Air conditioning compressor won't engage

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

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.

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?

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.