fluctuation on tachometer?

[Lord Warlock]

My tic toc tac tends to show rpms about double what it should when the engine runs (doesn't run often, but when it does the tach appears to be off quite a bit) what is the likely cause of this? is the ballast resister involved? or is there a resistor on the tach itself that needs to be swapped out to fix?  The clock will work for a short time if i pull out the stem and let it snap back into place, but it rarely works very long.  I never really cared much that the clock worked when i drove the car, but i did use the tach to gauge the speed sometimes when the speedo started acting up....usually above 100mph the speedo tends to bounce around, below 100 it seems to work fine,  for the past 25 years it hasn't been driven much so it hasn't been an issue.  Its slowly getting back to the point where i'll need to fix the gauges, wish there was a shop nearby that did rebuilds, as its more complicated than i'd be comfortable doing myself. 

The speedo problem I always assumed was a cable issue, I had replaced the cable back in the 70s when i drove it,  but the gear is likely worn out.  Not really sure what to do with it at this point. 

[A383Wing]

little more info needed here...like what kind of ignition system are you running? Did this start all of a sudden? Did you change anything recently?

the resistor has nothing to do with the tach....that is only for reducing the 12v power to the "+" terminal of coil....tach reading is taken off the "-" side of coil

the clock sounds like it needs a new set of contact points in it...

Bryan

[Lord Warlock]

My tach is currently totally stock, stock ignition, stock coil (actually has an accel hi powered coil but it looks identical to a stock unit) stock ignition.  Stock alternator. 

would having a high performance coil cause it to read wrongly? It was an accel version of a replacement coil for our car that i bought 30 years ago and it still works fine.  I do have another stock coil i could hook up if necessary.

[Just 6T9 CHGR]

More than likely it needs to be rebuilt.  The tach's arent very accurate even when they are rebuilt either.

Mine was doing the same before I had it rebuilt.  I am running elect ign now with a blaster coil and it works fine after the rebuild 

[Ghoste]

Chris is right, in fact they weren't all that accurate even when brand new which is why very few of these cars were actually ordered with one (even though most have it now).  There are a couple of very reputable instrument guys out there that can help.

[Pete in NH]

Hi,

There are a couple of timing and averaging circuits that are controlled by electrolytic capacitors. As those capacitors age they start to go down in value. This may be causing your tach to double trigger and read high. I would bet replacing those capacitors would fix things up. If you post a picture of the tach printed circuit board I can show you the ones that are likely bad. Over 40 years in the summer heat under the dash board and those capacitors are likely cooked.

[Lord Warlock]

Getting a pic of the circuit board would be troublesome as I'd have to remove the cluster to get to it with a camera, I've learned from the past that disassembly of old brittle parts usually results in breakage.  getting the cluster out would let me get a look at the headlight switch to see about replacing the hoses to the vacuum canister, the switch appears to work turning on the lights, but the doors have never functioned since i got the car, I had to switch the tubes coming out of the vacuum canister to the headlights to open and shut the doors.   The actual rebuilding of the cluster itself is beyond me, as is redoing the wiring harness behind the dash, even though i know the harness is probably cooked pretty good at this point, the wires all feel hard and inflexible. 

I wish i hadn't given away the extra ralley cluster with tic toc tach I held onto for decades, gave it to a car club member who needed one, its faces were more faded/yellowed than the original cluster was in mine, although it had lower miles that matched the miles on the motor and transmission when i swapped them out in the early 80s.  (69k).  My car hasn't sat outside now for over 30 years, its been garaged since i parked it in 84, so at least the heat and sun hasn't taken its toll, and rust has been relatively minimal.

[A383Wing]

ya might as well just plan on either getting a new one, a rebuilt one, or having yours fixed....I doubt there is anything you yourself can do to get it reading right again

[mhinders]

I had a badly jumping tach in my 67. The reason was simply a bad grounding of the instrument cluster.

Regarding the accuracy of the original tach:
I tested the 2 units that I have with an accurate rpm-signal from a signal generator. Mine are pretty good in accuracy, one could be adjusted to better accuracy than the other. In my opinion they are good enough for a street car.

Unfortunately I cannot find the file with the results of the test. I posted the results somewhere but can't remember where... 

[Lord Warlock]

When i first started driving the car, while i wouldn't claim it was totally accurate all the time, it was close and didn't jump around, Its possible that i may have bumped a wire or two when changing the speedo cable a long time ago, the parking brake light came on and wouldn't go off so i just removed the bulb on it in the 80s.  Is there a graphic that would show where the ground wire is for the tach so i can take a look next time i get the cluster loose enough to see behind it?

deciphering printed circuit error is a bit more complicated than I'd want to do, i've found replacement circuits online as well as gauge replacements (not cheap) I pretty much use my ears for knowing what the rpms are, and since the car is an automatic it really doesn't need a tach that bad, but replacing the tach is a bit more involved than just taking one gauge out and putting it back in, it appears you have to disassemble the entre unit to get to the tach, and if i do that I'd rather replace/fix all the gauges at once.  Unfortunately there are no shops nearby that do this, and i don't trust sending off parts to rebuild...tried that with front calipers and they lost them. 

[mhinders]

My guess is that the housing of your tach is grounded in the instrument cluster frame, and that frame in turn is grounded in the body of your car.
I made a separate grounding wire from near the battery to the instrument cluster frame, since I couldn't trust those old rusty sheet metal screws anymore.

[bulleteater]

Have you checked your points? Mine was fluctuating along with My engine come to find out my distributor was bad but along with My distributor I have dual points and one point was getting spark where the other point no matter how far open or how close I adjusted the points I only got one to work.. I bought a new distributor from O'Riley's and I get no more fluctuating on my tach and my motor doesn't run rough at 2k rpms. So I'm going to have my original distributor rebuilt. O'Riley's only has single point and I want my dual points.. Hope you find the problem

[Lord Warlock]

I still use the single point distributor on the car, it works fine as far as starting and idling or revving the motor.  When I actually get the car driving again I'll likely spend more time and money on the gauge cluster, wiring harness, and a few engine components like a new distributor or coil.  Right now the coil isn't even hooked up so won't be able to test the tach for a bit yet.  I'm busy with the trunk floor, gas tank and brake system now.