Tachometer bouncing
 

On my '78 SC, the tachometer needle has started bouncing around. I noticed it the other nite driving about 2800 RPMS and it jumped up to 5000, than came back down. Now it seems to be doing it as I shift gears. I thought the tach has electrically operated, not gear.

Any suggestions?

Thanks,
Bob

The alternator output voltage is "spiking" above ~16 volts, maybe even charging the battery to that level. The battery is electrically a L....O.......N...........G way from the alternator so circuit resistance can often be a factor.

With supply voltages above ~16 volts the CDI, Capacitive Discharge Ignition will continuously self-fire and thereby cause the tach to jump.

Another early indication is that the e-brake light will not reliability extinguish.

Once it gets bad enough the over-rev limiter will lock up and kill the fuel flow. In that case you will need to switch off the ignition entirely for about 10 seconds to allow the engine to restart, fuel flow return.

Multimeter time.....

Ok so it would be a good idea to check the alt voltage Eh? I have a multi meter but am not sure where to hook up. Thanks.

If not driving....at the battery terminals.

12.5v at rest
13-13.5 at idle
13.5-14 at 2500 rpms.....

over 14v is/can be a problem........

Ok I just checked it at rest. Its about 14.5. So possibly overcharging . Is the regulator separate from the alt on the '78 SCS?


Thanks for the info.

That's a little high,, start it up. Test at idle and 2500 rpms.

If you have an external regulator it will be over to the left of the engine bay.

Ok I started it, at about 2200 RPM it was at 14-14.5. When the idle came down, it jumped to 18, and stayed in that range. Than I shut her down, battery went back to 12.6.
Is the regulator under the black box with the 2 wing nuts? Thanks Joe.

Take a pic and post it it....or look to see it has terminals labeled B+ and says Bosch or whatever. Below is what Pelican uses for a pic......but not confirmed. Fits 75-81. Mine was an 83 and now a 93 motor.....so I can't go look in the garage.....:rolleyes:

http://www.pelicanparts.com/cgi-bin/...160320613-M260

$36.25 and in stock....

http://www.pelicanparts.com/cgi-bin/smart/more_info.cgi?pn=911-603-206-13-M260&catalog_description=Voltage%20Regulator%2C%20 external%20%28for%20Marchal%2055W%20Alternator%29% 2C%20%20911%2F911%20Turbo%20%20%281975-81%29

I will check it out, have to go now. So this sounds like it could be a regulator eh? Bouncing the voltage. Thanks again Joe.

Cheap fix IF you still have a separate VR. No telling if a PO replaced it with a newer alt with an internal VR.

Good Luck.....

Ok I checked it out. The regulator is gone. Suspecting the PO replaced the alternator with an internal reg. So my next step is to take it to a shop where they can check alt output.

You already have....replacement of the combined unit is advised.

Yes: both!!

It's your regulator, no question. 99 times outta 100 if the tach is jumping like that, regulator is suspect. Showing 18V cinches the deal. Replace it and all is well. don't well, have a tow-truck number handy when you're battery dies. :)

Couple things...

Get it fixed NOW. An electrical fire is bad news.

18V can also kill the CD, motors and bulbs.

And toast the battery. My Optima started 'melting'; you could poke the side and it was very flexible.

BTW, 14.5V or even 15V is no problem for a short while after you start the car as it re-charges the battery.

There are almost no separate VR's left. By now almost all alternators have been replaced on 81 and older cars.

+1 tcar..

Just a thought... I had some charging issues (started with bouncing tach needle) and eventually the car would quit running. After some testing, I decided my VR was at fault ('78SC updated w/ internal VR) and tore into it. I went through EVERYTHING, including replacing an aging battery and ended up finding the problem at my starter. There was a wire/ connection that was starting to break and interrupting the circuit intermittently. A new connection crimped on solved the issue. Wish I'd started there.

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

I had the exact same problem at the end of the driving season last year with my '79 SC.

Started with the tach bouncing, intermittent alt light which became brighter with driving, e brake light difficult to go off then finally would not go out at all. Then after start up one evening while waiting for the engine to warm up, a line of smoke came up from the defrost and the smell of rubber burning. I subsequently shut down the engine and it has sat until this morning when I uncovered to assess where to start to fix.

According to 101 projects for 911, it recos repalcement of the alt. Guess there is no other option.:confused:
Have you sourced one out yet?

It certainly does.

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.......

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

every 911 needs a volt gauge

I like analog



http://forums.pelicanparts.com/uploa...1299482616.jpg

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.

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 ?

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.

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.

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!