pcm problems ????

[lazyjoe]

I have a 1993 W350 auto. All at the same time my alt., a/c, and overdrive quit. The engine still runs. But these things won't work. Also the "Wait to start" light comes on AFTER I start the truck, and it does not go out. I was told it could be my crank sensor. I put a new one on, but it did not fix it. Now I'm bieng told it is probably my PCM. Any input ?? If I do try the PCM, is there some place that can rebuild mine, and maybe tweak it a little. Or is that possible? Any suggestions would really be welcomed. I really need to get this thing running.

[Philip]

I would check the auto shut down relay. It is mounted on the left fender by the hood hinge. It should be the first relay of the three mounted there.

There isn't much you can do for a bad PCM. It is a sealed brick inside.

[lazyjoe]

You were right. It was the auto shutdown relay. I cannot thank you enough for your suggestion, it saved me alot of money. I was getting ready to send my pcm off for rebuild. Thanks again. I will check back when I can, to see if I can help anybody else. This site is great!!!

[Philip]

Nothing like a satisfied customer.

[jogl]

Good call Philip.

I wish that this thread was a couple weeks older, I was talking to a guy that wanted to buy the PCM out of my wreck. He had similar symptoms and had already put one new (reman'ed?) PCM in and had a bunch of other work done.

I did tell him about this site and others so maybe he's reading.

JP.

[Philip]

Here is what I was thinking on his post. When our PCM's die it is normaly one circuit at a time. Mostly the charging circuit.

The OD and alt problem would indicate a crank sensor bad. But he had replaced that all ready. A bad crank sensor wouldn't shut off the AC. It doesn't go threw the PCM.

The wait to start light wouldn't come on in normal running with a bad crank sensor.

So that left the auto shut down relay. It was the only thing that could kill all the circuits listed.

Joe if you live close to Indiana check out the events section of this forum. Look under the SOP thread. If you can make it I'll let you buy me a MT Dew.

[PToombs]

Philip? The PCM controls A/C also, remember MMillers wire problem? His A/C quit too. Must have RPM before clutch engagement.

[Philip]

Pete I remember Michael's little furry chain saw working on his wiring.

Look at the PCM outputs in the FSM. There is nothing for AC from the PCM. All the PCM does in interupt the AC signal threw the auto shut down relay. I wouldn't call that control. Control to me is a PCM output for the acessory itself.

[PToombs]

Ahhhhh so! I will check this, Obi-Wan!

[lazyjoe]

Well, I was haulling hay today. And all of the same things quit again. What would be making the relay go out? I'm going to order another one, but I don't want to put it on until I find out what's going on. Any thoughts?

[jogl]

What has failed in the relay? Coil burned out or shorted? Contacts welded shut?

First thing I would do is try to determine how the relay failed.

JP.

[lazyjoe]

The contacts are free. I wasn't sure how to check the coil. Joe

[lazyjoe]

I got a new relay today but it didn't fix the problem. I had the pcm off when I got the first relay, and then put it on to check the relay, and it worked. So I thought maybe it was a bad connection at the pcm or wire between the relay and pcm. I'v messed with the plug at the pcm and I checked the wires between the relay and pcm, and they are good. So then I checked the power reading at the relay. I get power at all the terminals , with the engine running, except the one with the two green wires. Is this right? I'm back to thinking pcm.

[Philip]

A couple of items I have found on my truck was loose terminals.

On the PCM look at the harness connector, are the terminals round or egg shaped. If they are egg shaped take a small screw driver and bend them gently till they are round agin. I chased a intermitant OD problem for a while on that one.

The plug the crank sensor plugs into has small tabs that contact and tighten the connection. Make sure those little tabs are making contact.
Also check the wire on the engine side of that plug. It is unsupported and could be broken in the covering.

Let us know how it goes after these items.

[lazyjoe]

The connections seem to be good. I took the relay I had apart and jumped it across my battery and it worked fine. But it won't work on the truck. I've got power to the solid blue wire like I'm suppose to. The blue/yellow wire, which I figure should be the ground, goes back to the pcm. My question is, where does the pcm get its ground? And what tells it to?