Tach jumping,..anyone know for certain..?
 

One of the symptoms of overcharging the battery is the tach jumping, flickering, intermittently. Then at some point the tach jumping exceeds the rev-limit and the rev-limiter locks up, CLAMPS, and the engine cannot be restarted until power is completely removed, ignition fully OFF, from the rev limiter module.

But what is the actual source of the tach jumping?

a.) Over-voltage breakdown of the internal tach circuitry otself....

b.) CDI generating extra firing pulses due to over-voltage of its ciruitry (***).

c.) Extreme electromagnetic field from intermittent shorting of the alternator stator winding coupled into the distributor spark timing pickup/sensor.

My experience has been that the CDI will fail if the over-voltage issue is not quickly addressed.

Do the CDI modules that are triggered by distributor points exhibit this symptom.?

*** This is what I presumed until I realized that the actual source of the tach pulses is the distributor spark timing sensor "buffered" by the CDI internal circuitry.

Sounds like you might have a later model, but a jumping tach on my '72 always meant the points were about to puke. In my case, there were no rev changes associated with the jumping around, it just started off with a mind of its own.

Mine used to become erratic as the points wore and became out of tolerance.

^ what they said.

Points on my '74

you did not say the year.

first and easiest is to put a meter onthe battery with it running and/or take it and have the charging system tested. there are places that will do it for freeeeeee.

i just had the O scope on my dist and tach for my 930. the CD does take the signal from the dist and clean it up to a square wave signal. i would think that if the CD was making the signal to the tach bad, since it is the same signal that fires the CD, the engine would run bad. just speculation.
on my car, the signal to the tach and the speed relay split at the CD unit itself. so if the speed relay is getting false info, it is from the CD.

if the charging system checks good, you can disconnect the tach and run it again. if it does it again, its not the tach.

same with the speed relay. pull the relay, jump it to make the FP run and drive it.

i cant say why the rev limiter would clamp. may be something wrong with it. you may try just pulling a fuse to it, or disconnect the CD and see if it resets.

This was on a rarely driven 1978. Initial symptoms, I think related, the brake warning light would "latch" on and I would have to disconnect the battery to clear it.

Then..random events of engine quiting, replaced the battery but no help.

Discovered the over-charging condition, repalced the battery and VR. Symptom seemed to abate, but again, car was rarely driven throughout the following winter.

Come spring....

Engine quits randomly but there seems to be a definite corelation with hitting bumps in the roadway.

Replaced the alternator/regulater(internal this time) but now the CDI is heat sensitive, keep it cool, cool spray, and engine runs fine. Off to loren and things have been fine since reinstall.

Alternator disassembly indicated severe wear, stator wiring insolators mostly toast, seeming from overheating, 90,000 miles approximately.

An important point...over-charging persisted even after external VR was first replaced.

Note: Rev-limiter latch up prevented the fuel pump relay from working.

what made the rev limiter latch up?

i think the speed relays are bad for haveing bad solder joints. i fixed several on my brothers and i have seen several here with bad solder joints.
hitting all the connections with a soldering iron would not hurt.

Since the brake warning light circuitry latches up due to over-voltage I have assumed the rev-limiter circuitry has the same design flaw...electrical by...Lucas.