I've been having some electrical problems with my voltage regulator and tach for some time on my 1993 auto trans truck, then a few weeks ago I lost the voltage regulator all together and the gauge dropped to zero. The alternator tested fine so I decided to have the ECM rebuilt by SIA Electronics, vs bypassing the voltage regulator because I really wanted the tach to work again also... I sent it off, $160 and a week later I get my ECM back looking like new. Put it in, turn the ignition key, wait to start light comes on, dash lights, grid heaters all come on just like it all should. I turn the key to engage the starter and everything goes completely out, no lights no starter, nothing. If I turn the key off and on again everything comes back on and starts the cycle over until I turn the key and everything blacks out again loosing all dash lights. I have power at the fuse box, power to the blue wire as well when I turn the key to the first position. I called SIA and told them the problem, they said send it back for a test. I just got it back today with a note that it tested fine. But I'm still having the same issue and can't start my truck. I'm at a total loss, the truck was starting fine the day I pulled the ECM with the exception of the altinator not charging. Any ideas?!

 

1st thing to check is battery connections- since you had to remove it to get to ECU, likely its a loose connection. Would explain loss of power

 

Have you checked and cleaned all your grounds...this should be done yearly. Remove and sand paper to shiny metal and re-install with a dab of di-electric grease.

Could also be bad battery connections at the posts on the battery. When you go to "start" the truck do you hear a slight click or pop? I never tighten my battery connections, just snug them up by hand so occasionally I get the same issue your describing....everything works right up until I got to turn the motor over, then everything does black. Sometimes it takes me half a dozen tries cleaning and re-positioning the posts until I get it right.

 

Thanks for the suggestions, I'll give them a look-see. What really concerns me is the ECM, is this just a coincidence the day I took it out this new issue comes up or could it be the ECM itself? They are impossible to find these days so trying another isn't an option, Id really like to know if an ECM could even cause this or not?

 

Question I have for you is why do you have a voltage regulator separate from the alternator? 92-93 did not have a separate alternator and regulator. The regulator is internal inside their alternator on those years. Was the truck rewired for a reason? I agree with the others to check and clean every single ground that's available to be found on the truck.

as matter of fact I would add extra grounds from the battery terminal negative side to the engine block on the driver side with a number 4 or thicker gauge wire. I followed Jim Lanes rule of thumb and added an extra 4 gauge wire from the battery negative side to the back of the alternator. Or to the alternator mounting bolt I can't remember. This guarantees the negative side of the battery is connected both to the alternator and to the block.

 

Once you add all the grounds being cleaned to your once-a-year tune up and if the issue still continues there are other things that you can look at


with amount of miles that you have on that vehicle you might want to check the ignition switch. 25 years and being started that many times the switches tend to wear.

 

What you're describing is almost always a bad battery connection. It may be tight, but if there's black oxide on the post or connector, it'll do that.

10 amps or so to light the dashboard goes fine. 500 amp or so starter draw opens up the weak connection and everything goes out.

It is remotely possible for the battery to open up like that internally. Usually the spark blows the top off, however.

 

All the talk about grounds reminded me Im still having a problem with my low beams with a bad flicker, can only use high beams, and they are on relays. NJTman, my 93 w/auto trans has always had the voltage regulator in the ECM, I've had the truck 25 years and I'm the only person that's touched it in that time! ECM controls voltage regulator, tach, grid heaters, and over drive from what I can find. But it doesn't sound like its my ECM then, just weird I unplugged it and had it rebuilt and now I can get my truck started. I will go over everything this weekend, thanks for the tips so far gents!

 

Yep.. That's the way its supposed to work, internally controlled in the alternator by the ecm. When you kept mentioning the vr as being separate, I assumed you, or someone else added a firewall mounted vr as others here have done in the past. The old square box style that Chrysler had in the older cars and trucks

Its a semantics issue, not doubting your trucks originality

 

I generally don't post in threads about ecm trucks when they involve the ecm, but I agree with John, it is probably a ground issue right at the battery or the ground connection on the vehicle. Do what the guys have suggested and it should clear the problem. I generally don't believe in coincidence, but they do happen once in awhile . You can never have too many grounds...Mark

 

I had that trouble with the voltage going up and down and seeing it in the headlights. I replaced the VR but it didn't help. Recently I replaced the ignition switch and the problem went away. The VR gets a voltage signal from the ignition switch and if the switch is old and flaky then the VR will pulsate like that. I have the VR that's separate on the firewall though. I don't know about the ones built in to the alternator or the ECM.

Edwin

 

Sure enough, cleaned the ground wires attached to the chassis first and that's all it took, truck fired right up! Thanks for tips fellas, nothing as valuable as a second, third, forth... opinion! That also cleaned up the headlights on low, but the voltage gauge is still jumping up and down just a little so I'll keep cleaning terminals this weekend and take hard look at upgrading ground wires and that ignition switch as well. Thanks for the help!

 

Very nice. It's crazy to think some corrosion, or something as simple, can cause so many problems!