A/C Control (and PCM - ?)

The car: 1997 Ford Escort. 255K, though not on the engine/transmission.

I pulled my battery terminals yesterday to clean them. I was also changing the oil and inspecting this and that so the battery was disconnected probably for a good hour.

I didn’t drive it until today, hopped on the highway, and had no A/C - zero. No matter the switch positions the compressor wasn’t kicking on. (This event is impossible to miss on this little engine). The A/C has never had a problem, including on several days just last week - cold and functional as ever.

Anyway, I left it blowing all the same just to move some air and about 50 miles into the trip (95% interstate) it just spontaneously started working again and was fine for the other 100 miles I did today.

I don’t really know how the electricals work for the A/C system, but have always assumed that the PCM is involved since it is set to cut power to the compressor under certain load conditions.

So I’m wondering if, when the PCM’s memory is zeroed out, it has to redo whatever drive cycles are necessary before it will allow the compressor to run.

Either that or when I have a chance I’ll start looking for a bad connection or switch.

When disconnecting a battery on an OBDII vehicle, it’s good idea to connect a back-up power supply system to keep modules/computer memories alive. It used to be that doing so would just delete the radio presets or some other minor electrical system. But now that more modules/computers are involved with various electrical controls in a vehicle not doing so can lead to further/expensive problems.

This what I do when I disconnect a battery from a vehicle. http://www.aa1car.com/library/battery_disconnect_problems.htm


Good link - thanks Tester.

I knew that this was a problem with some makes/models for certain reasons (e.g. my Haynes manual for my Silhouette is full of battery disconnect warnings), but never thought of it unless potential problems were specified (in, e.g., repair or owner’s manuals). Loss of battery power for one reason or another just happens quite a bit over the life of a car.

Though I have thought of this before mostly for convenience. I actually bought one of those memory savers at Autozone. But I was puzzled b/c it didn’t have its own power source. Apparently you plug this one into a 12V car socket and plug the other end into the OBDII connector. Then you have to leave power running to the 12V socket. The problem was that the cord for the thing was all of about 3 feet long. So I’m wondering…how do I leave battery power to my 12V socket to run the memory saver when pulling power is the reason I need the memory saver…?

Anyway, I brought that one back to Autozone and forgot about it for a while. I suppose I’ll just have to start looking for what will work for me. (I don’t like the little 9V battery version just b/c they don’t leave you much time).

But its also confusing because sometimes the purpose of pulling battery power is so that you don’t inadvertently mess up any of those systems while working on the car…? So it looks to me like the risks run both ways. Leaving power to various items is bad. Not leaving it to them is bad…

It’s possible the PCM is to blame, but I think it’s more likely that you have a bad connection somewhere, or perhaps one of the system’s pressure sensors is failing. The car is at least 13 years old, so it could be low on charge too, though I don’t think this is your main problem.

I suppose if it’s working and the problem doesn’t occur again, don’t worry too much about it.