'78sc Erratic Tach behavior

[franklinstower]

My tach has started going bonkers. Seems to occur more often after I have driven a few miles.

see it here:
https://youtu.be/OrqlAbQ9Sg8

I used to have random tach weirdness but it usually just would stick at about 3,000; not bounce around like this.

Recent changes (last summer) include:

• New engine wiring harness from Dennis (Timmy2)
  Replaced original alternator with rebuilt IR Alternator
  Cleaned engine bay grounds

Completely stock 1978 SC

The tach issue has started recently, there were no problems with it for about the first 6 months after the engine reseal; harness; new Alt.

Any ideas of where to begin looking for the problem?

Thanks,
Paul

[timmy2]

Tach movement itself or perhaps VR is going out? Any flickering of head lights?
Plug in a digital voltage gauge in the lighter socket and see if it is jumping all over while you drive to rule out the VR.
If all normal for charging volts, then maybe the tach itself is going out. If you tap the gauge does it behave normally? Mine will stick if I leave the car in the sun too long. Plastic gauge face needs a tap and all good again.

Make sure you have good continuity from purple/black at CDI 6 pin connector to the back of the gauge. (Wiggle harness around while checking)

[franklinstower]

Originally Posted by timmy2

Tach movement itself or perhaps VR is going out? Any flickering of head lights?
Plug in a digital voltage gauge in the lighter socket and see if it is jumping all over while you drive to rule out the VR.
If all normal for charging volts, then maybe the tach itself is going out. If you tap the gauge does it behave normally? Mine will stick if I leave the car in the sun too long. Plastic gauge face needs a tap and all good again.

Make sure you have good continuity from purple/black at CDI 6 pin connector to the back of the gauge. (Wiggle harness around while checking)

Thanks Dennis.

I'll try those suggestions. Before your harness, I would randomly get the needle stuck at about 3,000 rpms, tapping the gauge I could get it to drop a little, but it would just stick there for a while until it would just randomly start working again. If the voltage checks out, I will try to swap tachs with a friend.

[timmy2]

Good plan swapping tachs, once you confirm the other tests are all ok

[franklinstower]

I'm following up on this issue: My voltage seemed to go all over the place after about 10 minutes of driving - everything from 9-17 volts when I used a lighter socket volt meter. So my guess is the VR went bad. A new VR is only about $35, but i had a year warranty on this alt so I am returning it. I got a new (rebuilt alt) and upon inspection noticed the diode plate(?) was cracked exposing some current paths that I think need to be insulated. I am not sure if this is a problem or not? Can i get some stray voltage/arcing from this?




here is a link to bigger pict.

http://imgur.com/aqveUWS


Thanks,
Paul

[Charles Freeborn]

Does it still have points? If so change them.

[timmy2]

Charles, No points in a '78.

I don't know that I would trust the exchange alternator looking at that photo.
Not an example of a quality rebuild!