2nd Gen Non-Engine/Transmission Electrical Woes...Maybe Bad PCM?

[Dave M]

I have a 97 and the truck runs but all of a sudden the Tachometer, Speedomotor, Odometer don't work. The generator is not charging and the MIL light won't come on with key on for light check. The fan works but AC doesn't. I believe it is controlled by the PCM also.



All fuses in fuse panel and PDC are good (I think). I have 12 volts at the generator but nothing on the field terminals. Since I have a 97 the fuel inj system is not controlled by the PCM but all the above devices are.



I tightened and cleaned up both battery gnd post connections and tugged on the ground connections at the block and they seem tight.



I tried to check for DTC's but the key on/off won't work because the MIL light won't come on. My actron scanner will not communicate with the PCM.



How do I test the PCM?



Any suggestions are appreciated. I have to work tomorrow (Monday) but will try to work on it Tuesday.



Thanks,

Dave

 

[jtwcummins]

PCMs on 12v don't go bad often, but they can fail. If there is 12 volts at the PCM and nothing works you probably have a bad connection or the PCM is dead. I would try cleaning the connectors and using dielectric grease before reconnecting the PCM.



As a note of interest, I replaced my Calif PCM with one from a Texas truck to get rid of the MIL problems with the DTT trans. I was prepared to have to get a reflash, but I installed the PCM and everything is working ok so I am leaving it alone.

 

[Dave M]

PCMs on 12v don't go bad often, but they can fail. If there is 12 volts at the PCM and nothing works you probably have a bad connection or the PCM is dead. I would try cleaning the connectors and using dielectric grease before reconnecting the PCM.



As a note of interest, I replaced my Calif PCM with one from a Texas truck to get rid of the MIL problems with the DTT trans. I was prepared to have to get a reflash, but I installed the PCM and everything is working ok so I am leaving it alone.

Thanks for replying. Good idea to check the conecctions and look for 12v. I'm afraid to screw it up (if it isn't yet). It looks like 3 connectors on the PCM.



Do I disconnect the connectors and check for 12v or should I uplug battery cables first and disconnect the connectors at PCM and then reconnect the battery so I can check foe 12v at the connector which is unplugged?



My Calif truck I actually bought from Texas and had it registered in Calif, so its really a Texas PCM.



Any good sources for the PCM please respond since I need this ASAP. What do they usually cost?



My truck is my wifes only vehicle and I have a company vehicle.

Shes is waiting for me to fix it.



I assume I can't get the PCM from Cummins since it does not run the engine controls. The last place I'll look is the dealer.



Thanks,

Dave

 

[y-knot]

Dave,

Look around on line, there are many dealers that sell computers for cars and trucks, and they are generally cheaper then the dealer.

 

[Dave M]

I looked at my PCM and the PN is 56040500AC. The dealer told me 56040451 using my VIN... whats up with that. The dealer will sell a rebuilt one for $536.



There are 3 connectors (Gray, white and black). How do you clean the pins? The pins look ok on the PCM. I'm afraid to touch the connectors on the cables and screw them up. The holes are real small.



On what connector and pin is the 12 volts and Gnd? Each connector has 32 pins.



I'm still having trouble finding my PCM other than through the dealer. No luck on Ebay.



Thanks,

Dave

 

[Dave M]

I unplugged the PCM and checked the 2 B+ voltages using the gnd on the PCM connector and they were good. Nothing much else I can think of to do.



I noticed when I hooked up the battery (with the key off) I get a spark. Is it possible I have a short somewhere?



When I find a PCM what (if anything) has to be programmed into it... . VIN... mileage... ?



I'm out of ideas on what to do to verify the PCM is bad and not something else causing it to go south which may ruin the new one. :{





EDIT: Could a bad or greasy covered crack position sensor cause my problem? I have an oil leak and noticed the crank sensor looks like its covered in oil.



Dave

 

[jtwcummins]

When I put the non Calif PCM in my truck everything worked w/o any reflash. Plugged it in and everything worked. My scanner doesn't recover a VIN and never has. If you want I can UPS my Calif PCM to you so you can verify it is the problem. Email me contact info at -- email address removed --. I would like it back, but if it fries no big deal. You will get the MIL because you don't have the EGR this checks, but everything should work and give you time to find a PCM.

