Hillcountry

Once a dealer shorted the back of the alternator to ground while changing the oil filter and it blew a fuse (Which I think is on the back of the alternator) which caused me to have a similar problem. The diagnostic computer said it was the PCM...it was a fuse.

vzdude

That 150A fuse is in the PDC if you want to check it as well. The easiest way is to either verify by looking at it, or simply checking for power at the large red wire on the back of the alternator. Unfortunately, it doesn't affect the guages.

Katmandu

Yes, ALL the gauges are affected.

Ok, here's the latest update. I hope it makes sense.

First, I haven't had the alternator "bench tested" yet... I had it checked out on Autozone's outside machine that hooks up to a running vehicle.

It showed the alternator had NO output what-so-ever. The OBD-II scanner showed NO PCM present.

Now, here is the wiring I just checked out on the PCM harness connectors.

Black conn to pcm

- pin #2 light green & blue wire should have 12v w/ key on, 12v PRESENT

- pin #22 red white wire should have 12v all the time, 12v PRESENT ALL

- pins 31,32 black/tan should have continuity to ground. YES

Gray connector

- pin #12 light green black wire should have 12v key on. NO VOLTAGE PRESENT when key is ON.

I also checked out the following.....

I also verified that the 30 amp PCM fuse is GOOD. I check the PCM circuit (where the 30 amp fuse plugs in) for continuity (Key ON) and there was NO continuity.


So, is the PCM is NOT getting any power ??

Katmandu

You know what ? This did all happen very shortly after I had the oil changed at a local shop.

But! I was sitting inside the truck and all gauges etc... was working just fine.

patdaly

Double check your fuse #9 ( I "think" 98 used the same circuit). That is the PCM/Shutdown relay/ CPS yada, yada, yada, fuse at least on the 97.

I am pretty sure that is where the black with green stripe wire gets its power from.

As others have posted, use a DMM to verify.

ticki2

You can "full field " the alternator to check if it is good, basically bypassing the PCM. Can't remember off the top of my head , do a search , it's in these forums. Very common problem for PCM to not have output to alt. I hooked up an external voltage regulator and has been good for 3 years. The VR is built into the PCM. That being said , it did not affect my other gauges . Good luck.

Katmandu

I just checked Fuse #9 and it's GOOD. I also checked for power (key ON) with the Fuse pulled and there IS 12 Volts PRESENT in the socket for Fuse #9.

Also, with the key ON, there IS 12 Volts PRESENT at the Fuel-Shutoff Solenoid on the Lt.Green/Black wire. This is the same circuit as Fuse#9, the PCM, the Fuel Heater Grids and the Auto Shutdown relay.

I'm bewildered that there is NO POWER present (w/Key ON) at the Grey Connector Pin #12 for the PCM power. ???????? Why not ???

Also, where are the Heater Grid and Auto-Shutdown Relays located and what is the Auto-Shutdown anyways ??

Katmandu

Here's the schematic showing the circuits in question.

Katmandu

Roger that. I did see the about the external voltage regulator during my Research.

But, like you mentioned. That "should" have nothing to do with the other gauges.

???????????????

vzdude

Basically, if you have all power and grounds at the pcm, and there is continuity from the pcm connector to the alternator corresponding wires, you have a failed pcm. The pcm communicates to the instrument cluster on the ccd bus. This is why all you guages are messed up. You can keep chasing your tail if you want, but I think you already know the answer.

Katmandu

Roger that.

I am a bit unsure to the PCM Pin #12 issue. There "should" be 12 Volts with the key ON correct ?

When I checked it with the DMM, there was NO power there. ??

Am I overlooking something ??

sweeney186

Hang on dude-prob sent you on a wild goose chase.according to the chry wiring diag. the 98 should have 12v key on pin 12 gray conn-but the diag is the same for 12v & 24v-which is prob not true.there have been many mistakes in their diag's over the years.the 97 diag shows only 2 12v feeds pins 2,12 blk conn.We normally would not trust these diag's online & use the books instead-but all I have here at the house is online access.Since you do have 12v at the right places-one other thing can shutdown your pcm-a shorted crank sensor.Try disconnecting it & then turning the key on & see if your gauges come back-be sure to disconnect w/ key off-simply unplugging will not let the pcm reset itself

vzdude

I am assuming you mean c12 - from the ASD relay? You can try a couple of things.....make sure that fuse 4 and 9 are good. Then you can try to jumper power to c12 circuit. The c3 wire should command that relay to engage ( from the pcm ) . The c12 is actually an input to the pcm. Another thing to not::::: That relay is only hot for a few seconds after turning the key on, or while cranking/running. If the truck is just turned on, it won't be hot all the time. If that works, then you need to find out what is missing......a power into the pcm, or a fuse blown, ect.

Dave Miller

On my 97 I had power to the PCM...I believe there were 2 12 v B+ volts but replacing the PCM was not my problem. It was a bad crank senser. My Dodge factory manual for my 97 shows 12 vdc on the PCM black connector (C1) on pins 2 and 22. The PCM white connector (C2) has the 5 vdc to CPS on pins 31 and 32. I agree with "sweeney186" here in that unplugging the CPS for me gave me a check engine light or MIL (which I did not have when turning key on....with the bad CPS). Your 98 may be different than my 97 but the gage lights that didn't come on for the bulb check were:

Water in fuel
MIL
GEN
Airbag

The things that did not work (with my bad CPS) were the Alternator was not charging, Tach dead, Speedometer dead, AC dead but blower worked.

The gages working (with my bad CPS) were Oil pressure gage, water temp gage, voltmeter, fuel gage, headlights and turn signals all worked. The ABS light, oil pressure light, seatbelt light, Brake light, blinker arrows, headlight ind, are all gage indicator lights that worked with my bad CPS.

When I put in a new CPS everything worked. I believe the CPS was causing my PCM to not work because it shorted out the 5 vdc that the PCM was suppling to the CPS. The new CPS restored the 5vdc from PCM to CPS.

Hope this helps some. I have a PCM that cost me $600 that was not bad and a $100 CPS fixed my problem.

vzdude

98 was the change over year from having "real" guages to "guess" guages. The 98 uses the ccd bus to "tell" the instrument cluster what the guage should read. For example.....the oil pressure sensor on 98 and up vehicles shows a calculated oil pressure amount determined by run time, ambient temp, and engine temp. I have seen the input to the pcm be 17 psi and the guage reads 45 psi. That is why I now have a separate oil pressure guage on mine!