Electrical Problems--HELP!!!!

1992MustangGT · May 30, 2009

I'm trying to figure out some electrical problems on my friend's 1992 Mustang coupe. Car has a 2.3L 4 cylinder. Something is draining the battery, the fuel pump will not work and I can't get the computer to turn on when I attempt to read the codes. The battery is brand new; only 3 weeks old and the fuel pump is brand new also; only 1 week old. I attempted to read the codes on the computer with my scanner, but when I plugged it in, turned the key to the on position and turned on the scanner, the computer would not even come on. I tried the scanner on my 1992 Mustang GT 5.0 and it worked just fine, so I know the scanner is good.

Here's a few ideas I've been kicking around and need someone to help me out and tell me if I'm on the right track....

1) I'm wondering if the computer is bad and that's why the fuel pump won't come on???
2) Is there a way to test the computer to see if it's bad?
3) Could it be that the computer relay failed and that's why the computer won't come on?
4) If it is the computer relay, where is it located and is it possible to test the relay to see if it's malfunctioning?
5) The fuel pump relay could be bad as well, but that wouldn't explain why I can't read the codes on the computer.
6) I'd like to "reset" the fuel pump shutoff switch (enertia switch), but I don't know the procedure for doing so. How do you reset the fuel pump enertia switch?
7) What is killing the battery? I think there's more than one problem going on here. Something's drawing juice out of the battery. How do I figure out what it is? What could it be?

I really appreciate any and all ideas and suggestions. Please help me figure this out. Thanks.....

jrichker · May 30, 2009

Fix the current drain first.
Typically it is something draining the battery. Small things like glove box or courtesy lights are often the culprits. If you have an aftermarket stereo or alarm system, it is also suspect.

The ideal method is to disconnect the negative terminal, and connect a Digital Multimeter (DVM) between the negative terminal on the battery and the negative cable. Set the DVM on a low current scale of 2-5 amps if it doesn't auto range. Watch the current draw, and then start pulling out fuses. When you see a sudden drop in the current, that circuit is the likely culprit. Note that the computer, radio & clock will draw less than 1/10 amp to keep the settings alive.

Using the negative side prevents nasty accidents like shorting the wrench to ground while disconnecting the battery cable. Electrically, the test will work with either positive or negative battery cable.

1992MustangGT · Jun 1, 2009

Thanks for the advice on testing for the short. I'll definitely try that suggestion. I've got electrical problems on some of my cars too and that will also help me trace down the problems.

Any idea about why the computer won't come on or the fuel pump? Could it be the computer relay? Where's the relay located?

jrichker · Jun 2, 2009

The relay is on top of the computer, it provides power to the computer and fuel injector system.
It is somewhat difficult to get to, since it requires you to remove the computer from its 2 bolt mounting. If the relay or socket is bad, you will not have 12 volts on any of the red wires in the engine compartment or to pins 37/57 on the computer. The pins 37/57 are the main power feed to the computer.

See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring Mustang FAQ - Engine Information Everyone should bookmark this site.

Ignition switch wiring
http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif

Fuel, alternator, A/C and ignition wiring
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif

Complete computer, actuator & sensor wiring diagram for 91-93 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/91-93_5.0_EEC_Wiring_Diagram.gif

Complete computer, actuator & sensor wiring diagram for94-95 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/94-95_5.0_EEC_Wiring_Diagram.gif

Vacuum diagram 89-93 Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg

HVAC vacuum diagram
http://www.veryuseful.com/mustang/tech/engine/images/Mustang_AC_heat_vacuum_controls.gif

TFI module differences & pinout
http://www.veryuseful.com/mustang/tech/engine/images/TFI_5.0_comparison.gif

Fuse box layout
http://www.veryuseful.com/mustang/tech/engine/images/MustangFuseBox.gif

1992MustangGT · Jun 7, 2009

NEED MORE HELP.... PLEASE!!!!

Ok guys here's the results of the testing results I did today, thanks to all your advice....First thing I did was follow your instructions about testing to see what was killing the battery. As instructed, I disconnected the negative battery terminal. I set my Digital Multi-Meter on 2 amps, touched one prong to the negative battery terminal and the other prong to the negative battery cable. And much to my disappointment there was absolutely no reading on the meter, just all zeros. So I reversed the prongs and still no reading. So that test ended leaving me totally confused and not knowing what to do from there. So I still don't know what's killing the battery because this test didn't register anything. Did I misunderstand your instructions and do something wrong?

Ok the second test was to pull the computer out of the kick panel and see if the computer relay was malfunctioning. When I got the computer out I looked at the top of the computer and I saw a black piece of plastic that was flush with the surface of the compter, like it was installed into the computer or something. I was expecting to see something "setting on top" of the computer, not recessed and flush with the computer. But I'm assuming what I'm looking at is the relay, but I don't know for sure. Can you tell me if that is the relay? Does it set down into the computer and is flush with the ouside surface? How do you get it out, just pull it out with a little force? I don't see any screws or clips holding it secure to the computer.

Since I was not sure if that was the relay I was looking at, I decided not to mess with what I thought was the relay anymore until I asked you guys some questions and take my digital multi-meter and test to see if I had power to the computer. So I unplugged the wiring harness from the computer, turned the ignition switch to the on position, set my mult-meter on 20 volts, found the 2 red power wires in the wiring harness plug/connector, and stuck the red prong into the plug hole and the black prong to a ground. There was absolutely no voltage whatsoever. So I took the red prong and put it in the other red power wire hole and still no power. (just in case you're wondering if the mulit-meter works, yes it works. I tested the battery after we charged it up and it had 12.84 volts, so the meter works).

So I guess my question is this....If the red wires have no juice in them and therefore, the computer is not getting power, then what is causing the red power wires to not have any electricity?

And just a little background, the car was running and suddenly stopped running. I asked my friend if he had done anything to the car in regards to wiring or mechanical work and he said he hasn't touched it or done anything to it. He just drove it back and forth to school. So it doesn't sound like it's anything he screwed up, the car just quit running. So what would cause a sudden loss of electricity in the wires to the computer? HELP GUYS, I'M AT A LOSS AS WHAT TO DO NEXT!!!!

jrichker · Jun 8, 2009

If the computer power relay isn't working, you will not have power to the computer.

Diagram courtesy of Tmoss & Stang&2birds

chefrsg · Mar 19, 2010

95 mustang gt 5.0 just quits no power at all.. help

i have a 95 mustang gt conv. i have been having alot of problems so were to begin... lets start with the worst. about 2 months ago i got into my car and tryed to start it it clicked then no power so i figured the battery cables tighten them down crancked up....then about two weeks later i get in all power gone then this clicking noise in the trunk happens and no power to the car..i took the battery cables off then put them back on boom cranks right up.. for about two weeks it was fine then i changed the exhaust runs great.. then it started to idle funny and act wierd so i had plugs wires cap and rotor put on ran better but not good enough so i had the timing checked they said it was good but something electrical was causing a miss.... so now what then as i drive sometimes when i take off the check engine comes on and never stays on. i took it to the shop to see if they could get a code but no codes but the light dosnt stay on to get any unless im driving it. so this leads me to today.. i was driving to the store and bam no power everthing goes out no power nothing so i push the car to the side and try to start it and the power came on alittle then drained off, so i disconnected the battery and let it sit for 10minn hookie it back up vroom started fine .. put gas in started right up backed up died.. turned the key vroom just hauled ass home and this is were i sit so if anyone has any clue PLEASE HELP ME.... I dont have money to get a new car , my car is my life line without it i will not have a job or a way to get one.. thanks scott

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