I have been struggling with an intermittent loss of all electrical power when I try to start my truck. I have checked everything I can think of. My local O'Reillys thought the ignition switch was starting to fail so I replaced it. The problem stopped for about a month. Now it's back. Here's what happens. Turn the key to the run position. Everything works. Lights come on. You name it, it works. Turn the key to start and everything goes dead except for the flasher. I mean everything is dead. No power to anything. Disconnect and reconnect the positive cable. Power comes back on. Has to be the positive cable. Disconnecting and reconnecting the negative cable doesn't restore power. Most of the time the truck will start after I do this. Sometimes I have disconnect the positive cable a couple of times before the truck will start.
Anybody have any idea what is causing this?

 

Bad connection at the positive battery cable most likely.

It would also help us help you if you told us what year 2nd gen you where working on....12V or 24V make s big difference.

 

My truck is a 94 12V with just over 400K miles on it. I have cleaned all of the battery cable connections numerous times. Doesn't seem to help.

 

How hogged out is the positive battery terminal clamp? My guess would be it is loose and not getting good connection regardless of how clean it might be.

 

I have the positive cable connected so tight that it is hard to get off. I plan to clean it again this morning. What is baffling is that the truck loses all electrical power. Nothing works except for the hazard flasher. No lights come. The headlights don't come on. The brake light doesn't come on. There is no electrical power to anything. This isn't a case where the starter clicks but won't turn over the engine like if you have a dead battery or a lose connection.

 

So you are saying it acts like the battery has been disconnected? Exactly my point.

 

It's not like the battery has been disconnected because the emergency flasher still works. I can check power from the battery cable, not the post, to a ground and there is still power. Same if I check power at the cable on the starter. I think something is causing the power control module to shutdown so no power flows to anything. I can sometimes hear a faint click coming from the passenger side of the dash when this happens. It has been very frustrating to say the least.

 

Do you have a multimeter or one of those capacitance-type battery testers?

I was going to suggest doing a voltage drop test from the driver-side positive terminal to the starter (if you don't know what a voltage drop test is, google-search it and keep the knowledge for later), but since other circuits not connected to the positive cable are affected, I'm doubting internal problems with the positive cable to the starter. Instead, try starting it - and when nothing else but the hazard flashers work, check battery voltage and condition on both batteries. Perhaps there's something internal in one of the batteries on the positive side - especially in the driver side battery, since you said R&Ring the terminal "restores" it. You're likely getting a little movement or stress on the battery post that may be re-establishing contact inside the battery but it's not enough for the starter load. That leads me to another idea. If it isn't cold enough to need grid heaters where you're at, swap the batteries side to side and try starting the truck only on the newly-relocated driver's side battery. Supposedly, one battery is enough to start our trucks. If it starts or at least cranks. and everything else still works afterwards, your former driver-side battery has an internal problem.

What do you mean by "power control module"?

 

What I am calling the power control module (PCM) is the computer on the passenger side up against the firewall. I have seen it called several different names. It seems like just about everything electrical flows through it. I have poured over wiring diagrams for hours and hours. There is an automatic shut down relay in the power distribution center next to the battery. My understanding is that on a diesel it only affects to A/C and the horn. In looking at wiring diagrams I see a reference to an automatic shut down.in what I call the PCM. I don't know what triggers it or what it shuts down. My memory is that a wire goes to the automatic shut down relay in the power distribution center.

When this first happened last June I found the 20a horn fuse was blown. Replaced it and the truck started. Now when it happens there are no blown fuses anywhere.

I will try swapping the batteries. Both batteries are just over 4 years old.

What is frustrating is this doesn't happen every time I go to start the truck. Sometimes it will happen the next day and sometimes there will be a month between it happening. The truck is my daily driver. Being retired I don't drive every day. But when I do I am starting the truck multiple times. I am at a complete loss as to what is causing this to happen.

I live about 15 miles outside of Paso Robles, CA. We have some night time freezing weather in the dead of winter. Even then daytime temperatures are in the 50s.

 

That's the Powertrain Control Module. It controls alternator fielding, AC compressor engagement, transmission OD & lockup, the wait-to-start light, and not really that much more more. It really controls nothing to do with the engine - so it's really kind of a misnomer

 

Studied the wiring diagrams for over an hour. Power goes through the power distribution center and then gets send elsewhere. Lots of stuff is hot all the time. Didn't see anything that would shut down stuff that is supposed to be hot all the time once power leaves the power distribution center.. Maybe there is something going on with the driver side battery. Started the truck. Then I switched the batteries. Started the truck with just the previous passenger side battery. Hooked up the previous driver side battery which is now on the passenger side. battery. Started the truck. I guess I will just have to keep using the truck. I don't have any choice in that. It's my only running vehicle. Maybe I should work harder on getting my 66 MG Midget running so I have a backup vehicle. I won't hold my breath on that. I have been resurrecting it for the last 8 years..

 

Grounds.

When power issues arise, check and clean them all....

 

If you decide that you want to swap out the PCM, I have several '94/'95 Cummins PCMs from trucks I have driven in my shop.