 

[Dave M]

When I put the non Calif PCM in my truck everything worked w/o any reflash. Plugged it in and everything worked. My scanner doesn't recover a VIN and never has. If you want I can UPS my Calif PCM to you so you can verify it is the problem. Email me contact info at -- email address removed --. I would like it back, but if it fries no big deal. You will get the MIL because you don't have the EGR this checks, but everything should work and give you time to find a PCM.

I should have checked this site sooner... . I just ordered the PCM for $391 + $100 core. Thank you for offering the use of your PCM. That was very kind of you. I wish I had seen this post earlier.



The PCM should be here Thursday morn (1 day air). I hope this fixes it.



Thanks again,

Dave

 

[zstroken]

Does your water in fuel light come on?? Wait to start?

 

[Dave M]

Does your water in fuel light come on?? Wait to start?

I know the "wait to start" light doesn't come on but I don't remember about the "Water in Fuel" light. I'll check when I get home.



Anyone know what a bad "crank Sensor" will cause?



Edit: After reading the manual it says the crank sensor feeds the PCM which uses the crank signal to feed the Tach gauge. The PCM supplies 5 volts to the crank sensor. If my crank sensor is covered in gease and dirt would that cause my PCM to fail?



Dave

 

[Dave M]

Does your water in fuel light come on?? Wait to start?

MY gage lights that don't come on for the bulb check are:



Water in fuel

MIL

GEN

Airbag



Also the Alternator is not charging, Tach dead, Speedometer dead, AC dead but blower works.



The Oil pressure gage, water temp gage, voltmeter, fuel gage, headlights and turn signals all work. The ABS light , oil pressure light, seatbelt light, Brake light, blinker arrows, headlight ind, are all gage indicator lights that work.



Gonna try to clean the crank sensor tonight.



Dave

 

[zstroken]

I believe if the water in fuel and the wait to start don't light, your either not powering up the PCM or it is dead.

 

[Dave M]

I believe if the water in fuel and the wait to start don't light, your either not powering up the PCM or it is dead.

I agree. I checked for 12 volts and ground at the PCM connectors and it was good. All the devices not working are controlled by the PCM. My Actron OB11 scanner (1 year old) cannot detect the PCM.



What concerns me is if something caused it to fail and I don't find it and ruin the new PCM. I'm worried that something may be shorted cause I get a small spark when reconnecting the battery (key off). Is this normal?



I figure I may have to pull a fuse at a time till I find the one drawing the current and go from there... a long tedious chore I'm not looking forward to. I may start will the starter and alternator.



Any ideas how to be sure nothing is broke/shorted I'd appreciate it.



Thanks,

Dave

 

[Joe G.]

Have you checked fuse 9?

 

[Dave M]

Have you checked fuse 9?

I believe I checked all the fuses in the Fuse panel and PDC. I just did a visual check. I did measure two B+ voltages (12 vdc) at 2 pins on the Black PCM connector C1-pin A2 (Ckt F18 Fused ignition switch. . 12. 10 vdc) and C1- pin A22 (Ckt A14 Fused B+... 12. 36vdc). I used C1- pin A31 or A32 as GND, so I know the ground is getting there too.



My PCM should be here Thursday. Joe, do you think getting a spark when connecting batteries is normal with key off? That has me worried. What if the Alternator or something is shorted.



Fuse 9 probably is where those voltages comes from. I assume thats Fuse 9 in the fuse panel.



I'll verify again tonight more closely.



Edit: After further checking of the schematics, fuse 9 powers PCM C1-A2 (which I measured 12 volds at PCM). Per the schematic fuse 8 in PDC powers PCM C1-A22 (which I measured 12 Volts at PCM). Fuse 9 also powers the fuel pump relay and the truck runs so it has to be good.



Thanks,

Dave

 

[DPapworth]

A modest small spark at the battery terminals when re-connecting leads is pretty normal, especially if the leads have been off for more than a minute or two. There are capacitors in the air bag module and PCM (and maybe the radio as well) that present a momentary low resistance sufficient to draw a small flash. Your hood is probably up as you do this, so there's the hood light to draw current and make a spark as well.

As a test, once the capacitors are charged, the remaining IOD resistance should be high enough that it should NOT draw a spark if you then momentarily (1 to 2 seconds) remove and reattach the cable.

Automotive PCMs are well protected and rarely fail. Probably 75 percent or more of replacements are not necessary. I gather you already know this. It seems like in your case you've done enough testing to suggest your PCM is really bad.

