F-150 revs with Defroster - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 02-13-2008, 10:36 PM   #1
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Default F-150 revs with Defroster

Well my dad took the Van, since its much less practical for me than the Truck, being that I drive between 40-80 miles a day and the Van gets 8-10mpg and drives like crap on the highway.

Here'* our newest item in the driveway if you missed it
http://www.bonnevilleclub.com/forum/...ic.php?t=93509

Anyway so I have been driving dad'* '95 F-150. I noticed it seems more sluggish than usual, which could be many thing causing that. But The thing that is really go me stumped is this; I noticed today when i came to a stop light the truck acted like it wanted to move, as if i didn't have my foot on the brake hard enough and it rolled forward, so i pushed harder and it stopped. Then again it does it, lurches forward. So i'm like wtf? Put it in every gear (this is a long-ish red light) and it continues to lurch forward every few seconds. even in park it did this. Neutral the motor reved up a couple hundred RPMs.

Light turns green and I continue on my way, paying close attention. All the gauges are looking ok except the water temp looked kinda low. But going 45 mph now and i feel it come out of overdrive like i put my foot down on the pedal for a second. I started counting, its about 8 second period between each time it does this. So i called dad to let him know and see if he knows what it is or if he knew about it. He said turn the defroster off. So i did, and it stopped acting weird. I asked if he knew what it was and he said he doesn't know what would be causing it, he meant to look into it but never did, so he is unsure why it acts the way it does with the defroster on.

Any ideas or experiences similar to this? Why would it rev up like that in intervals?
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Old 02-13-2008, 10:38 PM   #2
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It'* drawing wayyyyy too much current.

Kinda had a similar issue in my install bay just recently, except an amplifier was internally shot, drawing an insane amount of current.

Probably shorted out somewhere.
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Old 02-14-2008, 01:15 AM   #3
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yup. something is most likely shorting out, and it is just drawing a ton of current to a ground in a loop. does the defrost still work?
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Old 02-14-2008, 01:23 AM   #4
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rear defroster or regular defroster? because it could the the ac compressor being bad causing the engine to rev, since the compressor is running when defrost is on.
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Old 02-14-2008, 01:54 AM   #5
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Quote:
Originally Posted by GoldenBullet
rear defroster or regular defroster? because it could the the ac compressor being bad causing the engine to rev, since the compressor is running when defrost is on.
my thought exactly, sounds like a compressor cycling to prevent lockup and just either pullin too much juice or straining.... i doubt an f150 has a rear defrost
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Old 02-14-2008, 04:37 AM   #6
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They usually cause the idle to step up to compensate for the load of the compressor. Just like when the air/defrost is on when you floor it, it will cut the compressor off to utilize the power at the wheels.
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Old 02-14-2008, 09:23 AM   #7
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It has nothing to do with the current draw.

It'* the air conditioning compressor engaging/disengaging. The additional load on the engine is causing the idle to bump up and down. You wouldn't typically notice the additional drag on the engine if the refrigerant pressures were correct in the system - it would be just one constant drag if they were correct (remember the old saying that a/c being on causes the gas mileage to go down? This is why.). Being a 13 year old vehicle, it'* a safe bet that you have a slow leak somewhere in the system. By engaging/disengaging like it'* doing, it'* protecting itself for seizing...and causing the looping idle.

Just run with the defrosters off. And, if you want better gas mileage in the winter, keep the defrosters off. The a/c compressor engages everytime you turn on the defrosters to remove the moisture from the windshield (in essence, a/c does two things - cools the air AND removes moisture/humidity).
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Old 02-14-2008, 11:26 AM   #8
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Alright thanks, some good info here. I knew when the AC is on the rpms pick up a bit, I didn't know the defroster was run off the ac compressor though, it makes sense i guess, never thought about it.

But thanks guys, we'll look into it.


Sorry about this being in the wrong section, though this section was only for pics and vid, just reread the description, won't happen again, i think.
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Old 02-14-2008, 11:58 AM   #9
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No problem on the sections. This new forum takes a little getting used to.

Not to beat a dead horse, but the water you see dripping on the outside of your car when the a/c is running is actually the moisture removed from the cabin area. Your home a/c unit does the same thing, but it'* plumbed to a floor drain or condensate pump.
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Old 02-14-2008, 01:59 PM   #10
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Ah..... I didn't read that correctly. I had thought rear defrost, not the windshield defroster.

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