PCM programing?

I’m thinking about replacing the PCM behind the glove box in a 1993 Chevy Corsica, as it is not turing on the Radiator Fan when required (power wiring and relay test fine;temperature sensor is also fine).

Anyone know if the PCM requires connection to a computer for initial programing or setting of flags? Is it possible that a new one is not needed and some flags just need to be cleared or set? Does anyone know of an easy way to get the setup-clearing… done?

How does one get the techical service bulitin?

You get the information with the new PCM. There is necessary and excess info with it. These things should not need any programming. If the computers still use a removable PROM chip, I can’t believe there could be programming required while using the old memory. Don’t change it on a 1993, just put in a toggle switch. Use the fan while in town, or whenever you need it. I see it’s a 93 now so there is a PROM. Save the money toward a newer car.

You could just wire in a temperature switch to turn the radiator fan on, directly, or through a relay.

ADDED: This would only be ok IF the engine coolant temperature sensor were still in place and sending signals to the PCM. Some cars have radiators with radiator switches.

Would it pass smog test next year in CA with just a toggle switch to ground on the control? Is it OK to leave the wire to the PCM with a switch shunting to ground?

The relay is there and functional. The engine has a temperture sensor that works wired to the PCM.

What do I buy for the temperature switch, and where do I install it?

By the way, thank you for the advice. It is more thurough than I’ve gotten most other places.

Forget the radiator switch. The PCM (engine computer) needs the input from the engine coolant temperature sensor to go into closed loop when the sensor tells it that the engine coolant is hot enough (maybe, at 160F degrees).
HOW are you trying to turn the radiator fan on? And why? Is the engine overheating?
The PCM controls the radiator fan from signals from: the engine coolant temperature sensor, vehicle speed sensor. It will energize the fan relay when the A/C is turned on and the A/C intermediate pressure switch closes. Without the engine running, the A/C intermediate pressure switch DOESN’T close, and the PCM doesn’t energise the fan relay.
You can cause the radiator fan to turn on by jumpering the ALDL connector (under the dash) as you would to get the check engine light to flash.
In summary, if the engine isn’t over-heating, there’s no problem.

No, the car will NOT pass smog testing if the PCM doesn’t go into “closed loop” operation. It won’t go into “closed loop” until the engine coolant temperature sensor tells it that the engine coolant temperature is over 160F degrees.

Either the PCM is bad, or the wire from the fuse is bad or the control wire from the relay to the PCM is bad ,or the PCM ground is bad. The PCM is not turning on the relay.

I’m thinking I need to replace the PCM for $79, but I need to check the control voltage at the relay first, and then check the wire from the relay to the PCM.

Still, can I just plug a new PCM in, or is there some setup required?

Terminals to jumper on the ALDL connector? This would indicate if I needed a new PCM if I knew where to jumper.

How does the PCM know if the engine is running for A/C operation? Is there another sensor or relay?

I would think that the PCM does not require any programming if you replace it.

To see if the coolant fan relay circuit will turn on the fan all you need to do is tie a jumper between ground and the point where the dark green/white wire ties to coolant fan relay. If that works then the relay control is ok. You stated the sensor is ok but is the sensor getting hot enough to turn on the fan?

I see you have at least two other threads going besides this one pertaining to the same problem. This makes it difficult for everyone to keep track of things and what is going on to help solve the trouble. I recommend that when you start a post about a certain problem you do not start additional ones for the same thing.


Once more, IF the engine isn’t overheating, and the A/C is switched off, the radiator fan SHOULD NOT come on. If the A/C doesn’t work, even if SWITCHED ON, the radiator fan should not come on.

WHY should the radiator fan come on?

Yes; I waited until after the thermostat openned, and then waited even longer as the temperature indicator on the dash continued to rise; still the PCM did not operate the relay. The relay is fine. Also checked for control power at the relay and continuity of the Geeen wire with white stripe to the PCM; the wire is fine.

The A/C does work; it just doesn’t turn the fan on, thus the condensing unit eventually gets overheated.

Of course the car would overheat if driven in present condition!!! The PCM is not turning the fan on! That is why the car is not being used, which is why I’m checking the parts!!!

The last time the car was driven 50 ft, the car overheated.

hellokit -

You posted that I could jumper the ALDL connector to get the PCM to turn the fan on (if the PCM is working). Please give me the pin numbers for jumping, or at least a reference source that I might have access to that could give me the pin numbers.


Checked the control power voltage at the relay last night after work (with the ignition switch on). The control power voltage is fine, indicating no broken wires or problems on that side of the control wiring.

Then, pulled the PCM out, and checked continuity of the wire from the geen/white control wire from the relay to the PCM; it also is fine.

But the PCM doesn’t look anything like what the part store showed me, so I went back to the part store to see if they gave me wrong information and to actually find out what the PCM would cost. Well it turns out the gave me pricing for an ignition module and showed me an ignition module. They do not stock nor can they order a PCM. They have no way to look it up; so now I need to check a couple other competitor part stores. A friend checked the dealer, and stated the PCM is obsolete and will be difficult to obtain.

The PCM also has something plugged into the circuit card inside the metal box that looks like a microprocessor and EPROM. Don’t have any way to verify. Would that be transfered to a new unit?

Yes, I started additional threads because I wasn’t getting answers, and the question is slightly different. Old threads do not generally get posts. Similarly, thread titles have a lot to do with whether people open a thread or not, just like newspaper article titles determine whether people will skip an article.

But cougar, I haven’t seen any suggestions from you on how to get the fan running properly yet on any of the threads.

I may have assumed incorrectly about your car. The fan does turn on on a 1990 Gran Prix (a Chevolet). Your Chevrolet (Corcica) is, I’m assuming, the same in that regards. Under the dash is an electrical connector that you jumper across two holes to get the check engine light to flash. When this connector is so jumpered, the radiator fan comes on on a Gran Prix, and, supposedly on your car.
See repair manual for particulars on getting engine codes to flash.

I’m assuming you are stating you found it in a Haynes manual? I’ll look at it again and see if it indicates what to jumper.

I temporarily have the PCM out; but it will not be that big a deal to plug it back in for a test.

How long will the fan run with the pins jumpered? Did you buy one of those code testers and if so, did you find it was worth the investment?