Tachometer bouncing

[wright77]

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

[wwest]

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.

[Joe Bob]

Multimeter time.....

[wright77]

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.

[Joe Bob]

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........

[wright77]

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.

[Joe Bob]

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.

[wright77]

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.

[Joe Bob]

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.....



$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%20exter nal%20%28for%20Marchal%2055W%20Alternator%29%2C%20 %20911%2F911%20Turbo%20%20%281975-81%29

[wright77]

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.

[Joe Bob]

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.....

[wright77]

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.

[Joe Bob]

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

[dshepp806]

Yes: both!!

[cstreit]

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.

[tcar]

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.

[Neel]

+1 tcar..

[b. hilliard]

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.

[Joe Bob]

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

[Coach House 911]

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.
Have you sourced one out yet?