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Discussion Starter · #1 ·
Ok, i think i figured out why i have been blowing ignition boxes. It seems i have been overcharging for some reason. Problem is, i can only get a little run time in before the ecu blows. So where am i to look for the problem? I cleaned the grounds for the regulator, i switched with a known good regulator (which made the voltage jump 4 more volts till a wire started to fry). Put the other regulator back in. It was charging at 14.3-14.8 volts at the battery. Now for whatever reason, it seems to be down around 13.8-14.1, which i fell is acceptable, but i think its still borderline. What should i look for? BTW, the alt is a newer nippendenso style, as found stock on a magnum motor (i swapped in the entire engine). The magnum's are regulated thru the pcm, im using the older external regulator as i am carburated.

--chad
 

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Discussion Starter · #3 ·
But is 14.5 or so overcharging? I realize and know it charges more than 12 voltes, but i never had a vehicle that charged more than 14-14.1 volts before. Am i looking in the wrong place then for the blown ignition ECU boxes? Anyone have the factory wiring diagrams for an 85 truck? Id like to know what wire i fried when it went to 18 volts. o[

--chad
 

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Oops, just replied to your ECU post.

Sounds like one of the field post on the alternator is getting full voltage, and/or one side is ground/shorted out. You have to check the voltage at the field terminals on the alternator, you can disconnect one of them and run the car. This should keep the alternator from charging. Try that first. Your voltage at the battery might stay the same for a while, since the battery is fully charged, try turning on things like the headlights and see what happens, should come down.

I have a 84 pickup and those fusible links are always a pain, so I wouldn't doubt if that's part of the problem.
 

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I had intermitant electrical probs with an old 78 truck I had. Can't recall if it was over or under charging, but after replacing everything I could the problem persisted. Finally found an older mechanic who told me to replace the ammeter, it was internally failing. Problem solved. If your truck has an ammeter it might be worth looking into.
 

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Been trying to remember what the deal was with that truck. I kinda recall all lights going super bright for short periods of time, blower fan going supersonic, etc... so it must have been overcharging. I don't recall ever burning up my fusable link, though. But then again, this was almost 20 years ago.
 

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cu440da said:
I had intermitant electrical probs with an old 78 truck I had. Can't recall if it was over or under charging, but after replacing everything I could the problem persisted. Finally found an older mechanic who told me to replace the ammeter, it was internally failing. Problem solved. If your truck has an ammeter it might be worth looking into.
It's always a good idea to check mopar amp meters and the wires. Half the cars I've pulled apart, the amp meter wires are half melted.
 

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Check that there is not a short to ground on the wire going to the regulator. That would full field the alternator. It also could be an intermittant short that does not happen unless you are driving so look closely.
 

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Discussion Starter · #10 ·
nope, no pulley change. I think my best bet is what i had to due with the dart----rewire everything in the engine compartment. I had a bunch of problems in the dart and i finally just took a weekend and replaced all wires being used and removed all wires not being used---its probably the best thing to do. As i said, after messing with it, it came down to an acceptable 13.9-14.0 volts.

Ed, do i remove the field wire and measure at the field terminal or am i measureing the wire itself?

--chad
 
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slow_motion said:
nope, no pulley change. I think my best bet is what i had to due with the dart----rewire everything in the engine compartment. I had a bunch of problems in the dart and i finally just took a weekend and replaced all wires being used and removed all wires not being used---its probably the best thing to do. As i said, after messing with it, it came down to an acceptable 13.9-14.0 volts.

Ed, do i remove the field wire and measure at the field terminal or am i measureing the wire itself?

--chad
ya thats what i did after a few electrical gremlins,just rewired the whole car my way.the car is far from original and it was easier to do that then deal with the old electrical system all the time.
 

