ABS light and rpm tach

[xnetcrash]

Hi guys, any idea why sometimes during driving the abs light comes on and the rpm tach stops, than after a minute or two the abs light goes off and the rpm tach starts working again.
I am guessing is a connector somewhere.

Ideas where to look are apreciated.
Thank you

Oh and one more question , the compressor cycles on and off during normal operation, So if the system was not filled to the proper pressure level than the compressor would run continuously or it will still work normally? And what side effects would be running the AC with low pressure.

Thank you