The other night, I started up my '71 Valiant after it had sat for a week. I put in the battery, and started it and let it run. I found that if I took the + cable off the battery, the engine would start to idle a little higher, and the tach was bouncing between 4000-5000 rpm until I hooked up the battery again, at which time all returned to normal. I checked the alternator and coil connections and all seemed ok. What makes this happen?

Tim the reason the rpm went up is because you unloaded the alternator when you took off the + battery cable. Therefore it did not have no load on it. This type of test is not recomended because you could cause damage to the alternator.

rpracing@catt.com

Must have been a lot of load, the battery needed to be charged. And why did the tach go crazy?

The alternator actually produces a high frequency pulsing DC voltage. The battery filters and dampens those pulses. The reason your tach went crazy is bacause it was not designed to work on the high frequency DC pulses you created by disconnecting the battery. Your problem(s) was caused by disconnecting the battery.

cfield@ll.net