1998 Dodge Dakota Replacing the PCM on 1998 Dakota

Tiny
ROCKETMAN1
  • MEMBER
  • 1998 DODGE DAKOTA
  • 6 CYL
  • 2WD
  • AUTOMATIC
  • 98,000 MILES
I got a check engine code on my 1998 Dakota a few months ago P0340, which is cam shaft position sensor malfunction. I cleared the code and it reappeared yesterday morning. I stopped for gas, and then the truck would not restart. (Crank/No Start).

I had it towed to the shop (Firestone Auto Care), and they said that the PCM had power in, but no power out. After messing around a while they got it started, and they then had power out of the PCM. They checked the cam shaft sensor and it had power in, but no power out. They recommended replacement of both the sensor and the PCM. (Sensor $200 and the PCM $600) Ouch!

I have doubts about the PCM being bad, so for now I had them replace the sensor only. Truck ran OK running a couple errors driving back from the shop.

1) What are you rthoughts on my problem?

2) How hard is it to replace the PCM, and where is it located? (Have not looked yet) It appears that they need to be factory programed, and I do have a digital odometer that may be controlled by the PCM. Is this something that a do it yourselfer could replace on his own? It looks like I can buy a new or rebuilt one for $250 to $300, which sure beats $600.

1998 Dakota
6 Cylinder A/T

Thanks!
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Saturday, February 28th, 2009 AT 6:25 PM

2 Replies

Tiny
CARADIODOC
  • EXPERT
Sorry it's been three weeks since you posted this, but I'm here now.

I doubt your engine computer is bad, after all, it did what it was supposed to do. It set a code indicating the area of the fault. Many engines only use the camshaft position sensor to figure out which cylinder to fire first when you crank the engine. After that, it is not needed. Not sure if this applies to your engine. If the sensor fails while the engine is running, the computer runs the engine off the crankshaft position sensor, but it memorizes the code for the cam sensor when it sees that it's not sending signals. It will continue to run fine until you turn the engine off and try to restart it. It won't restart. Some GM engines will pick a random cylinder to start firing, then go in order after that. On a V-6 engine, you have a 33 percent chance it will pick the right pair of cylinders and run.

I hate people who run down their competitors, but it sounds like either your mechanic doesn't understand how this circuit works, or he gave you a poor or dumbed-down description of what he found. The cam sensor needs a ground and voltage supply wire, commonly 8 volts. The third wire is the pulsing signal wire. You can not measure the output voltage with a digital voltmeter. If the signal is missing, the "Check Engine" light will be on and the fault code will be memorized. You might be able to retrieve the code yourself by cycling the ignition switch from "Off" to "Run" three times within five seconds without cranking the engine. On older vehicles, you counted the flashes of the Check Engine light. On yours, you should see the Chrysler code number in the odometer display. It will not be a "P" code; it will be a two-digit number that you can look up in the service manual.

Also, when they said that "the PCM had power in, but no power out", it suggested they don't know how it works. The fact that it started after they fiddled with it a while goes along with your earlier symptom of the cam sensor being intermittent. It probably cooled down in the shop and started working.

When you turn on the ignition switch, the engine computer turns on the Automatic Shutdown (ASD) relay for two seconds, then it turns back off until pulses arrive from the crankshaft or camshaft position sensor. These pulses only occur when the engine is rotating, (cranking or running). It looks to novices like the computer isn't turning on the ASD relay if you're troubleshooting the system while just leaving the ignition switch turned on, but that's what's supposed to happen. That two-seconds that the relay turns on is enough to run the fuel pump to insure fuel pressure is up and ready to start.

When crankshaft sensor pulses arrive during cranking, the engine computer turns on the ASD relay constantly. Voltage from the relay feeds 12 volts to the ignition coil(s), fuel injector(s), alternator field winding, oxygen sensor heater, and the fuel pump or fuel pump relay. Ever hear of Ford's silly inertia switch? It will kill the engine if you hit a big enough pothole. Chrysler accomplishes the same thing much more effectively with the ASD relay.

If you are in a crash that ruptures the fuel line with roughly 50 psi, the in-tank electric pump will pour raw fuel on the ground creating a severe fire hazard. With a ruptured line, you won't have fuel pressure. Without pressure, the injectors won't spray fuel and the engine will die. When there is no rotation, there's no crankshaft sensor pulses so the engine computer turns off the ASD relay and removes the voltage supply to the fuel pump.

To troubleshoot this system, you have to bypass the ASD relay with a paper clip or you have to work on it while cranking the engine, (not very practical).

It sounds to me like your sensor just failed, and that's not too uncommon. By now you've probably noticed your truck is running fine without having to replace the engine computer.
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Thursday, March 19th, 2009 AT 8:08 PM
Tiny
ROCKETMAN1
  • MEMBER
Caradiodoc,

Thanks for responding.

Well so far so good after three weeks, I have not had anymore problems after having the cam sensor replaced.

I think I will be finding a new mechanic. As you mentioned either the mechanic didn't understand how the circuit works, or he gave a poor or dumbed-down description of what he found.

I ended up consulting with an old friend who lives out of town that is a mechanic for Chrysler. He gave me much the same advice that you have given me. He said that the PCM supplies 5vdc to the cam shaft sensor and the crank shaft sensor, and when the sensor circuit goes bad the 5 volt supply from the PCM (can, but not always) be shut down by the PCM making it appear that the PCM is bad. He also sent me the complete diagnostic tree for the cam shaft sensor circuit and the crank shaft sensor curcit.

I think I may take the diagnostics tree over to my mechanic and so he has the info on the correct way to trouble shoot this problem. I doubt it, but just maybe he will learn something and it could keep the next person from spending money on unnessesary repairs.

Thanks again for your post.
Take Care
Rocketman1
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Friday, March 20th, 2009 AT 5:10 PM

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