Unintended Acceleration with A/C Running Wasn't by Design

Dear Car Talk

Dear Car Talk | Jul 21, 2016

Dear Car Talk:

I used to own an '81 Ford Fairmont. Although it was a great first car, I recall it having a rather peculiar issue (perhaps it was by design?): While the air conditioner was on, if I pressed the gas pedal enough, the pedal would lock, such that if I were on a level grade, the car would accelerate to 45 mph (even after removing my foot from the pedal). If I pressed hard enough on the brake pedal, the accelerator pedal would release. Now, here's the peculiar part: Turning off the air conditioner would immediately release the gas pedal as well. Was this by design? Thanks!

-- Jon

Not unless Ford was trying to get a jump on Audi in the unintended-acceleration race of the 1980s, Jon. Maybe Ford wanted all the free publicity that came with that?

The '81 Fairmont, like most cars of that era, had something called an idle-up solenoid. The purpose of the idle-up solenoid was to raise the engine's idle speed when a heavy load was imposed on the engine. And the air conditioner was exactly the kind of heavy load they were thinking of. It demands a lot of power from the engine, so it can drag down the idle speed and cause the car to stall.

So, by design, whenever you turn on the air conditioner, this idle-up solenoid kicks in, and raises the idle speed.

However, it's only supposed to raise the idle speed a few hundred rpm. It sounds like in your case, it was raising the idle speed a few thousand rpm! So it was out of adjustment ... OK, way out of adjustment. But an adjustment probably would have fixed it, Jon. 

I don't blame you for ignoring it, though. I'm sure it was quite a thrill to have your '81 Ford Fairmont accelerate to 45 mph under any circumstances.

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