This morning on my way to work it was pretty warm so I hit the AC button on my 1987 EFI 4.9. When I did that I was on the highway and it started jerking back and forth pretty bad. It almost felt like it wanted to shut off.

When I hit the AC button again to turn it off it stopped and I was able to accelerate without any issues. Then, again I turned the AC back on and the same issue continued.

I use the AC daily and it has not had this issue in a while. Two weeks ago I changed the plugs wires and rotor so those are not the problem.

Any ideas?

The check engine light and the brake light have been on for a while now. I'm going to pull the codes tonight and post them up. Any info in the mean time would be great.

 

Sounds like the A/C compressor is cutting in and out which is normally caused by low freon. A low pressure switch turns off the compressor to prevent it burning out.

 

Would the switch cause it to go on and off repeatedly? The jerkins constant, not every few seconds. Thanks for the quick response.

 

Yes it will do that, the A/C compressor can slow down an engine when it kicks on, so the switch is reading slight pressure then it drops off again and so on. If you find a set of refrigerant gauges, you can test to see the amount of freon you have left. Run the engine, open the hood and hit the A/C on, you should see it turn on and off repeatedly.

 

Thank you I'll try that tonight. Do u think a can of freeon with gauges from an auto parts store will work?

I don't have access to any professional gauges.

 

never had that problem on my ford, but I had a honda on which the compressor seized it would shake violently when I turned the AC on.

 

It will give you an idea what pressures you have left. But If you have a fairly large leak, its only going to be a temporary fix. You can buy a set of refrigerant gauges for about 25$-35$ at some supply stores.

 

Make sure to check the pressure when the compressor is running. Also you have to make sure to set the guage with the outsie temp.

 

Thanks for the info guy, a funny thing happened yesterday. On my way home there was no problem, it was about 80 outside and the ac kicked on without a problem.

Then just now, on my way to work, I had the AC blasting and it worked just fine. There was not any of that bucking and jerking it was doing yesterday morning.

This is very weird, specially because today it was cooler outside than it was yesterday morning.