Tachometer trouble - repair options?

[David F]

My tachometer recently decided to stop working correctly.  It all started after a long day of driving... got home shut the car down and restart was difficult, once started, car died.  I put it away to diagnose later.  Subsequently, I pulled the distributor, changed the condenser and coil.  Upon restart, I notice the tachometer was reading about 300 rpm when actual was about 900 rpm.  Revving the engine and the tach will only display a maximum of about 1000 rpms.  I cannot find any obvious fault.

Anyone have any ideas on how to troubleshoot, or is it more than likely that my tachometer finally died?  Can the tachometer be repaired readily?

[7tooZ]

My tachometer recently decided to stop working correctly.  It all started after a long day of driving... got home shut the car down and restart was difficult, once started, car died.  I put it away to diagnose later.  Subsequently, I pulled the distributor, changed the condenser and coil.  Upon restart, I notice the tachometer was reading about 300 rpm when actual was about 900 rpm.  Revving the engine and the tach will only display a maximum of about 1000 rpms.  I cannot find any obvious fault.
Anyone have any ideas on how to troubleshoot, or is it more than likely that my tachometer finally died?  Can the tachometer be repaired readily?

David before you do anything else go back and double check the wiring you touched when changing the coil and condenser. Check all the wiring on the ballast also.

Ps always helps if you tell us what you are working on.


72 body and block, everything else 71, Tokico springs, Illumina, R180 CLSD, 83 close ratio, 3.90 gears, Ztherapy SUs, BRE 15X7 Libre wheels and BRE front spoiler.

[Zed Head]

4 hours ago, David F said:

 I pulled the distributor, changed the condenser and coil.  Upon restart, I notice the tachometer was reading about 300 rpm when actual was about 900 rpm.  Revving the engine and the tach will only display a maximum of about 1000 rpms.  I cannot find any obvious fault.

I had this problem with an electronic ignition system.  A condenser/capacitor added to the system fixed it.  It wasn't the tach it was the signal on the line.  My guess was based on logic based on accumulated random knowledge.  Can't explain in detail why it works.

Like 7 too says, it's hard to tell what you're working with.  Why would you pull the distributor to change those parts?

[David F]

Sorry, 72 240z.  Condenser replaced, ballast resister wiring checked.  Not much you can screw up wiring wise.   No electronic ignition.

[Zed Head]

I'm going to guess that your new coil is different than the old coil.  Can you give specs on the two, take a picture, and/or measure resistance across the primary circuit?  Might be a clue there.

[7tooZ]

I realize it seems so simple. I have been driving and working on early Zs for 30 years and made a wiring mistake on my current 72. It took a friend 5 minutes to look at it and move one wire to cure a similar problem.
Ps I probably have five working tachometers if you need one. Let me know and can send you one. Changing out a tach is no very much fun.


72 body and block, everything else 71, Tokico springs, Illumina, R180 CLSD, 83 close ratio, 3.90 gears, Ztherapy SUs, BRE 15X7 Libre wheels and BRE front spoiler.

[cgsheen1]

Right, the Tach in the earlies is "current sensing" so the wiring path to the ballast and coil is very specific and it requires a coil with the same value as stock.  There are two B/W - one that goes to the ballast and one that goes to the coil.  Be sure to reference the full circuit diagram because there are simplified diagrams in the BE and EE sections that don't show all the wiring involved in the ignition and Tach circuit...

[Zed Head]

Just a follow-on to cgsheen's post - the ballast resistor is in play while the engine is running.  So if you had the ballast wiring wrong, bypassing the ballast like the Start circuit does, you might get too much current through the tach.  You can get the diagrams and/or you can see which post on the ballast has power with the key on, then measure resistance from it to the negative post.  Resistance should be the ballast plus the coil.  Or it might be that you used a "Blaster" or some other low resistance coil.  72 uses a pretty high resistance for the coil.  Or you used a 3 ohm coil and don't have enough current through the tach circuit.  Many possibilities.

[David F]

On 11/26/2018 at 7:34 PM, Zed Head said:

I'm going to guess that your new coil is different than the old coil.  Can you give specs on the two, take a picture, and/or measure resistance across the primary circuit?  Might be a clue there.

Tach worked fine on new coil for months.  Re-installed OEM type coil and no difference.  Neither coil had any internal resistance and requires the ballast resistor.  I can provide resistance measurements across the primary winding.

[David F]

On 11/26/2018 at 7:43 PM, 7tooZ said:

I realize it seems so simple. I have been driving and working on early Zs for 30 years and made a wiring mistake on my current 72. It took a friend 5 minutes to look at it and move one wire to cure a similar problem.
Ps I probably have five working tachometers if you need one. Let me know and can send you one. Changing out a tach is no very much fun.


72 body and block, everything else 71, Tokico springs, Illumina, R180 CLSD, 83 close ratio, 3.90 gears, Ztherapy SUs, BRE 15X7 Libre wheels and BRE front spoiler.

Appreciate the offer.  Sure would like to figure out why the one I have is acting up.  But, if no luck, I might take you up on your offer.  I have had the tach out once before to change out the lamps.  Long ago I labeled the coil wires + and - so I would not have to worry about swapping them inadvertently.  So, I have them installed as labeled when the tach worked fine.

[David F]

On 11/26/2018 at 11:01 PM, Zed Head said:

Just a follow-on to cgsheen's post - the ballast resistor is in play while the engine is running.  So if you had the ballast wiring wrong, bypassing the ballast like the Start circuit does, you might get too much current through the tach.  You can get the diagrams and/or you can see which post on the ballast has power with the key on, then measure resistance from it to the negative post.  Resistance should be the ballast plus the coil.  Or it might be that you used a "Blaster" or some other low resistance coil.  72 uses a pretty high resistance for the coil.  Or you used a 3 ohm coil and don't have enough current through the tach circuit.  Many possibilities.

Thanks.  But, ballast wires not disturbed between working and not working.  Jumper across ballast makes not difference.  I think the voltage sensing circuitry in the tach might be the culprit.

[Zed Head]

5 hours ago, David F said:

Tach worked fine on new coil for months.  Re-installed OEM type coil and no difference.  Neither coil had any internal resistance and requires the ballast resistor.  I can provide resistance measurements across the primary winding.

Might be that the tach (edit - wrote coil before) was damaged by too much current.  Good resistance measurements might tell something.  Only superconductors have no resistance and they require liquid nitrogen and/or helium.

Edited November 28, 2018 by Zed Head
wrong word