Erratic electrical problem--1986 Mustang GT

M

ms fowler

Senior Member
I have posted this on Fox-body specific Mustang forums with no success. Its time to see if the old guys over here can help..
Erratic charging system problem symptoms-
1) Ammeter shows wide swings D to C
2) Engine surges at idle---my guess is due to #1 above as the load on the engine changes so does the RPM
3) W/S/W makes random sweeps while the W/S/W switch is in the OFF position
4) The interior lights on the bottom of the rearview mirror flash on and off

I have tested the alternator at parts stores and it tested good--I replaced it anyway because "everyone" said it was a regulator problem. No difference

I have cleaned and tightened battery connections, solenoid connections and both grounds from the battery--one to the engine block and the other to the fender apron

Last week I started the car up to move it and everything behaved as NORMAL. I had done nothing immediately prior to that. It just suddenly worked.
Yesterday I had it outside for pictures. I lowered the top and the windows and everything worked perfectly.
Then i went to drive it inside and NOTHING. Not a click or a grunt---nothing.
In checking the connections (again), I noticed that the Circuit Breaker on the B+ post of the solenoid was hot. AFAIK, that CB supplies current for the windows and top only. I disconnected it anyway and after a few more minutes I had headlights and door buzzer. ( I had pulled the headlight switch "on" and left the key in with the door open to help me know when I had voltage again)
I tried to start it and all was fine--I mean it started and ran, but I still had the 4 symptoms listed above.

It sure seem to me to be a "Bad Ground"--but I don't know where else to look.
Are there any other CB that might be overloading and tripping?

This car is pretty much "as Ford made it"--At least there are no "boy racer" mods that I can see.
Any help would be greatly appreciated.
I am stumped. I am "an old guy" too.
 
I had an '86 F150 that I fought with similar symptoms to your car. It ended up being a ground issue at the pcm. One of the ground wires was corroded inside the insulation about 4" from the pcm connector. Spent hours doing "wiggle" tests with an ohm meter to finally find it
 
Alaska Jim has a good point. Check that out. Other than that, sure sounds like a ground issue to me. Fox body Mustangs are notorious for bad grounds. On all my builds, and especially my 2 Mustang builds, I make the entire chassis a ground plane. I weld studs to the frame and run my NEG batt cable directly to that. I run two 4ga cables from each side of the engine to welded studs on the frame. I also run a 4ga cable from the body to a welded stud on the frame. Finally, every accessory ground wire I run to a terminal strip and ground that to a welded stud on the frame. In all my builds never have I had a ground issue.

https://www.fordbarn.com/attachments/494841/
 

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Thanks for the responses.
I have 3 or 4 volumes of the Factory Service manual. I "thought" I had the Electrical/ Vacuum one, too, but I cannot locate that on. I do have a Haynes manual which does have ( very tiny) wiring diagrams.

I will check out the bulkhead connectors as well as the ground at the PCM.
Thanks.
 
You need to locate in the MANUAL the circuit that is causing the problem and what accessories are on it. The WIRING DIAGRAM will give you that and the location of the circuit ground.

If the CIRCUIT BREAKER is overheating, that is an indication that it may be bad as if there is an overload on the circuit, that circuit breaker should trip almost immediately to protect the circuit and the rest of the electrical system.

If the CB protects only POWER GLASS and CONV TOP (it should be identified in the OWNERS MANUAL), maybe remove it and drive it to see if the problem goes away.

If you are lucky, there will be a DIAGNOSTIC SEQUENCE for the circuit and you just follow that.

Yesterday I had it outside for pictures. I lowered the top and the windows and everything worked perfectly.

Then i went to drive it inside and NOTHING. Not a click or a grunt---nothing.

In checking the connections (again), I noticed that the Circuit Breaker on the B+ post of the solenoid was hot. AFAIK, that CB supplies current for the windows and top only. I disconnected it anyway and after a few more minutes I had headlights and door buzzer.
Click to expand...

Can you post a photo of the CIRCUIT BREAKER and its' location, especially if located at the STARTER SOLENOID?
 
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I went to look for the firewall bulkhead connectors, expecting to see what I knew Ford used on Falcons and Mustangs-- a couple of blocks of 10-12 bullet connectors, keyed to prevent plugging them in incorrectly. Well, "time marches on"---there were no connectors that I expected to see. Evidently, Ford eliminated those connectors and just shoved large bundles of wires thru the firewall and protected them with large grommets.
What I did was to uncouple every connector I could find under the hood, wiggle the wires and reconnect them a few times to attempt to clear out any corrosion. I didn't see any corrosion, but plugged and un-plugged each one several times anyway.
Afterward, i started the car and it behaved normally with no more weird symptoms. I am not sure I solved anything, but maybe I did.
I will keep you posted if anything changes.
Thanks for taking the time to read and respond.
 
There can be corrosion that you cant see. I had a pickup once that would start when it felt like it. Took off the wires on the starter, polished posts and connectors with emery cloth and never had that issue again.
 
The 86 should have a full electronic alternator with internal control. They are usually either good or bad. Not much in between. Circuits coming alive when switches are off are disconcerting. This makes me think there may be cross shorting of a wire or two in the system. Wipers carry through the firewall in most cases but the interior lighting should be in cab only unless there is an under hood light but that should be a separate circuit with it's own switch.

All it takes is one wire shorting to ground or shorting to another wire to give an electrical problem such as described. Plastic electrical plugs can melt. A hot wire can melt other wires in the same bundle and cause a cross short. A person needs a volt ohm meter to test each suspect circuit so a good diagram is essential to know where to isolate each circuit and the wire colors and such. Full systems going on and off could be caused by loose main connections such as battery or computer connections. I don't remember what an 86 Fox body has for systems but I know that Ford went to throttle body injection in 1984. These systems were some rudimentary systems by todays standards. I don't know how well the computer control systems held up in those cars either. A good diagram will show all the connections to sensors and functional systems.
 
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