Vacuum reading for wipers?

What is the minimum vacuum needed to run windshield wipers (or does this vary wildly from car to car)?

I've had an accessory, fanbelt-driven vacuum pump in my 1937-era car for several years, and it always kept the wipers working when I drove up a hill. Lately, the wipers stall when going up a hill, and I'm trying to pinpoint the problem.

The wipers work fine under normal engine vacuum (when idling) so I'm assuming the wiper motor is okay. When I test the accessory vacuum pump it seems to be pulling about 10-12 (inch-pounds?) on my vacuum gauge but when it's hooked directly to the wipers, they don't move.

Should 10-12 pounds be sufficient to work the wipers? (In that case the accessory pump is working, and the problem may be in the wiper motor after all.) Or do you really need full manifold vacuum (I think that's about 22 on my car) to budge the wipers?

Hi- a healthy engine produces 18-20" of vacuum. Your auxiliary pump is probably not producing enough pressure to power the wiper motor.

Hook that vacuum gauge up to your engine and see what you get.

Marty Lum

Actually, the engine is pulling good vacuum and the wipers work great when they're hooked to it. I was just curious what the "threshold" was, in terms of vacuum needed to work the wipers. 20"? 18"? 12"?

If I knew that 12" was all it took to run the wipers, (which the pump now puts out), I would then suspect that the wiper motor itself might not be up to par.

I should have measured the vacuum on the accessory pump back when it was working okay, to get a benchmark against which to compare. But of course back then, I had no reason to do so!

You might email Rob at Ficken Wiper Service. He overhauled my 38 wiper motors and they are like new. He tests all motors before returning. He surely will know the vacuum range to work your motors. His email address is wiperman@wiperman.com.