Ha! From one problem to the next! I'm almost beginning to understand the frustration that leads some to abandon the 1st gens.... but I'm not there yet, got too much time and love invested.

Just got done fixing overdrive and leaky throttle shaft seal on Bosch VE, when I get massive electrical problems.

Seems like it's not charging, lots of dimming lights and truck won't start without jump, even after running a while.

Pulled out alternator, had it tested, and it tested strong. Started putting my 12Volt test light all over the place, and got a short to ground on the alternator 12V output wire, even with alternator removed from truck.

Anyone had this problem before? I don't really want to pull the wiring harness apart to visually inspect the wire, that seems like no-fun.

I'm thinking of just replacing the output wire with a new one and running it outside the harness straight to the battery. Is there anything special about this wire that I should know? Does it do anything fancy while inside the harness? Does it do anything special when it passes through the little black box on the alternator itself?

Thank you everyone, as always, for your invaluable help.

Christian

 

I can't think of any way the output wire would short to ground. How did you determine that with a test light?

Check the crank sensor and its wiring very carefully. Gap on the crank sensor should be 0.050". The PCM uses the crank sensor to decide whether the engine is running - if the PCM doesn't get the signal it does not turn on the ASD relay and you have no field current.

If the crank sensor stuff looks good, swap the ASD (front) and A/C (middle) relays on the driver's fender and see if that clears up your problem.

Failing that, put an external regulator on it.

 

Well, It seems like I made one assumption and it seems that assumption was wrong.

With the positive lead removed from the battery, I put a test light between the lead and the positive post. I got a light on the test probe, so I determined I had a short. I carefully un-did each of my accessory wires, checking for a light each time. Only when I un-did one particular wire, which is a single conductor, black, fat lead did I not get a light. This particular lead has a latched plug and then disappears into the wiring harness near the battery. On the other side of the engine bay, the fat black wire coming from the alternator looks exactly the same, and also has a latched plug and also disappears into the wiring harness.

The assumption I made was that these are the same wire. When I tested for a short to ground at the plug on the alternator side of the engine bay, I also got a short, further reinforcing my idea that these are the same wire.

I did not have the positive lead on the battery at any time during these tests, and so I did not notice that with this particular wire (the one on the battery side) unplugged nothing in the truck works. After spending a day messing around with the alternator and running a separate wire for the output, I finally did notice that the truck will not do anything without this wire - it now seems to me that this wire basically gives power to ALL accessories, including ignition.

To my way of thinking, that just means my short could be anywhere!

Anyone have any good ideas on how to track down shorts? It seems daunting to me.

Dave, I will try the various things you've suggested. At this point I don't know where the crank sensor is, nor do I know which relays you are referring to. Any help appreciated.

Thanks,
CR

 

Wrong assumption.

The connector on the driver's side is the feed for the whole truck.

If you had a short it would have fried the fusible links. I think the alternator is just not getting any field current.

 

Right.

Got the truck running with the help of battery charger (on boost/start setting). I put a meter on the battery as the truck was running, and the load seemed huge. Everything was dim, meter was fluctuating between 5 and 7 volts.

To my mind, this seems to suggest a short. But I see the point about the fusible links being gone if it had been a short. Another thing that confuses me is that the truck was fine 2 days ago, and then trying to start it yesterday, the battery was dead, as if something had drained it overnight.

However, I guess that could really just be lack of alternator output (due to loss of field), but somehow I thought the battery would do better than that on its own. (Battery is relatively new.)

Question1: Would you suggest putting a 12V jumper to the field to see if everything comes back? Question 2: Of the two small studs with 7mm nuts on the alternator, which is positive, top or bottom?

From what I understand, it sounds like the crank sensor could be to blame. I looked in my Chilton's manual and, unsurprisingly, couldn't find the location of it. Question 3: Any help locating it?

Question 4: And also, what does ADS stand for? I understand it controls field voltage to the alternator.

Question 5: If the 12V jumper to field brings it all back, it sounds like it could be the crank sensor, PCM, or ADS relay, right?

I never thought I'd get to know quite this much about this truck!!!

 

Also, with the feed wire for the whole truck (the one on the battery side) disconnected, is there any reason I should see 12 volts between that wire and the positive battery post?

Thanks,
CR

 

1. Yes - but only to test the ASD relay. The PCM regulates by switching the field ground on and off rapidly - so if the crank sensor is bad applying 12 volts to the field will prove nothing.

2. To full field the alternator (assuming the ASD relay is supplying power, ground the bottom terminal. Top terminal should have battery voltage when the engine is running.

3. Above the crank damper, slightly to the passenger side of center.

4. ASD stands for Automatic Shut Down.

5. Depends on what jumper fixes it. If 12 volts to the field wakes it up, then the whole PCM/crank sensor/ASD relay thing needs going over. If you are getting battery voltage on the top terminal, and grounding the bottom one spikes the voltage, then the regulator circuit in the PCM is toast. Install an external regulator and be done with it.

12 volts between the connector question:
On a multimeter or LED test light, or with a good high-amp test light? If the doors are closed and the key is off, there's no reason a high amp test light should light. A multimeter or an LED test light will show voltage.

 

Thanks, Dave, for your thorough answers.

Just one thing, though.... luckily enough my girlfriend has the identical truck (except with 80,000 fewer miles, hers is usually working) and with the engine running in hers I measured about 1.5 volts on the top terminal of the alternator and battery voltage, i.e. 12.5 or so, at the bottom terminal.

Was her truck doing something weird, or is battery voltage really supposed to be coming in at the bottom stud?

Thanks,
CR

 

Nope. I screwed up. You have it right.

 

Ha! The worst way to get a problem fixed!

I put everything back together to have a starting point before putting jumpers everywhere, and the truck is working just fine!

Yesterday it wouldn't start without 250 amps from a charger, and the radio wouldn't work if the lights were on, and the pyrometer was all over the place, and now everything is fine! And I changed nothing!

I don't know whether to feel really lucky or really scared......

 

Sounds like it was a cruddy connection. I hate that kind of stuff, because even though it's working it's not FIXED.

 

Exactly.

Another interesting piece of the puzzle I put together in my head driving (my Scout) home this evening....

When I did put it all back together, at first I forgot to plug in the alternator output latched plug over on the passenger side of the engine bay. While the truck was running I had a meter on the battery and was seeing the voltage slowly decreasing from 12.8 to 12.6 or so. Then I realized the mistake and plugged the alt. back in, and got the expected 14 volts.

However, in the midst of the problem a few days ago I had a meter on the battery while the engine was running and the alternator WAS plugged in, and I had a battery voltage reading of 5 - 7 volts, fluctuating all over the place.

That seems to indicate to me that it's something other than an "alternator not getting a field" problem. Hmmm, still thinking about it.

CR