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Clutch Disengages Over Bumps

lute on Mon May 02, 2011 12:20 AM User is offline

Year: 1997
Make: Chevy
Model: Silverado
Engine Size: 6.5
Refrigerant Type: r134
Ambient Temp: 90
Country of Origin: United States

A friend of mine was telling me about a problem he has with his '97 Silverado pickup. A/C works perfect until he hits a bump at speed, and then the clutch disengages. He stops, pops the hood and taps the face of the clutch with a hammer handle. Then the clutch re- engages and the A/C goes back to work.
Seems he needs a new clutch, but I am curious what part of the clutch would cause this. Weak coil?

GM Tech on Mon May 02, 2011 3:42 AM User is offline

Try a severly worn armature plate driver and pulley face. If you were to measure that air gap between the two surfaces, you are probably near .080" Factory spec is .020" (business card thickness)

The front clutch plate driver is indeed a press fit. So if you were to "borrow" the clutch installer tool from Autozone, you, yourself can set that air gap back to .020"

Note: usually hitting the bump makes the clutch engage, not disengage. Worn clutches engage when coil is cold, but once the coil warms up, and compressor cycles off due to normal operation, then they don't re-engage so forget the bump theory.

Coil is not weak- you don't need a new clutch.

The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

lute on Tue May 03, 2011 11:33 AM User is offline

lute on Tue May 03, 2011 11:43 AM User is offline

Thanks for the quick reply.
I have some more data. He measured the gap and it is correct. The clutch will not engage at idle or low rpm, whether cold or hot. He says at speeds over 60 mph the clutch will engage and generally stay engaged until he slows down. However, if he hits a bump at speed the clutch disengages.

lute on Tue May 03, 2011 11:55 AM User is offline

A little more data: Clutch will normally engage if tapped lightly with screwdriver handle, but wont stay engaged unless at speed. Coil ohmed out at 6.5 Ohms.

Dougflas on Tue May 03, 2011 1:23 PM User is offline

and the voltage at the clutch is ........?

lute on Thu May 05, 2011 8:21 PM User is offline

He says .8 volts at the plug with engine running and a/c switch on. Jumped the clutch from the battery and it would not engage. Now it won't engage if he taps it, either.

Assuming gap is correct, why would rapping on the drive plate cause the clutch to engage?

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