PCM issue? Bad ground? No tach, speedometer, alternator charge

[BlameCandida]

I have a 1996 12valve 5-speed with 4 wheel drive. Everything is stock. Yesterday while driving, the truck shut down in the middle of the freeway. I got to the shoulder, and the truck would turn over just fine, but wouldn't stay running. I took off the fuel shutoff solenoid and zip-tied the arm up. However, it looks like there was more of a problem than just the fuel shutoff. During my 7 hour drive, I did not have tachometer or speedometer, and the alternator was not charging. I assume there is a computer issue somewhere. When I stepped on the brake pedal, the tachometer and speedometer returned. Also, the Check Engine and Gen lights came on. Once I let off the brake, the lights disappeared, and the tachometer and speedometer dropped. I know the brake sensor works by closing a ground, so I'm wondering if the PCM lost its ground and only works when grounded through the brake sensor.

I ordered a voltage regulator for the alternator so the truck will at least be useable until I figure this out. What is the next step in troubleshooting this?

 

[brucejohnson]

I have a 1996 12valve 5-speed with 4 wheel drive. Everything is stock. Yesterday while driving, the truck shut down in the middle of the freeway. I got to the shoulder, and the truck would turn over just fine, but wouldn't stay running. I took off the fuel shutoff solenoid and zip-tied the arm up. However, it looks like there was more of a problem than just the fuel shutoff. During my 7 hour drive, I did not have tachometer or speedometer, and the alternator was not charging. I assume there is a computer issue somewhere. When I stepped on the brake pedal, the tachometer and speedometer returned. Also, the Check Engine and Gen lights came on. Once I let off the brake, the lights disappeared, and the tachometer and speedometer dropped. I know the brake sensor works by closing a ground, so I'm wondering if the PCM lost its ground and only works when grounded through the brake sensor.

I ordered a voltage regulator for the alternator so the truck will at least be useable until I figure this out. What is the next step in troubleshooting this?

What is the voltage reading for the batteries? If you were running for 7 hours without the batteries charging you may be experiencing low voltage issues. When you activate the brake lights it may be causing the voltage to decrease enough to trigger the check engine and alternator lights. But you say it would turn over OK when it quit, and you had to pull over. Something going on there for sure. Definitely check all the grounds and wiring.

 

[BlameCandida]

What is the voltage reading for the batteries? If you were running for 7 hours without the batteries charging you may be experiencing low voltage issues. When you activate the brake lights it may be causing the voltage to decrease enough to trigger the check engine and alternator lights. But you say it would turn over OK when it quit, and you had to pull over. Something going on there for sure. Definitely check all the grounds and wiring.

Thanks. I'm going to be away from the truck for about a week, but I had the batteries on a charger all day. By the time I got home, the batteries were about drained. I had to turn on my headlights for the last half hour and that about finished off whatever charge was in the batteries.

When it died initially, I think it was due to the fuel shutoff solenoid releasing. I was able to crank the starter, the engine fired, but once I let go of the key, the truck died again. I did this a few times before I remembered this happened on the last Ram I had, and it was the fuel shutoff. After I ziptied the arm up, I was hoping that would be the end of the story. But once I got started driving again, I noticed that the tach and speedometer were obviously lower than they should be. It took me a while to realize the batteries weren't charging.

 

[Matt42]

Check the positive cable that goes from the alternator to the battery for corrosion. Check both ends.

Something much like this happened to me in a gas F350 on the Dalton Hwy at Toolik Lake some years ago. We had been charging up the truck's battery at night, but it just ran down and politely quit running there. We did a field diagnosis. It turned out to be an assembly line defect where the cable eye had been painted so the only current getting thru was via the bolt threads. Due to the climate, those had corroded. Using a lock-back knife, I scraped the paint off and put it back together. Then we jump started it from one of the other F350s and drove directly back to Coldfoot. That drive charged the battery back up and it was the end of that trouble.

Edit: I wasn't the driver of the troubled F350. So if there were instrument gremlins, the actual driver never told me. I'm not certain he would have noticed, anyway.