The dirty CPS might cause the alternator to not charge, and the tach to not work, but the MIL and WIF lights would still light up at least initially. A dirty/greasy CPS should not be able to damage the PCM.

One possible testing option would be to try a common $15. 00 junkyard gasoline engine PCM to see if you can get the MIL light to work. Obviously many of the rest of the functions won't be right, but it doesn't take much to get the computer up and MIL to come on. The pinout of the donor vehicle's PCM connectors has to be close enough, this requires analysis of the schematics of your truck and the donor vehicle. This might be more trouble than it's worth.

 

[Dave M]

A modest small spark at the battery terminals when re-connecting leads is pretty normal, especially if the leads have been off for more than a minute or two. There are capacitors in the air bag module and PCM (and maybe the radio as well) that present a momentary low resistance sufficient to draw a small flash. Your hood is probably up as you do this, so there's the hood light to draw current and make a spark as well.



As a test, once the capacitors are charged, the remaining IOD resistance should be high enough that it should NOT draw a spark if you then momentarily (1 to 2 seconds) remove and reattach the cable.



Automotive PCMs are well protected and rarely fail. Probably 75 percent or more of replacements are not necessary. I gather you already know this. It seems like in your case you've done enough testing to suggest your PCM is really bad.



The dirty CPS might cause the alternator to not charge, and the tach to not work, but the MIL and WIF lights would still light up at least initially. A dirty/greasy CPS should not be able to damage the PCM.



One possible testing option would be to try a common $15. 00 junkyard gasoline engine PCM to see if you can get the MIL light to work. Obviously many of the rest of the functions won't be right, but it doesn't take much to get the computer up and MIL to come on. The pinout of the donor vehicle's PCM connectors has to be close enough, this requires analysis of the schematics of your truck and the donor vehicle. This might be more trouble than it's worth.

Thanks for the ideas. I'll check the battery spark again tonight. I looked at the crank sensor and it doesn't appear as bad as I thought. It is too much work to discnnect it because of the tight space so I cleaned it and checked the wires for shorts to GND or each other and am calling it good.



My reman PCM will be here tomorrow so I find out then if its really bad. First time on any vehicle for me replacing a computer. I'm going to disconnnect the GND battery terminal prior to replacing it. That should be adequate to protect it.



Thanks for the responses. Any more ideas let me know. Tomorrow night after work I'll be replacing it.



Thanks,

Dave



Edit: Oh. . by the way, I had the hood light disconnected and was sure all lights etc were off.

 

[Dave M]

A modest small spark at the battery terminals when re-connecting leads is pretty normal, especially if the leads have been off for more than a minute or two. There are capacitors in the air bag module and PCM (and maybe the radio as well) that present a momentary low resistance sufficient to draw a small flash. Your hood is probably up as you do this, so there's the hood light to draw current and make a spark as well.



As a test, once the capacitors are charged, the remaining IOD resistance should be high enough that it should NOT draw a spark if you then momentarily (1 to 2 seconds) remove and reattach the cable... ... ...

I tried this test tonight and... you were right on. After the battery was connected for a few minutes I disconnected it and did not see a spark. Of course, last night I checked it when it was dark outside and tonight it was light but I looked closely and did not notice a spark when re-connecting. Thanks for that tip... I feel better about putting the new PCM in tomorrow.



Tonight I took out every fuse in the PDC and Fuse Box and used an ometer to verify they were good... all tested and looked good.



I can't thing of anything else to check that could cause the PCM to fry. I checked fuse 9 again Joe and its good.



Dave

 

[Dave M]

HELP!!..... Replaced the PCM and still no change in symptoms! What am I missing here.



Thanks for any help you guys can shed on this. Somthing is preventing the PCM from working. This truck is our olny vehicle so I'm stuck till its fixed.



Thanks,

Dave

 

[jtwcummins]

There should be 2 12v w/ign off and a third with key on. If you don't have a manual, or access to one close by, I would consider alldatadiy.com. The $24 is worth the cost and it has good electrical drawings and troubleshooting guides. You might not be getting the 12v from ign switch or a bad ground. There are 2 ground posts at the radiator support area, 1 on each side. I had 1 so rusted the bolt had to be replaced and the area scrapped clean.



What is needed is to loosen and inspect/repair ground connections. Disconnect wires to Alternator and see what that does, short to ground will kill PCM