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If you use this basic diagram (link), you get a better idea what's going on. Basically the voltage reg just grounds out one of the field terminals, it controls the amount of voltage going to the field terminal on the alternator. If the reg sees say 15 volts (on the key side), it opens the circuit so no voltage goes to the field wire being controled by the reg. If the voltage drops, the reg will ground out the field, allowing voltage to flow thru the field terminals and the alternator starts charging. Now you can see if that one field wire going to the reg is grounded out (it might be shorted out to the engine), the alternator field gets full battery voltage and it goes full charge. Or if there's a problem with the keyed wire going to the reg, say a bad connection, that makes the reg think there's a lower voltage then there really is, and it will tell the alternator to charge more. So the trick is to be able to check the voltage at the field terminal while it's running without frying the ecu. The best is to look in the back of a big chilton manual, they have all the test procedures. You can disconnect either field wire at the alternator to keep it from charging. If it still charges with the field wire off, you got a very strange alternator.

http://www.bigblockdart.com/techpages/dualfield.shtml
 

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Discussion Starter · #13 ·
Ok, so do i disconnect the wire with it running, or disconnect first and then start it up?

I am looking for voltage at either terminal of approx 12-14 volts?

And not try to fry the ecu? i hope not, im on a third and i didnt realize they had gone so high in price (jumped about $10 since the last one ibought years ago.)

--chad
 

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slow_motion said:
Ok, so do i disconnect the wire with it running, or disconnect first and then start it up?

I am looking for voltage at either terminal of approx 12-14 volts?

And not try to fry the ecu? i hope not, im on a third and i didnt realize they had gone so high in price (jumped about $10 since the last one ibought years ago.)

--chad
I would run it first with both fields unhooked, so it shouldin't fry anything. Once it's going, then check the voltage at both field wires, one should be 12volts. Then I'd hook the fields back up to the alternator and check the voltage at the battery right away, if it's over 15volts, then you'll have to find the problem in the charging system. Like going thru the wires, checking the reg etc. But first get it running.
 

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Discussion Starter · #15 ·
Ok, here is what i got

Here is my initial readings 12.53 volts at the battery--before starting.

upon starting, it went to 14.6+v at the battery, alternator B+ post was 14.85+v

While running, the coil + was 6.89, and the one side of the ballast was also at 6.89v

Shutting off and Disconnecting the field wires, i started it up

the battery started at 11.83

the one field wire measured 12.3V

I connected both field wires quick and the battery voltage read 13.85v

So now im am baffled---at first the thing was damn near 15 volts and after testing with shutting it on and off a couple of times it was charging normal at under 14 volts. Bad wiring? Voltage regulator? Alternator? Im lost and getting fustrated.

PLEASE HELP!!!!!!!!!!!!!!!!!!!!!!!!! **(*)

--chad
 

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Since you suspect it might be an intermittent short in the wiring, try moving the harness while the engine is running and see if there is any change in the alternator output. If you have it narrowed down to just a wire or two you could use an ohm meter between the wire terminal and ground to check for the short, this is a little safer method but much more time consuming.
 

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This is why everyone loves electrical work.  Really have to have all the connections clean and all the wires in good shape.  I'd peel all the tape off the field wires and see what's what.  If you're running a voltmeter all the time inside the cab, you could just try driving it for a while and see what happens.   At least short trips and you could always pull a field wire if the alternator starts going crazy again.  If all the wires look good, I'd say it's the reg.
 

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Chad, it sounds like the regulator is tripping out... If the wiring is not hacked to hell, I would put my sights on that.
 

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Electronic voltage regulators control the ground for the field windings. One field terminal recieves full battery voltage. The other is grounded by the voltage regulator. The regulator varies the ground to control the output of the alternator. If the wire from the alternator to the regulator shorts to ground it full fields the alternator or in other words overcharges. I think this is likely where your problem is. Bad regulators usually don't ground the field at all causing no charge. I hope this helps.......Dave
 

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Discussion Starter · #20 ·
Well, i think i have made my decision to just rewire the ignition and charging circuts in the truck. I had problems in my dart and got to the same point and never had a problem after i re wired everything. Plus i'll lose all the other wires that dont go to anything anymore.

--chad
 
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