PCM Replacement

I have a 99 Grand Prix. Due to several odd things it has done over the last two years (door locks constantly locking while driving, spedo and tach not working) we just live with it until it goes away. Now the service engine light is on with a cooling fan 2 circuit problem. The fan is working fine. I need to smog this car next month and it cannot be done with a light on. I beleive this to be a PCM problem. I found a used PCM the same year model and engine. Can I install this without reprogramming? I read in the Chilton manual that a replacement would not work unless it was programmed. Does this also apply to a used PCM? Will I cause any damage if I put it in to try?

Hold on here,Some of your issues could be caused by a BCM problem and you have not even mentioned making use of the OBD2 diagnostics that your cars is equipped. You are attacking this problem in the most amature way possible.Set me right here and list the diagnostic procedures you have used. You dont want that MM to stand for MickeyMouse do you?

No damage will occur by stubbing in a PCM.

On this car, are the door locks supposed to lock once you start moving? It may be the problem with the speedo, tach, and door locks are all common to a bad speed sensor. (but don’t quote me on that) If the computer came from the same model and year car, with same engine, there shouldn’t be any harm in swapping it in for testing—at the worst it may not start or run like hell if the programming is different. Your car may also have a “Body Controller Module” or BCM, which is like a second computer concerned with different tasks—this could be failing as well. I’m not familiar enough with your car to know what the PCM vs. BCM controls. But as oldschool mentioned, reading the codes from your existing computer is the best start—the check engine light is trying to tell you something. It looks like you must have read the codes at some point or you wouldn’t know that you have a “cooling fan 2 circuit problem” What are the rest of the codes you got? I doubt the fan problem is related to the other problems.

It’s easy to see why you think the PCM is at fault, and it’s clear that you want very badly to swap it out. Nonetheless, I think it is more likely that you have multiple electrical and sensor problems than a bad PCM.

Auto computers are proprietary and very little information is provided to the public about how they are configured. But my impression is the controlling the door locks with the PCM would be unusual. More likely they use a different computer or relays for that function. If you care, you can probably tell by checking the wiring diagram in a Haynes or Chilton manual and seeing what controls the door locks. I’d guess that you have a broken or intermittently shorting wire or electrical connection.

I’d guess that the speedomter/tach problem is a bad speed sensor, wiring problem or defective instrument cluster. Likewise the fan circuit problem is probably a fan sensor, not the PCM. You can probably identify the suspect components from the wiring diagram.

Can you replace the PCM without reprogramming? Good question. If the cars are identically configured, probably. If there are minor differences, maybe. However, tinkering with expensive components that may not be broken is not always a good idea.

What would the symptoms of a bad PCM be? Check Engine light constantly on. Throwing random error codes. Or refusing to read out codes.

The code I get is PO481 cooling fan 2 circuit malfuntion. When I clear this code it comes back again. This is the only code in computer. I checked all relays in circuit and they are OK. No mention of a BCM in any repair manual. As far as the door lock, speedo,tach, and I forgot to mention the traction control would also turn on and off, these problems would happen at different times not at the same time. After one of these problems would start it would last for a few days or weeks in the case of the door locks and never happen again.

I think you better look harder for that BCM even lower end GM products had BCM’s by 99. I hope this is not all caused by a acid damaged postive battery cable (the acid can wick down to the starter). Because you are able to communicate with the PCM I think it is healthy. Is the fan code associated with a fan failure or is it just a code? You should be able to see on your scanner if there is a BCM and talk to it and pull codes and perform other diagnostic activites. Are you using just a code reader and not a scanner? if yes this is part of the cause for the long diagnostic times. These cars are made to be repaired by a trained mechanic using a scanner,any thing less and the process gets way slowed down.

What is the “cooling fan 2 circuit”? The circuit which controls #2 fan? Could be.
There are two wires which come from the engine computer (PCM) which are for fan control(s). What a repairer would do is check the wiring from the PCM, to the fan relay, to #2 fan. What if the problem is just a poor wiring connector?

Hello, Mike;
your car has a “underhood electrical center” wich holds 3 relays for your fans, the pcm controls the ground side of the circuit. It will activate relay 1 and have fan 1 and 2 on for low speed fan mode; it will activate relay 2 and 3 and have fan 2 on for high speed mode. Please check the ground connections at the PCM. the connections for the two round control wires for the relays (dark blue and dark green wires) and the fuses for fan 1 and 2 (30 and 40 amps - look for corrosion in the socket also). Please post your results.