Tachometer Help!!!!
 

Can someone tell me just how to chase down this tachometer problem. The tach. occasionally bounces around, does not show the idle rpm properly (stays at zero, bounces up a bit and settels in). This has been a very recent but intermittent problem. I noted in my 73.5 T engoine compartment that their is a rpm transducer relay next to the CD Unit and was wondering if this could be the culprit. Just what makes a tach. go bad? Others have told me its a sign the coil is going out, maybe even the CD unit itself! It is the newer VDO model (that is compard to the old 60's version)and is a @#$% remove form the dash. Can anyone shed some light on tachs. going bad, or relays screwing up and how I can test or better access this problem.

Thanks
Bob 73.5 T

Did somebody rebuild your gauge? Either way, make sure that the needle actually is seated tight to the shaft.

Good luck!

My 930 was doing the same, check your alternator. Mine was putting out 14-15 volts which causes the tach to go nuts. A worn out battery may also be the cause. Check this ASAP, because if the alternator is putting out too much, you could fry your CD box (happened to me) and you'll have to replace that too.

My alternator is fine.......
Isn't there a relay of some sort that I can check or replace as a first step to correction? I tried to pry that tach. out of the dask and its nested so tightly it stays wedged in between the steering column and dask overlay! How do you get that sucker out?

Thanks
Bob

Bob, remove the clock first (usually easier), then push the tach out from behind.

Randy Wells

The 2.4 CIS engine does NOT have a speed relay, or rpm transducer!

The points are the most likely culprit, as the CDI-uniit and tachometer share the same trigger signal from the distributor. The coil or CDI-unit can't make the tach. act eratic! Older tachometers of 1969 vintage used a ballast unit -- a Voltage divider from the coil primary winding, but that was discontinued.

Have you monitored the system Voltage level during a test drive? Either low or high Voltage can cause the tach. to be eratic.

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Warren Hall
1973 911S Targa
1992 Dodge Dakota 5.2 4X4 parts hauler

I agree with early_S_man. It is likely the points. If you ignore it, it will get worse (more tach bouncing) and then the engine will start to run like hell. Plus you can burn the points. We have a 73.5 and 76 and a bounching tach means time to get the dwell meter out and adjust the points or if you put it off long then replace the points! If the points have a lot of miles on them (10,000 miles plus) I'd just go ahead and replace them as they're inexpensive. Cheers. Jim

Hey Bob!

I had a similar "Dancing" tach problem that originally was diagnosed as a bad battery, but it ended up to be a coroded ground strap to the battery that was interupting the flow of "juice" to the car. New $ 10.00 dollar groung strap solved the problem!

huey581
86' 911 Cab

Thanks Guys.............
My mechanic, who I trust advised the same with a checkof the points and checking the movement of the rotor button arm in the distributor. Too much play can be a culprit too. Has anyone installed that perma-tune unit in place of the points? The scuttlebutt is that they are superior to points, are tied into the stock CD and never dwell to check. And only $90.00! Thoughts, please.....

Bob
'73.5 T

Hmmmm, Warren may have answered the question I have had for a couple of weeks.

I installed an MSD unit on my 69 and the tach doesn't work when connected to the coil (as it was) - the multiple sparks of the MSD make it show 2500-3000 revs at idle!

The tach also doesn't register if connected to the MSD tach output or the distributor. Matt Smith installed an MSD in his '72 at the same time and it works fine from the distributor - therefore I have assumed the tachs are different from '69 to 72.

Warren's remark seems to back it up.

Best bit of news is the MSD tach adaptor is in my father's hands as of this morning and I can install it this weekend! Woohoo - working tach!

And Sunroof, sorry to jump on your thread. I have read that bouncing tach can be either battery or grounding issues or points. If it happened all of a sudden it is probably not bad grounds (which would worsen over time).


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Cameron Baudinet
1975 911S (in bits)
1969 911T