v8440

Hi all,

I'm starting to look more closely at the '96 12v truck I got recently. I was pressure washing in the engine compartment, and something jumped out at me-the old style separate mopar voltage regulator on the driver's side fenderwell. That ain't supposed to be there, as the charging on these is controlled by the ecm. The truck charges, but as you rev the engine up, the lights have a slight flicker to them, and the voltage gauge wiggles with it.

I have also noticed that neither the speedometer nor the tachometer work, both also things controlled by the ecm. Thankfully the truck is a manual, or the transmission would probably act crazy too. So, I'm thinking that either the ecm has died, or a wiring fault has rendered it inoperative and someone didn't know how to fix it.

Where IS the ecm on this truck? If it's under the hood, I haven't seen it. Also, I could use some basic troubleshooting steps to see if the ecm is even getting power. I have done nothing to the truck yet-not even checking for blown fuses.

If I need an ecm, where is a good place to get one, and roughly how much should one cost?

phox_mulder

There is no ECM in a 12 valve truck, only a PCM.
Yes, the PCM does do all you said the ECM does.
The PCM is located on the passenger side firewall, behind the air box.

More than likely, it's something simple like the Engine Speed Sensor.

Not real familiar with 12 valve engines, but the part in question is mounted to the harmonic balancer and tells the PCM that the engine is turning so it can turn on the alternator, the Tach gets its signal from this sensor as well.

As to the speedometer, I think the 12 valvers have an old fashioned style speedometer, cable going from the transmission to the dash, but I could be wrong there.


phox

v8440

Yeah, Pcm-that makes sense. Would the engine speed sensor kill the speedometer AND the alternator? I know it would kill the tachometer...

phox_mulder

If the PCM doesn't know the engine is running, it doesn't let the alternator charge the batteries.

That old style regulator must have been put in by the previous owner, not knowing why the alternator wasn't working.


phox

dslpower

My 95 has an electronic speed sensor to run the speedo. Check the crank position sensor before replacing the PCM.

wannadiesel

It sounds like the PCM is dead/unplugged or both the the CKP and VSS circuits are bad/unplugged. Does the WTS and WIF lights come on when you turn the key to the "run" position? If the WTS and WIF bulb check work, then the PCM is likely OK and the fault is with the sensors.

v8440

Yes, the wait to start and the water in fuel light come on, and then go out. Last night the water in fuel light stayed on, but I had just pressure washed the engine. Turns out, that is totally common, and generally does not indicate that water is actually in the fuel. Sure enough, a few hours later the water in fuel light quit staying on.

I will check the crank sensor today.

wannadiesel

Also check the VSS on the trans.

Gap on the crank sensor should be 0.050". Use a brass feeler or matchbook cover folded over once.

v8440

I pulled the crank sensor, and cleaned it off. I was going to substitute the sensor off my '98 12v, but they changed the connector between the two years. Is there a procedure to test the sensor, using an ohmmeter maybe? I'm pretty handy with electrical stuff, and could easily do that if I knew what values to look for.