This wiring doesn't make any sense

I have a '75 Firebird and decided to put a tachometer on it. The tach unit is a four wire (black-ground, red-power, yellow-light and green-sending wire). Everything went together smooth. The car has a factory HEI distributor. There was a brown wire attach to the TACH outlet. I attempted to trace it but it got lost in a wrapped loom and never came out. I couldn't find any use for it. So, I removed the wire and inserted the green tach wire. The car wouldn't start. I reinserted the mysterious brown wire and the car started right up. The BATT wire on the distributor is easy to identify (heavy gauge, red), so I know I am not getting things confused and plugging the green tach wire into the BATT terminal. I gotta be missing something but what? What is the mysterious brown wire suppose to do other than give me grief?

Tom

 

Did you try it with no wires on the TACH terminal?

 

Jim-

I tried no wire on the TACH terminal and the car won't start! With the brown wire on the distributor the car will start.

Tom

 

the brown wire could be the 12v "hot" from the starter - throws 12v to the HEI while cranking. Flip the Cap upside down and see if there are actually 3 terminals in there , tach could be right beside the brown .








.

 

I've never heard of a cranking wire on an HEI, since the pink wire gets full 12v.

1975 was the first year that HEI was used in most GM cars. Some cars had it earlier.

The brown wire usually goes straight to the tachometer in the dash.

Very strange....might try on an F body forum? or get the factory shop manual and see if there are any clues?

 

Where is the ground on the distributor? I understand some used a wire for ground (rather thanthe dit to engine block).
Could your brown wire be the ground?
(If so where is the tach wire.....?)