How to Bypas Fuel Pump Relay 94 Jeep GC 4.0

docsplace · #1 02-22-2006, 08:42 PM

Anyone have a way of bypassing the fuel pump relay to see if a relay or pump is faulty.

I replaced a pump a while back and a Pickup coil that was open and faulty. After a couple hours of use the 1994 Jeep GC shut down and now I get nothing, Maybe a relay or I'm thinking that filter could be plugged and smoke the other pump. The pump relay ran about 2-3 minutes before I got a check engine light and it started. Sounds like it is another faulty pump or had back pressure and smoked the pump. I believe I;m getting spark or at least I tested the voltage to pickup coil and had 6 volt there, and after cranking engine had voltage pulsing. Can not get a check engine light, No Pump Noises, and Last Time I checked voltage at relay had 12v there.

Oh Yea I forgot to mention that the old pump would work on a test. I took a 12v DC Adapter 350ma and Run Jumpers to the hot and ground. Hooked it to a switchable Surge Bar and The pump turn on no problems.

Thanks In Advance
Tom

dksob81 · #2 02-23-2006, 07:11 AM

Does the CHECK ENGINE light come on now, when you turn the key?

docscomputers · #3 02-23-2006, 08:36 AM

No it does not,

But a another strange thing is that when i said there was a constant 4.9v to pickup coil. that is gone. Did the crank test, turned key on got 5 volt cranked engine and got a volt pulse. so that is working. I'm thinking that there may be a short in the relay box wiring or the ASD relay is bad or pcm ground. going to try those things today.

Tom

docsplace · #4 02-24-2006, 03:04 AM

Is the Throttle Control Sensor suppose to be closed? When I stick the ohm meter on it it O's out like it us shorted. That is the sensor on the Throttle Body With 2 Wires.

Tom

dksob81 · #5 02-24-2006, 06:49 AM

There shouldn't be any wire on the Throttle Body with 2 wires, all should have at least 3 wires. TPS - 3 wires, MAP - 3 wires and IAC Valve - 4 wires.

You might be talking about the IAT (Intake Air Tempature) Sensor, it is located on the intake manifold, it has 2 wires, You should never have 0 ohms from this sensor. @ -40 degrees you should get a reading of 291,490 - 381,710 ohms and at 120 degrees you should get a reading of 370 - 410 ohms.

docsplace · #6 02-24-2006, 06:59 AM

Yea thats right it is on the intake manifold, The throttle body is actually the replacment of the carb right. But that 2 wire sensor 0's out would it cause a ASD and create no spark of it is shorted.?

Tom

dksob81 · #7 02-24-2006, 07:03 AM

don't think so but it might short out some other sensor which might be needed to start.

Did you check the resistance with the electrical connector disconnected? It should be disconnected when testing.

docsplace · #8 02-24-2006, 07:12 AM

Yes I pulled the plug and testd the wires and the sensor and got nothing on the wire and 0's on the sensor. I wonder if that might be my battery drain problem also?. Shorting that plug with a jumper may not work either becuse a jumper would be a dead short with 0 resistance to probalbly. I will double check it but tested it a couple times. It may have some carbon on corrorrsion on it causing it to short.

Tom

bringselpup · #9 02-24-2006, 01:14 PM

Open the PDC and pull the fuel pump relay. Use a dental tool or some such small prying thing and carefully take the cover off. Looks like this.

http://img.photobucket.com/albums/v4...t/DCP02124.jpg

Put it back in with the cover off. You can see the spring loaded contact on the relay that closes when the computer tells it to close. You can:

1) Watch it and have someone do a key on to see if it closes like it should right in the beginning.
2) Close it yourself carefully just for a moment and either have someone up the back listen for the pump to spin, or be real quiet and listen to the fuel rail. I could hear it pressurize.

With it open like this you'll be able to see which two pieces of the circuit is being closed by the relay contacts.

docscomputers · #10 02-24-2006, 03:37 PM

Thanks, That a good piece of info, I will try that and see if that is part of my problem. I think I'm having other problems because I powerd the old pump with a 12v DC Adapter and it would pump. Thanks for the info, I know that will work because it gets the 12v to the relay.

Tom

docscomputers · #11 02-24-2006, 04:08 PM

Well that worked which I thaught it would it did pump fuel and I could here the fuel rail fill up. But still no check engine light and no spark. I thaught that my iat sensor was dead shorted but in fact it is open, No ohm reading at alll, I pulled the sensor out and hooked a couple wires to it and tested it that way and I get know ohm reading at all. I still have something draining the battery, As soon as I hook the battery up it arcs a little and if I leave it overnight it has real low charge in it. Got to figure out where my short is. Someone told me to hook a meter up to the battery and start pulling fuses to see if I get a voltage drop. Anyone ever try this.

Tom

docsplace · #12 02-24-2006, 08:48 PM

Quote:

Originally Posted by docscomputers

Well that worked which I thaught it would it did pump fuel and I could here the fuel rail fill up. But still no check engine light and no spark. I thaught that my iat sensor was dead shorted but in fact it is open, No ohm reading at alll, I pulled the sensor out and hooked a couple wires to it and tested it that way and I get know ohm reading at all. I still have something draining the battery, As soon as I hook the battery up it arcs a little and if I leave it overnight it has real low charge in it. Got to figure out where my short is. Someone told me to hook a meter up to the battery and start pulling fuses to see if I get a voltage drop. Anyone ever try this.

Tom

A funny thing happened today I decided to pull a few fuses at the relay box and when I pulled a few fuses on the right side, I think was the PCM Fuse, ABS Fuse, and ASD Fuse whatever the first 3 fuses are at the far right of the box. My dash lights come on, The Check Engine Light, ABS Light, and The Seat Belt Light. The key was turnded off. I had to unhook the battery to get them to go off. Is this normal?????

dksob81 · #13 02-25-2006, 06:19 AM

I'm not sure if this is normal haven't pulled all my fuses yet....lol

But do you get the CHECK ENGINE light when you turn the key to the ON position now? After pulling the fuses.

If you turn the key to the ON position and wait a few minutes does the CHECK ENGINE light come on?

docscomputers · #14 02-25-2006, 08:38 AM

No It still does not come on. But why would the lights come on when the fuses are pulled? I'll pinpoint the one that actually causes all the lights to come on and there may be something in that circut and let you know later.

Tom

bringselpup · #15 02-25-2006, 08:57 AM

And when you do a key on are you getting that initial fuel pump burst?

Have someone do a key on and see if you can see the computer trying to close that relay right at key on. If it doesn't and yet you've said you can manually fire that pump the question is why isn't the computer doing that. That's an essential part of the start sequence and maybe you have no spark because you don't get far enough down the sequence in the computer.

I'd then be asking myself is it all related to the funky fuse business, or is it a toasted computer and the fuse business is because of that.

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