Check to make sure you have adequate pressure then try bypassing the low pressure safety switch on the receiver/dryer. My compressor seized last summer and two weeks later when I overhauled and fixed the system, it wouldn't engage because that switch wasn't working. I have it bypasses because I for darn sure wasn't going to open the system up again just for that. Now I just check the system whenever I think about it, maybe once a month or so to make sure its not leaking out. Its been good for about nine months now, knock on wood.

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Seth

1984 300D 225K
1985 300D Donor body
1985 300D Turbo 165K. Totaled. Donor Engine. It runs!!!
1980 300SD 311K My New Baby.
1979 BMW 633csi 62K+++? Dead odo