Hey guys so I really don't have any luck with vehicles at all. Although this truck hasn't given me any issues till now after a couple years of owning. I'm having a voltage issue. I'm kinda assuming it's the internal voltage regulator in the pcm, but not 100%. Voltage is surging quite a bit, but not over its normal voltage only under and it's causing the tps/cps/overdrive to malfunction. Jumping back and forth from 3rd-4th gear. So my assumption is either the cps or the regulator. Getting kind of pissed off because my other truck(2nd gen) is already down and it's to cold up here in the north to be working on them.

 

that trans is hydraulically controlled. the system voltage wont have anything to do with that. you should have a voltage regulator on the firewall. They didnt put the regulator into the computer on a Cummins truck til at least mid 90s. well into the next body style.

 

Sadly after the 91.5 trucks they switched the regulator into the pcm instead of an external regulator which is on the 89-91.5 trucks. The only part of the transmission that's controlled by electronics is the O/D which is controlled by the Pcm and the tps. I have found out though that the Crankshaft position sensor can lead to issues with the Overdrive as well thats all the cps if for on these trucks. Charging and O/D.

 

The major things to check with a voltage issue on these with a PCM controlled alternator is the cross over cable, battery connections in general for corrosion, tightness, cracks, ect. Next is a good clean connection for the alternator cable, next make sure all chassis, engine and body grounds are clean and solid. Wouldn't hurt to test the battery with truck running for A/C voltage indicating a failed diode trio. A/C voltage in a DC electrical/computer system does some wacky stuff

Im not sure if these 1st gen bodys are the same as newer ones in that they read voltage off drivers side and charge passenger side? I believe the PCM is located behind the drivers headlight in the fender but I might be incorrect on that, assuming im correct that would mean it reads on charges the other. Wouldn't hurt to run a jumper cable between battery posts to see if the issue is resolved that way indicating a poor crossover cable. Personally I had my alternator wire extended to charge the drivers battery, after boiling a new battery, I decided that wasn't going to happen again

 

Good on corrosion, all grounds are clean, I actually decided to plug and play with the pcm. I unhooked the connector and cleaned it out at the pcm behind the driver side headlight. Same with the battery cables I unhooked them they have anti corrosive on em, but I figured maybe it'd help with resetting the pcm and stuff. Everything looks fine voltage when the truck is off and voltage when the trucks is running. When I was driving today though it was going all wacko.

 

have you actually tested he voltage with a multi meter to verify that it is dropping enough to cause it to drop out of overdrive? i would assume it jumping in and out of overdrive may likely be a completely separated issue like the temp sensor on the transmission line going bad. as long as the battery has 12v it should be enough to activate the overdrive solenoid. could also be a sensor issue. some people even swap to aftermarket transmission controllers. i got a friend with 2 of these trucks with overdrive on a toggle switch. if you think the voltage regulator in the computer is bad best to just install an aftermarket voltage regulator. they are easy to wire up.

 

Yes voltage is good with multimeter. I double checked with it 14.4 while running I mean not 12.4, I forget what the reading was when off think it's 12.5. Usually the transmission only acts up if I turn overdrive off then back on. It'll go nuts between the 3rd and 4th gear when I turn it back on. This time it was shifting back and forth with the voltage if the voltage moved it'd go to 4th then if it moved again it'd go to 3rd then it'd move again then go to 4th then to 3rd when it moves again and so on. I had tried a potentiometer in replacement of the tps this past summer and that didn't help with the issue. I had planned when spring comes along to replace the crank sensor because that can lead to issues with the O/D. It's to cold for me to do much I know how I get when it's cold and I'm trying to work on either of my trucks. I start breaking stuff because I get impatient. Usually the tps and cps will malfunction when I turn overdrive off then back on. So let's say I leave overdrive on and never drive the truck again with overdrive off it'll be fine. Problem is these trucks don't like to tow in overdrive so that's why I'm gonna go with replacing the crank sensor first before and and if that doesn't help I will wire a switch instead and bypass the tps.

 

obviously I wouldn't be able to test the voltage with a multimeter though while going 40-50mph

 

i don't think the crank position sensor should effect overdrive but it will effect the charging system if bad. it will not ground the asd if it doesn't sense that the engine is running. stupidly enough on the diesels all the asd relay does is power the alternator 12v field wire.

who said you have to always have to test for power at the battery? last time i checked there is many places in the cab that you can hook up a multi meter to check voltage while driving.

 

yea I mean the thing is though is that it doesn't have any issue charging whatsoever. I ordered the internal regulator kit for the alternator for now. It was only $25 for the transpo 8315 would rather see if that fixes the issue because this doesn't seem like it's the cps to me. I checked the gap not to long ago as well as it needs to be gapped correctly and it was just about spot on. Tommorow I'm gonna reinstall the new potentiometer as the one I had gotten from Genos shorted out so they sent me a new one.