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Idle stuck at 3000, lost anti-dieseling, low voltage


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Hello. '86 KC Z24 carb CA model. Suddenly all of the following occurred:


1) Idle stuck between 2000-3000


2) Voltage gauge reading about 11.5v, normally is at 13.  Wipers and turn signals very slow.  Charge idiot light is on.


3) Lost anti-diesel function, truck diesels when turning it off


4) Exhaust smells rich


Only thing I recently did was swap the steering box, but truck has run fine since that repair was done.


Any ideas on where to begin the diagnoses?

Edited by MaddieCycle
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Fix the charging system. Yes...check that there is a ground from the negative cable to the body. Often, battery cables are replaced and this ground isn't available on the generic cable.


 Does the interior or horn work? If both do not then the Black fusible link is blown and this plays into the charging system. A guess.




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Next time you have running briefly remove one of the battery cables. If charging at all the engine will, keep running. Briefly, don't keep it off. My guess is the engine will quit because there is no alternator output. The alternator has it's own ground wire but mostly it's grounded to the block through the mount bolts. 


Belt tight?

Plastic two wire plug in the rear of the alternator snugly plugged in?

Ground (Black) and power output (White) connections are clean and tight?

Inspect the fusible links attached to the positive battery post. Specially the Black one. If burned, melted or crispy it's hooped. The Black one connects to the charging system. If burned out the battery cannot be recharged.


The Red charge light has battery on one side when the ignition is on. The other side has 12v or power when the alternator is charging. As there is equal potential on both side of the bulb, it does not light. If the alternator is not charging the alternator side is grounded through the diodes and the bulb will light.


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There are three main  grounds on the 86 720 in the engine compartment. 


These are pictures of my 86 720 2wd king cab.  I do apologize for the dirty engine compartment


The first and most important one is some what like Mike was describing except it is not a wire coming of the battery cable it is the battery cable.  The ground cable on the factory cable  has a second grounding lug that bolts to the edge of the battery box and then the ground cable continues on.  On my Z20 the ground cable attaches to the block and on my Z24 it attaches to the intake manifold.






The second ground attaches to the inner fender well and then attaches to the alternator it is a black 10 or 12 gauge wire.






The third ground cable attaches to the drivers side rear portion of the head and then attaches to the firewall.  The factory cable was a braided cable.  This braided cable can be replaced with a 10 gauge wire with eyelet ends.








If you want to learn a little more about automotive wire then read this it has a lot of basic information that is vital to adding and repairing circuits to you vehicle.



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Welp, the local rebuilder wants $250 to repair, and 2 weeks.  They want to sell me a Chinese new replacement for $189 which they stand behind. It is a reputable shop.  The question is: Do I have them try to rebuild mine, do I buy their "reliable" Chinese part for big $, or do I get a NAPA reman for $100?

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You could try it yourself with something like this. This is not the exact kit you need. Some on here have had good luck with the new chinese. Nobody ever has good luck with parts store rebuilds. They have lifetime warranties but you have to like changing alternators or getting stranded. I try to get my originals rebuilt whenever possible. I know their history and they always bolt right back into place.

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For $44 get the kit. There are any amount of you-tube how tos. The brushes are probably the only thing worn out, next the bearings (noisy) and last the diode rectifier if the light is on or poor output.

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The YouTube's even show you how to remove the bearing seals to repack the bearings with grease. I am guessing that is what happens with the autozone ones. Old bearings but new grease. I did do a rebuild on a marine diesel alternator I have and it was easy. So if you have some confidence and I have seen you have Maddie I second the kit.  Just double check you are getting the right one. I just threw that one up as an example.

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Well I just spoke to the "reputable" alternator shop and they are giving me a hard time about the rebuild, takes too long, blah blah blah.  They originally told me 2 days now telling me 2 weeks and when I expressed frustration that I wish I'd known that in advance so I could have moved my truck somewhere it can be parked longer term, he told me it was my fault for wanting the rebuild vs just putting their Chinese replacement in.  So I am done with these clowns and there are no other local rebuilders, they all just replace.

So I am going to get a NAPA reman for $75 and then also when I have time in the next few months will rebuild my OEM Hitachi and either swap it then or just carry it around in the truck like I do with a fuel pump.

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Instead of spending $44 on a parts kit I would rather spend a $100 on having my alternator gone through at a shop I have been using for decades.


By doing this all parts are tested visually inspectected, brush area cleaned up, and all parts are cleaned, cases glass beaded and painted parts are repainted.  The alternator looks and performs like new.  Oh I get a 3 year parts and labor guarantee because of all the years I have been using them.

Edited by Charlie69
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Got a reman from NAPA for $85, it is a Hitachi, so that is good news. I will keep my Hitachi core as well.

Now I am having a miserable time getting the lower bracket lined up.  I can not get it lined up to get both bolts in line for the holes.  I can get it lined up for either side but not both.  I am thinking either I need to take it off and grind down some of the new paint the reman put on the mounting on the alternator, or buy a longer M10 bolt and grind down a CM at the end to use as a guide to slide the bolt into the threads. I spent 2 hours today trying to line this up using all sorts of leverage and pry bars and I am at wits end.

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