Air conditioning compressor won't engage

[HotRodr56]

1994 C3500 1ton dually 454 V-8 gas burner R134a system. The air quit working and I assumed low refrigerant. Hooked up the gauges but looked ok. I found the compressor clutch would engage for about a second sometimes but mostly none at all. I bypassed the low pressure cutout on the accumulator but still wouldn't engage. Pulled the power to the compressor and read 11.59 volts with a meter. I thought this was enough to engage the clutch but apparently not, perhaps amperage is low. Ran a jumper direct from the battery (12.49 volts) and clutch engages fine. Any ideas or suggestions why I have low voltage at the power source, what to look for, any experience with similar problem.
Thanks, HotRodr56

[J-Ri]

You've got some corroded connections which is causing the voltage drop. Measure the amperage, that will tell you if you're getting enough power. You could have just 1/2 a strand of wire and it will have low ohms and no voltage drop, but won't flow nearly enough amps. The amperage for the compressor should be around 6A.

[HotRodr56]

Thanks,
That is sort of what I was thinking. Where are the common places to look for corrosion? The plug to the compressor looks fine and I cleaned it thoroughly. Somewhere upstream must be a problem but where do I look?

[J-Ri]

There is a relay for it... I would try bypassing the relay and see if that does the trick (but don't leave it like that for long or you'll damage the A/C system). If it doesn't, just keep testing the voltage drops across the connectors (sorry, I don't know offhand where they are) until you find the bad one. Up to .1v is ok for a low amperage connection. Just remember the system has to be intact for the voltage drop tests to work, back probe the connectors with a safety pin or a straightened paperclip.

It could also be a problem with a pressure or temperature sensor.