alternator feedback?

[slammd72]

my truck doesnt seem to stop running even after i turn the key into the off position and pull the key OUT! after the key is out it begins to idle slower, probably because the power is cut to from the battery? right?

the only way i can get it to quit is by putting my hand over the tbi or flipping the tank selector switch to the right tank. (the right tank is removed along with the tank selector valve)

ive replaced both relays on the top of the fire wall. and the battery is brand new.

could "alternator feedback" be the problem? i dont know what alternator feedback is, but i saw a relay kit in the latest summit that is supposed to fix this problem. as stated in the small description.

the specifics on the truck are:
1988 CrewCab Dually 454TBI all stock engine compartment

[shadetree]

Your alternator is "self exciting" the ignition circuit. The factory solves this problem with the "idiot light" in the dash. What this light does, is when you shut the ign off, the alternator tries to keep itself "excited" through the alternators second (or the charging) wire. The installation of a bulb in that circuit makes the light glow when the power is trying to go backwards, but the current after that bulb isn't enough to fire the ignition coil. Another fix can be a relay, as you saw in Summit catalog. Either one, if wired corectly will fix your problem.

[stueber68]

I wonder if somebody could install I diode inline to that circuit.

[slammd72]

Originally Posted by shadetree

Your alternator is "self exciting" the ignition circuit. The factory solves this problem with the "idiot light" in the dash. What this light does, is when you shut the ign off, the alternator tries to keep itself "excited" through the alternators second (or the charging) wire. The installation of a bulb in that circuit makes the light glow when the power is trying to go backwards, but the current after that bulb isn't enough to fire the ignition coil. Another fix can be a relay, as you saw in Summit catalog. Either one, if wired corectly will fix your problem.

so then i need to put in this "idiot light"? if you have any specifics i would really appreciate it. i.e. which wire to tap into or what kind of light

would the problem stop if i temporarily unplugged the second charging wire? im gonna try that right now. thanx for the help.

[slammd72]

well i guess the alternator is not the problem. i completely unhooked all wires on the alternator and there was no change in the way it acted.

[shadetree]

The wire that "excites" the alternator is a 16 ga brown wire, comming off the back of the alternator. It goes into the ignition switch circuit, to allow voltage from the battery to "excite" the alternator (alternators will not produce ele unless their feild coils have been charged with voltage from the vehicle battery). The issue of "feedback" is your brown wire is allowing voltage to go backwards through the ignition circuit (from the alternator) this feedback keeps the ignition powered. A diode can be inserted in this circuit, yes. I used one, but after about 8 months, the diode blew, leaving me with an engine that wouldn't shut off. I switched to an in-line 12 volt bulb in that brown wire, and THAT permanently solved the feedback problem.
Now, your truck keeps running with the ignition switch off, AND the brown wire disconnected? Huston, we have a problem. I'm not sure now, what sort of diagnosis track to start you on. I would start with the ignition switch, using a volt meter to see if it was really shutting off power to the system.

[stueber68]

You need to see if there is a short in your ignition switch or if another power supply has been added for the ignition system, when you trun the key off that should be it

[slammd72]

YEAH! this has been a real weird one! ok ive got it to quit running by turning off the key and pulling it out but...i took out the "ECMB" fuse. what is the fuse fusing?

also a few months back i went to autozone to see if they had any insight and they sold me a $10 relay. one of the two on the firewall. that fixed the problem for about twenty minutes.

now i unhooked both firewall relay and took out the fuse and it will start (after alot more cranking) and will turn off by the key.

i am SO confused. im tempted to go carburated

[jimfulco]

Is it dieseling? Could a dirty injector be dribbling enough fuel out to keep it running?

[slammd72]

i dont think it could "dribble" the same amount of fuel to keep the damn thing running at the same rpm. im convinced its something electronic. because of the fuse removal temporary fix anyone got any suggestions.

btw anyone herd of these HELM manuals? if anyone has got one i could sure use a wiring schematic for the engine wiring and sensors.

[stueber68]

I have some of the original GM sercive manuals with the wiring diagrams. I will look at them tonight and see what I can find out for ya.

[slammd72]

Originally Posted by stueber68

I have some of the original GM sercive manuals with the wiring diagrams. I will look at them tonight and see what I can find out for ya.

thanks man...i could use some accurate diagrams

[stueber68]

Hey, sorry. Last night turned into date night for my wife and I. I didn't get the chance to look that stuff up for ya. I get off work at 1 then I will get right on that. Stay tuned, same bat channel!

[stueber68]

Okay, something is really screwed up. You said that you unplugged both of the relays at the junction block on the passenger side. There are two relays for that area #1 the cold advance relay (optional) #2 the fuel pump relay. If you can still start this truck with the fuel pump relay unplugged something is very wrong.
The GM manual that I am looking in is for a 91 and it does not have a listing for an ECMB fuse, however I am thinking that the fuse you are speaking of is the INJAB fuse with my info. Is it right below the ECM/IGN fuse? The vehicle should not run at all with this fuse out. I really think you have a short in or right after the ignition switch on your truck. You have a 12v lead touching some ECM or ignition demands. Check to see if some wires are stuck together. Also I would say put the fuses and relays back and start the truck. Turn the key to the off position and if it continues to remove the two bolts that reain the fuse box to your firewall. Sometimes the wires can get hot behind the box when they are all crammed together such as they are.

Let me know what happens here.

[stueber68]

I can't help but think that this is going to end up being something so simple and that I am going to feel like an idiot.

[stueber68]

Did You Get This Thing Taken Care Of?

[LostMy65]

Originally Posted by shadetree

Your alternator is "self exciting" the ignition circuit. The factory solves this problem with the "idiot light" in the dash. What this light does, is when you shut the ign off, the alternator tries to keep itself "excited" through the alternators second (or the charging) wire. The installation of a bulb in that circuit makes the light glow when the power is trying to go backwards, but the current after that bulb isn't enough to fire the ignition coil. Another fix can be a relay, as you saw in Summit catalog. Either one, if wired corectly will fix your problem.

I know this is an old thread....
But this post helped me understand the possible reason why I don't have the 'Shut-Off' issue.
I had been reading much on the topic of switching to an internally regulated alternator on my 66, and went and bought some of those Radio Shack diodes - number 276-1661. I somehow misplaced them and figured I'd still do the change to the internally regulated alt. I figured if the truck didn't shut off, I knew what to do, and then go buy another diode.
Either way, my truck never gave me the shut off issue.
I bypassed the external regulator by doing one of the suggestions in this forum of taking the wires: brown to blue to alt, and red to white to alt.
I have a dummy light in my gauges that works, so I'm guessing that's why I don't have the 'Shut-Off' issue, despite not having put one of those diodes inline.