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Hello Everyone,

I was woundering about engine vacumn at Idle, which I think is supposed to be 18 in Hg. My question really is what should it be when you put the car in drive (I have a C4 automatic)? My vacumn drops to about 15 in Hg. Is this normal or is the transmission putting a big load on my engine to drop the vacumn.

Thanks,

Craig.
 

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The load of being in gear drops the vacuum . YES it DOES:bigthumbsup.
 

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Discussion Starter #3
vacmun for c4 transmission

When you place the car in gear the vacumn drops, but in gear the vacumn is suppose to be at 18 in Hg to provide the correct control pressure. The lower the vacumn the higher the control pressure devieates from the desired transmission setpoint.

Can anyone explain if this is correct?
 

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Their is no actual set point to the amount of vacuum an engine makes. Another words it does not have to make 18 Inches. The modulator works on a set of of parameters. The modulator controls line pressure. When the line pressure equals the pump pressure the transmission shifts. Some modulators are adjustable like the B&M units. Most modulators are set for app 16" of vacuum. But all transmissions have a governor that overides the modulator when the output shaft is not turning. This can be checked with a hand held vacuum pump like a mighty vac.
 
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