 

[brucejohnson]

I found a service manual online for the 96 Ram. A little challenging to jump back and forth between pages vs. a paper version, but so far it could very well be a low voltage issue causing most, if not all of your problems.
Check both of the battery's terminals for condition of wires and connector security/tightness. Then follow through to the PDC (Power Distribution Center).
A possibility could involve the ignition switch, but I would put that thought off until the wiring and battery/charging issues are resolved.
It looked like your voltmeter was hovering around the low end of the center-gauge sweep line, so that may be in the area where the PCM (Powertrain Control Module) may not be receiving enough voltage/current to adequately feed all the digital circuits their minimum required signal. When you applied the brakes the extra current used caused the comparator circuits to trip the Check Engine and Alternator instrument warning lights. The Tach and Speedometer increasing may have to do with the comparators tripping and feeding "extra" voltage to their driver circuits allowing them to indicate the increase; or, maybe triggering an initial start instrument check due to low voltage threshold at the tach and speedometer drivers, but the two gauges on the right did not move. Curious for sure. Also, insufficient voltage at the PCM can cause the ASD (Automatic Shut Down) relay to drop offline causing fueling and injector issues.
Just some thought on a quick review and scan of the situation. I could be way off, but "the simple things" are always the best place to start.
So, charge batteries, verify wiring condition and connections, resolve alternator/charging issue and go from there if needed. Hope this helps.

 

[GAmes]

If you don't have a factory service manual now would be the time to get one for the wire diagrams. I prefer the print version but if you have a lot of printer paper the CD will work too.

1997 Dodge Ram Factory Service Manual (genosgarage.com)
1997 Dodge Ram Factory Service Manual CD (genosgarage.com)

I would spend some time under the dash looking for a loose or chaffed wire next to the brake pedal too.

 

[Matt42]

Geno's also has factory service manuals for 1996 trucks. There are a few differences between the 1997 and 1996 models.
https://www.genosgarage.com/product/dfsm96-cd/dodgeram-factory-service-manuals (1996 on CD)
https://www.genosgarage.com/product/dfsm96-print/dodgeram-factory-service-manuals (1996 paper)

I have both. I'm on my second paper FSM. I bought my first copy from Chrysler Tech Authority in 1996 and wore it out. The FSM has saved me more time and money than I can calculate.

 

[BigPapa]

The brake thing makes no sense to me.

As mentioned, start with the simple things. First on the list is the alternator fuse, charging cable, and other electrical connections at/to the alternator.

Have you done ANYTHING else in the engine bay lately?

The symptoms you describe, except the brake thing, are typically indicative of an issue with the ESS (Engine Speed Sensor) which is located at the top of the crankshaft damper. Check all the wiring, connector, and the gap. You can use a credit card to check the gap, not a metal feeler gauge.

 

[BlameCandida]

Thanks all. I am away from the truck, but will check these things after i return. I did initially check the alternator fuse, and it was intact. I did a wiggle test on all the wires I could reach, and did not find any to be loose.

I have a copy of the 94-98 wiring diagrams on my laptop and have started to reference it.

Prior to this, I had not recently touched anything in the engine bay beyond checking and topping off fluids. The previous day I did drive through some pretty wet mud for about 50 feet, but not sure if that had anything to do with it.

While searching the forum, I did see a lot of references to ESS failure. I’ll take a look at that, as well as the alternator wiring, and under-dash wiring.

 

[brucejohnson]

Geno's also has factory service manuals for 1996 trucks. There are a few differences between the 1997 and 1996 models.

Oops...I corrected my reference from the online 97 manual to the online 96 manual. I have a 97 hidden deep in my library of FSMs for many of the vehicle I have owned in the past and mistyped the referenced year.
https://www.rammanuals.com/1996-ram-1500-2500-3500-owners-manual/ <<<< It's actually the Service Manual, not the Owner's Manual.

 

[GAmes]

Geno's also has factory service manuals for 1996 trucks. There are a few differences between the 1997 and 1996 models.
https://www.genosgarage.com/product/dfsm96-cd/dodgeram-factory-service-manuals (1996 on CD)
https://www.genosgarage.com/product/dfsm96-print/dodgeram-factory-service-manuals (1996 paper)

I have both. I'm on my second paper FSM. I bought my first copy from Chrysler Tech Authority in 1996 and wore it out. The FSM has saved me more time and money than I can calculate.

Thanks. My brain said '97 when I was in Genos website. I bought mine from Chrysler when the truck was new also. It has lots of greasy pages.

 

[GAmes]

The symptoms you describe, except the brake thing, are typically indicative of an issue with the ESS (Engine Speed Sensor) which is located at the top of the crankshaft damper.

