I couldn't find any thing with an ohm meter.
Take all of the readings if you want to find the problem

A computer ground blk/wht is connected to front of left cyl head. Fuel pump ground left rear wheel in engine compartment. Both front & rear wheel house must be a misprint. Check both until located.Base of left A pillar behind kick panel. Rear of right cylinder below exhaust manifold.
You should have a pressure regulator around the rail with a vacuum hose on it if it leaks gas when pulled it is bad. You can pinch off the return line in a rubber hose section with vise grips. Use a 12 volt test light on the coil neg. will it blink check cold and hot. If the computer loses a ground it will shut down. On the black & white on the module check blk/wht wire to the ground touch the wire at module and touch a ground.
Go out in the morning and heat the module with a hair dryer if it won't start get it replaced. I had a friends car I was checking out and as I have a module tester I asked him to get me one. He brought 3 PDQ modules in a row that were bad new ones. As he had had a kem I told him to insist that parts man to replace it with another kem. It was good.
Was the computer prom replaced or just the computer.

I like to check all the circuits. I realize it may lead back to the same problem. But, there is no ignition control because, there is no low resolution signal, because? Are you losing ignition feed to the distributor? Red wire terminal c. There might be issues involved with trying an alternative source for the ignition feed, maybe not.

If you try adding feed to the ignition circuit, I would disconnect the ecm end.

I bought a fuel injection pressure tester to eliminate the fuel system. Pressure is fine when cold or hot, pressure regulator also works fine.So the fuel system, including all grounds for the fuel system is eliminated.Will any of the hand held engine diagnostic computers diagnose an OBD 1 system while running to detect if the optical sensor, or one othe supporting chips are opening up on the rebuilt distributor that I put in. Will it aso check all the sensors? Did I tell you the car seems to be overheating, at least that is what the gage is saying. The thermostat has just been replaced. The radiator is not very warm at all. Water is comming out of the cap when hot, and the cap is not that old. The fans work when the radiator gets to the proper temperature. This car is the worst piece of junk I have ever owned. It is a true lemmon, or maybe just a bad year. Both my Dodge Caravans, my Ford Probe, and my Toyota Corrola opperate fine. My 89 Caprice that I had previous to this car was a 1000 times beter car. If this is the kind of junk GM insists on puting out, I won't own another GM. I guess land fills are good for something.

I would pull the water pump and see if the impeller is spinning on the shaft or fell off. Stick a water hose in the thermostat housing remove lower hose and flush out the system.
Before doing this I would run the car until it trips off. Immediately pull the distributor cap. Then I would feel the optical sensor. After it cools down I would use a heat gun like a pistol type hair dryer or lineoleum heat gun,and heat up the optical sensor until it feels the same as when it shut down. I would then install the distributor cap and see if it will start.
If your engine got hot enough on top to get the sensor to 212 deg. it most probably took out your unit.
I never used the new gensis enough to know what it can completely do.
I don't think my otc 4000e will say what is causing the problem but you can monitor the system. Most parts stores will warranty a defective part. The GM dealer may have a tester for the dist. sensor call and ask. If they sold it to you and they can do this insist. But it will have to be at shutdown temperature.
Even using a module tester you have to heat it up before it will fail. This is why I suggested to try it in the car.
I ohmed my mazda wiring more than once and never could find a problem but I used a new piece of two wire shielded cable and it run perfectly. You have to ground the shield. If your losing signal it has to be a computer,optical sensor or wiring. My car would run properly for 30 min shake then run properly and shake again but never shut off.
This is why I think your optical sensor is bad. Computers don't normally heat up and fail. Especially two even if from a junk yard.

You never did say if you were losing ignition feed to the distributor? The low resolution and high resolution signals are digital signals. If you have a dmm with tach--dwell--frequency signal, any of these will react to those signals. A digital logic probe should react to a pulse on those circuits. If the pump runs and you have ref voltage to sensors while cranking, the computer isn't shutting down, at least not on those circuits.

Even though this is an old post i just came across it. My best educated guess would be . water temp sensor. tok me forever to figure it out on my trans am. after replacing damn near everything down to the timing chain. it ended up being a $5 sensor