TR6 Swapped Lucas to GM alternator

[birkline]

I just finished changing from a Lucas alternator to a GM alternator on my 75TR6 engine. The swap was simple BUT, now my tach is wacho. It acts like it’s reading double the actual RPM and jumps all over. Anyone have this happen to them?

 

[poolboy]

Did you replace the mechanical cable driven Smiths Tachometer with an aftermarket electronic tachometer ?

 

[birkline]

No, the car has an electronic tach.

 

[poolboy]

Somebody did, then.

 

[birkline]

Could be, it’s been around a bit

 

[bobhustead]

Contact the tach maker. They probably have the answer.
Bob

 

[Madflyer]

Did you get a new / new Alt or a rebuild Alt it may have a bad Diode. Check all DC volts or drive the car to where you got the Alt and they can check it in the car
. Rebuilds are a kin to seconds

 

[birkline]

I’ll check it out.

 

[Sarastro]

Unless you did something dumb, like disconnecting the tach lead from the coil's points terminal on the coil and replacing it on the (+) terminal, I suspect that the new alternator is putting voltage spikes on the power line. That's a concern, as a modern alternator's output should be pretty clean. I agree, it might be a good idea to take the alternator somewhere to check it.

If the alternator is OK, and you can find someone with an oscilloscope, it would be a two-minute task to confirm the spikes. Then adding a filter capacitor from the tach's power lead to ground should fix it. I would use about 100 microfarads, with a breakdown of 25 volts or more. It will be polarized, so the (+) terminal must be connected to the power lead and (-) to ground.

 

[birkline]

Thanks, added to the list!