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Discussion Starter · #1 ·
This is on our 91 YJ. Heat on, blower on the first click as in when its set to or past defrost. Drive along with little gas pedal and the amount of air coming out of the floor vents & fan is OK. Hit gas, as in drop vacuum, and its like I just turned the fan control to high. I assume this is related to the cowl vent opening and closing upon changes in engine vac, but why does it do this and can it be made to blow similar to when Im on the gas?
 

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So does the air movement increase or is your fan increasing speed? What does you amp gauge do when this happens? Is it 10v or lower and then increase to 14v or more? Could be low voltage and when engine rev's up, your alt could be increasing voltage causing fan speed to change. When this happens, have you switched it to high to see if blows even harder? I could be wrong but your vacuum should not make a difference, it's enough at idle to open the vent. Could be a weak battery or alt. Check to see if your headlights get brighter too.
 
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Where is the vacuum check valve? I don't have this problem, but just in case i do someday and it's not electrical.
 

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Under the hood between the engine and the tube going through the firewall
 

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Discussion Starter · #6 ·
This is what is in the jeep heater system explained sticky. "It should also be noted that the right Cowl Gate defaults to the open position when the vacuum motor, which controls it, looses vacuum". I can say my heater controls are pretty screwed up and I manually switch from heater to defrost by hand. The vent cable is broken loose and the blend door gets done by hand as well. The only thing still hooked up is the hot/cold flap. So according to the heater sys explained the heater pulls air from the left side all the time which is operated by cable. Then in fact the right side defaults to open once the Vac dropped when I hit the gas allowing both sides, if the right side even works, to allow 100% open air supply. That would make sense then GottaGoFast about the check valve. should have figured that one as several of the ford trucks I've had were notorious for a similar problem. They instead switch from defrost to dash vents back and forth as vacuum goes up and down.
 

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yep, don't sound electrical to me. LOL My explanation is what happened to me on my dodge ram. Alt was failing and the voltage would go up and down with RPM's
 

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Discussion Starter · #8 ·
yep, don't sound electrical to me. LOL My explanation is what happened to me on my dodge ram. Alt was failing and the voltage would go up and down with RPM's
Seems dodge products are really sensitive on the gauge etc with things like blinkers or? I had a few 70ies dodge vans that ran for years just fine but simply putting the blinker on made the volt gauge dip or bounce like I maxed the charging system. Ive been over my YJ end to end redoing cables, fixing or checking grounds etc, but like I mentioned the blinkers on make the idle fluctuate a wee bit when sitting at a stop light. Alt charges at 14.3 and it always starts and everything electrical works as it should.
 
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