extreme steel

Could you please post a picture of relay location? i disconected pos+ bat cable turned key to on position and touch pos+ cable on battery when i did that you can hear relay clicking so assume its working. now this is the relay closes to the battery.

thrashingcows

Take a look in this thread....from the Sticky.

https://www.dieseltruckresource.com/...d.php?t=128911

You are looking for the relay #5...it calls it an shut down relay, but what it does is turn the power on/off quickly between the alternator and the ECM to control the charging....or some such thing.

patdaly

Not exactly, it is what provides the 12v to the field, the PCM modulates the return ground to control the charging.

It took me long enough to get that thought pounded into my head........

thrashingcows

That's why I added..."or some such thing." Cause I figured I was not 100% correct.

Thanks for the "proper" explanation. But I'll still probably forget it for the next time I give faulty advice....

j_martin

Best to systematically troubleshoot.

with the engine running, check the 2 field terminals on the alternator.

If one is 12v, and one is 0v, and it isn't charging, the field is open in the alternator

If both are 0v, the automatic shutdown relay isn't supplying the field.
a. Relay bad
b. PCM not energizing the relay

If both are 12V, the PCM is not energizing the alternator field.

The PCM looks for engine rotation (CPS) before it will energize the alternator. I think the same is true for energizing the ASD relay. Otherwise, what's the point of the ASD.

extreme steel

I tried both fields, when I turn the key to on position it lights up then goes out not running. when I run the truck reads no voltage. I traced wires and everything seem to have power. I swapped relays and same thing. should I just replace all the relays? I'm stump on this. everything was fine then a/c and charging take a crap at the same time.

NJTman

I guess you haven't checked the fusable links like I suggested.....eh ? you have to test their continuity from one side of the FL to the other. when they go bad, you cannot always "see" the burn, as it happens within the FL's Casing..

extreme steel

i check power on passenger side and i have power coming across if the signal wire lights up and shuts off does that mean i've lost power from a FL?

j_martin

There is one field, with two terminals on it. I'm assuming you are talking about both terminals. Here goes the diagnosis based on that guess.

The PCM is turning on the ASD relay, then turning it back off. That means the PCM is working, but it is missing something to tell it the engine is running.

That would be the signal from the CPS. There are 3 connectors between the CPS and the PCM. There are 3 conductors, making a total of about 9 connections that have to be good. (might be 1 less in the ground circuit)

At the last connector (Engine top front)
1 wire should be at about 8v all the time (orange)
1 wire should be at ground all the time (Black/lb)
1 wire should toggle between approx 5v and just above ground as the gap passes it. (I forget the polarity, either it works or it doesn't.) (gray)

You can pierce the wires there and test for function with an ordinary multimeter. It's really simple with an oscilloscope.

8V is supplied by the PCM. If it's missing at the PCM, either the wire is shorted to ground, or the PCM is bad.

Ground comes from the instrument splice in the harness.

The pulse is supplied by the CPS, and needs to show up at the PCM.

If the pulse gets to the PCM, then the PCM is bad.

In your favor, the odds are it's a connection or sensor rather than the PCM. Resist the common impulse to throw the most expensive part at it in an attempt to fix it. Do the diagnostics, or hire someone that can.

The first time my new to me first gen encountered some salty foggy weather, OD, tachometer, and alternator all failed. In my case (after diagnostics determined the connections were suspect) I cleaned the connections with DeOxit, then lubed them up with dielectric grease. Haven't had any further trouble. (with that particular circuit) WD40 will clean em up also.

Angry Johnny

Have you checked for codes? That can save a lot of time and at the least point you in the direction of the problem many times.

extreme steel

Checked all fusable links and all have power. one thing i'm goint to do is get rid all the fusable link and replace with fuses.

extreme steel

Thanks for the rundown will try again today. was needing a break so took a few days off to go fishing and get drunk!!haha also when checking cps should i check power from pcm to cps? any hoot thanks for the help guys.

extreme steel

Here is the deal PCM is bad, friend has a shop and put it on his computer no bueno. he has a guy who rebuilds PCM's and said he'll have it done in 2 or 3 days. gonna drive to Oregon in 3 weeks and hit some bone yards for stuff i cant get down here. anyways guys thanks for all the help. BEERS on me if yer ever around here.