The ESS doesn't affect the speedometer either. The tach and the speedometer, IIRC, share a ground but I'm not sure which one. Perhaps the PCM ground?

OP, when you first turn the ignition switch on do you get all the self test lights on the dash? Have you done the key on/off sequence to look for codes? Just to let you know, '94-'98 wiring is not the same. Every year has differences, most of them the incorporation of switched grounds, and '98 is a whole different animal.

 

[BigPapa]

I blew through the speedo comment. He has a ‘96. I don’t think the key-on/key-off will display any codes on a ‘96.

 

[GAmes]

I think it will. Same PCM as a '97 and mine does.

 

[brucejohnson]

I think it will. Same PCM as a '97 and mine does.

IIRC from reviewing the schematics last night the driver grounds appeared to be common through the PCM.

Woops, wrong post responded to...Grounds are the subject. I'm batting a thousand...

 

[BigPapa]

My ‘95 didn’t.

 

[GAmes]

The '94s and '95s had two plug PCMs. '96 & '97 have three.

 

[brucejohnson]

After re-reading the thread I have...
More questions:
Isn't the voltage regulator in the PCM? I assume you plan on having an external regulator in that case.
After the truck died and you pulled over and ended up tying up the relay, did the truck turn over strong when restarting?
Were all the lights, including dash warning lights, strong or dim as time went by?
Did you take a voltage reading from the batteries and/or alternator output after arriving at your destination?
Did you notice any odd dash indicator/warning lights or symptoms when using your turn signals?
How about 4-way flashers? Probably didn't try them.

When you get back to your truck see if it starts, and if it does try the 4-way flashers and turn signals to see if any odd lighting occurs like it did with applying the brakes.
I seem to remember there are small ground wires on the negative battery terminals in addition to the large frame ground wire. (Not sure on the 96 model - seem to show something in the schematics. I'll have to relook.) Feel for internal corrosion, usually lumpy and/or crunchy.
If there is adequate power somewhere, it should be everywhere...Your assumption about a ground (or power) issue makes sense. Just need to narrow it down.

 

[BlameCandida]

After re-reading the thread I have...
More questions:
Isn't the voltage regulator in the PCM? I assume you plan on having an external regulator in that case.
After the truck died and you pulled over and ended up tying up the relay, did the truck turn over strong when restarting?
Were all the lights, including dash warning lights, strong or dim as time went by?
Did you take a voltage reading from the batteries and/or alternator output after arriving at your destination?
Did you notice any odd dash indicator/warning lights or symptoms when using your turn signals?
How about 4-way flashers? Probably didn't try them.

When you get back to your truck see if it starts, and if it does try the 4-way flashers and turn signals to see if any odd lighting occurs like it did with applying the brakes.
I seem to remember there are small ground wires on the negative battery terminals in addition to the large frame ground wire. (Not sure on the 96 model - seem to show something in the schematics. I'll have to relook.) Feel for internal corrosion, usually lumpy and/or crunchy.
If there is adequate power somewhere, it should be everywhere...Your assumption about a ground (or power) issue makes sense. Just need to narrow it down.

I didn’t have a multimeter handy, so I wasn’t able to get readings at the batteries or alternator. When I restarted the truck on the shoulder, it was cranking strong, which gave me false hope that it was just the fuel shutoff that failed.

I didn’t have any issue with turn signals. They appeared just fine on the dash with no dimness. For the first five hours of the drive, I had the radio on, was charging my phone, and was using my turn signals as normal. Only issue appeared to be the incorrect tach and speedometer. At that point, I knew it would be getting dark soon, so once I got to civilization, I decided to try the headlights. At that point, the instrument panel did begin to dim, until I made it home with barely any light.

I’ll be back to the truck after new years and will test voltage and check for corrosion.

 

[BlameCandida]

I finally got a chance to get under the hood today. I had the batteries out to charge, and took a look at the wiring. The cables do not look bad. Connections at the PDC and starter are tight. However, on the driver's side (left) battery, the ground cable appears to be missing a connection. The heaviest cable is attached firmly to the engine block, and the small wire runs back to the tow kit. However, there is a medium gauge cable that is not attached to anything. I am looking at the wiring diagram, and it looks like this is G115 or G116. But the wiring diagram does not picture where this cable grounds to.

The voltage regulator and FSS wiring harness arrived from Larry B, but I have not touched these yet.