Engine/Transmission (1994 - 1998) Gen light on and mechanic says I need a new computer?

[AMPolley]

Truck is a 1997 3500 automatic. The last couple of days the truck is not charging a normal amount. Gauge is low and the gen light comes on. When it happened it happened on the highway and at highway speed it still would not fully charge. I'm thinking alternator. No time to work on it so I took it to the shop and that's what they thought too. $370 later (I know, ouch), new alternator and go to drive the truck home and nothing is fixed. Took it back and now they want to replace the computer. They are saying it's not letting the alternator charge the batteries. Needless to say, I don't really believe them and am looking for other things that might be causing it. Wondering about grid heaters etc. I did check the 120 amp fuse in the main panel by the drivers battery and it appeared to be good. The only thing that makes me think grid heaters is that one of the covers at the connections by the air horn was melted. Is there a way I can disconnect the heaters and see if that is what is causing my drain? Start throwing me some things to try please before the truck goes back Monday.



Thanks a lot in advance.

 

[KcoleKennyc]

Does the tach work? Other things can cause this. I would not change the expensive computer until I had eliminated the cheaper ones first. Good luck

 

[DBraunig]

Kcole is right, if the tach is not working the alternator will not charge. Crank sensor on front of timing cover at harmonic balancer is what works the tach, should be gapped at . 010". also you said the fuse "looked" good... test it! you shold have battery voltage on both sides of that big fuse in the PDC. Alt bad is possible but is more rare than these other problems. also need to test both batteries separatley if the other stuff checks ok. dodge charging system won't act right even if just one battery is defective. hope this helps.



Dave.

 

[biggy238]

Just unbolt the cables at the solenoids down under the P-Pump on the inner fender. The look like ford solenoids. If that doesn't fix the problem, check into the solendoids in the transmission. I had one that was causing problems and I was losing power.



Good Luck,

Kyle

 

[ABorchard]

Your mechanic is most likely right, it needs a new PCM. It's actually quite common for the PCM to lose the charge regulation function and nothing else. 12 valve PCMs are a little hard to come by, dealer ($600) and junkyard are about the only options.



Not to worry, the problem can be fixed by adding an external voltage regulator from any other vehicle for under ten dollars. Buy a wiring harness with the regulator. Remove the two small wires from the back of the alternator and cap them off to keep from shorting and hook up like the picture. . It makes no difference which wire is which from the alternator but the 12 volt supply can only go to the top pin. You don't need the type of regulator in the picture but most electronic regulators have the small triangular plug setup.



Picture won't upload, try copying and pasting this to your address bar, it doesn't work to click it either #ad

 

[JLC1]

Thanks for the ideas. I posted from my brother in laws computer when I was at his house, hence the different username. Anyway, the tach is working. I also tried disconnecting the grid heater leads at the intake and it was still not charging. Maybe the dealer was right with the regulator thing. I'm interested in the external regulator idea. Is there a particular part # I should ask for that someone knows works? $10 is a lot cheaper than $500. Will it cause any funny things in the PCM to regulate the voltage externally as opposed to through the PCM? Let me know as the truck is supposed to be taken back to the shop tomorrow. Thanks!

 

[BTighe]

this must be the day. my rig had the gen light come on this morning for about 10 minutes. it also accured at about 80mph. 10bucks. 500bucks. hum. what to do, what to do.

 

[ABorchard]

Any 12 volt electronic two post regulator will work. It won't effect your pcm because you disconnect the two stock wires. What is most important is the 12 volt positive lead is connected to a source that is only on when the engine is running, otherwise it will drain your batteries. I have found the hold up wire for the shutdown solenoid as an easy one to get at. Also make sure you have a good ground, a wire from the body of the regulator to the body of the alternator is a good idea. Make sure the 12 volt positive is only connected to the pin at the top of the triangle, if it touches the other pin for even a second it will fry the regulator.

I've done this to several trucks, owners are happy. Even did one for a guy who tows all around the country who wanted it for a backup in case his pcm should ever fail. He runs with the wire harness removed from the external regulator waiting for the day.

 

[Garry Verheyden]

Check the wire that goes from the 120A fuse to the grid heater. Sometimes it will rub through on the drivers side shock tower. This will cause the alternator to charge the frame instead of going back to the battery. I cant remember the exact wiring since I dont have my 96 anymore, but check the shock tower to see if a main wire has rubbed through there.



Garry

 

[K5IP]

Rather than guessing, measure it!

The alternator field requires a battery voltage supplied through the auto shutdown relay, and a return current sink through the pcm.

So, if you have something close to battery voltage across the field terminals (smaller wires), or above 10vdc, the alternator should be cranking out maximum output or over 100A at 2000rpm.

If you don't have nearly 10vdc across the field, then you need to determine which side is missing. If the battery voltage is not present on either of the field leads, you have a auto shutdown relay problem or the pcm is not pulling it in. If the other side of the field is not within a volt or so of zero, the pcm is not sinkiing current and you may indeed have a pcm or associated wiring problem.

