Tachometer bouncing

[wwest]

Originally Posted by Joe Bob

Yeah that can happen but would not explain the 18v spike.

It certainly does.

[Joe Bob]

A loose wire in the charging circuit would on occasion show less than 12.6 volts which was the resting battery charge. The OP stated it didn't dip below that level.....assume, VR nit alt.....but who cares, he will likely get an exchange unit and not rebuild this one.......

[TonyFlaco]

My needle jumps at start and occasionally between shifts as well. Oh boy, I will have to go check it out tomorrow morning.

[RoninLB]

every 911 needs a volt gauge

I like analog

[wwest]

Originally Posted by RoninLB

every 911 needs a volt gauge

I like analog

Thats fine for monitoring battery voltage, meter wiring directly to the battery up front. But you also need a peak voltage "sample-and-hold" meter wired to somewhere in the rear very near the alternator to monitor the alternator "spike" voltages that are most likely to adversely impact electronic components, CDI, etc.

In an ordinary car with the battery and alternator nearby each other electrically one meter would be fine.

[RoninLB]

never heard of a "spike voltage meter"

i'm not worried about the 400v spike from improperly jumping another car

how about more info on this meter ?

[wright77]

I havent sourced one yet. BUT, tonite I drove the car, no spiking of the tach. I checked voltage, an even 14.4 at idle. I do now hear a grinding noise near the fuel pump. No fuel starvation it runs fine. Can a shop check alt output separate from the battery?

Thanks.

[Coach House 911]

I venture to bet it will happen again. Several times the tach stopped jumping in my '79 ...but like I say the last time I ran mine I melted the wires behind the instrument cluster. Wish I knew now what I didn't know then -hoping the problem would just go away.

[bmoody33]

my car - 1985 - 911 - Cabby - 94K miles - My tachometer was sporadic for about 6 months bouncing all over the place then stopped completely. I tested the dme for signal, ground, etc and everything showed fine but thought I'll just send it to Palo Alto speedometer and them test it for a diagnostic fee of $35.00 which can be applied to a repair. They bench tested it and it worked fine for them but they offered to rebuild known parts that go bad over time in our tachs for $85.00 So I said go ahead. And guess what? When I got the tach back and installed it worked perfectly again! I was very happy! Thank you Palo Alto Speedometer!