Rog

 

[AMPolley]

Thank you to everyone for their help on the truck. So far my brother-in-laws truck is working now. He did end up replacing the PCM. The truck is used for a work truck and needed a quicker turn around than searching further possible problems. The truck appears to be working fine now and is charging as it should be. Thanks again.

 

[dieselworld]

I had the same problem on my 91 and it was the PCM. The voltage regulator is built into the computer.



What a stupid design.

 

[yenga80]

Just unbolt the cables at the solenoids down under the P-Pump on the inner fender. The look like ford solenoids. If that doesn't fix the problem, check into the solendoids in the transmission. I had one that was causing problems and I was losing power.

Good Luck,
Kyle

Well my gen light might be caused by the fact that while checking for intake leaks with a homemade rubber boot on the turbo it blew off because it was not secured tight enough. This in turn took out one of the field studs on the alternator. I was able to retrieve the little screw and put it back in; also noticed it had a burned mark on it from shorting out inside the alternator. Anyway put it back together and the alternator seemed to be working; voltage meter up at 14. Well today the light came back on and the voltage on the dash gauge was 12 and it didn't go back to 14 for quite a while until after a few key cycles. Funny thing is I also noticed that the auto transmission didn't want to shift back into 1st gear after coming to a stop. So it makes me wonder if what kyle wrote above about a solenoid in the transmission is causing this. Of course I will check the whole grid heater relay deal and fuses. Also will measure voltage at the field terminals on the alternator. Hope it's not the PCM going. IF so I'll just add a cheap voltage reg. My question is how would I check a transmission solenoid and which one and where would it be located. Seems it would be the 1st gear relation as all other gears had no issue. Lock up was fine also. thanks in advance for insight.

 

[yenga80]

Just unbolt the cables at the solenoids down under the P-Pump on the inner fender. The look like ford solenoids. If that doesn't fix the problem, check into the solendoids in the transmission. I had one that was causing problems and I was losing power.



Good Luck,

Kyle

Well my gen light might be caused by the fact that while checking for intake leaks with a homemade rubber boot on the turbo it blew off because it was not secured tight enough. This in turn took out one of the field studs on the alternator. I was able to retrieve the little screw and put it back in; also noticed it had a burned mark on it from shorting out inside the alternator. Anyway put it back together and the alternator seemed to be working; voltage meter up at 14. Well today the light came back on and the voltage on the dash gauge was 12 and it didn't go back to 14 for quite a while until after a few key cycles. Funny thing is I also noticed that the auto transmission didn't want to shift back into 1st gear after coming to a stop. So it makes me wonder if what kyle wrote above about a solenoid in the transmission is causing this. Of course I will check the whole grid heater relay deal and fuses. Also will measure voltage at the field terminals on the alternator. Hope it's not the PCM going. IF so I'll just add a cheap voltage reg. My question is how would I check a transmission solenoid and which one and where would it be located. Seems it would be the 1st gear relation as all other gears had no issue. Lock up was fine also. thanks in advance for insight.

 

[yenga80]

Well I disconnected the grid heater relays with the four small wires and my battery voltage is still reading low (12) on the dash gauge. Meter showing 12. 1 on batteries with truck running. Field terminals read 11. 8 volts. Gonna do some more testing. Might have ruined the generator when that boot popped off when pressurizing the intake system.

 

[yenga80]

I ruled out the grid heater relays with the multimeter. Also ruled out the ASD relay with meter and test. PCM is supplying 11. 8 volts to field terminals and I'm not getting anymore than 12. 5 volts from the output terminals on the generator. Gonna have to bite the bullet and put a new one in. Question to you guys is do I really need a 136 amp or will a 120 do as all the auto parts stores around only have 120s. Should I try to fix what's broken on this original denso?

 

[MLee]

Is it rebuildable? I'd stay with the currant rating 136 Amp. Geno's has kits, maybe a local shop to help you out.

 

[yenga80]

Is it rebuildable? I'd stay with the currant rating 136 Amp. Geno's has kits, maybe a local shop to help you out.

I'm not sure but I did take the alternator apart today finding the rotor shaft where the brushes ride to be worn down pretty good. Bearings are good. I know the stud for the field terminal that broke off and went inside the alternator shorted out somewhere on the coil windings I'm going to put a new one on here shortly.

 

[yenga80]

Is it rebuildable? I'd stay with the currant rating 136 Amp. Geno's has kits, maybe a local shop to help you out.

I'm not sure but I did take the alternator apart today finding the rotor shaft where the brushes ride to be worn down pretty good. Bearings are good. I know the stud for the field terminal that broke off and went inside the alternator shorted out somewhere on the coil windings I'm going to put a new one on here shortly.

 

[yenga80]

Well it's fixed. New dura last alternator put in. Nice thing is at autozone gave me one from a 98 ram that was 136 amp. Different part number from the 120 amp one that came up for a 96.