Erratic Tach

[Rickrey]

1972 Elan+2– Recently purchased car. The tach works properly about 10% of the time. Otherwise, there is a wildly swinging needle. ALSO, I just noticed that when I use the signal light there are large swings of the tach needle in time with the signal flashes.
I have checked and cleaned the wire connections at the ignition coil.
Help ! Any suggestions ?
Thanks

[billwill]

Search this forum for Tachometer or tacho it has been discussed many times.


Probably your car has converted the tacho to voltage sensing and you are feeding erratic 12 volt supply to the supply terminal on the tacho. That one should be smooth, the RPM 'signal' is the only one that should have pulses on it.

[gjz30075]

Poor points adjustment, if you're running them.

[Rickrey]

Yikes ! This is more complicated than I thought . I did read some of the other threads.

The car has the Allison electronic ignition and a Facet fuel pump. I don’t know the source for the fuel pump feed.
This car also has a ballasted coil.

Would it be useful to run a direct wire from the battery to fuel pump to test if this part of the interference ?
Why would the the directional switch cause large swings on the tach ?
Last, what is the easiest way to get to the back of the tach— there is no radio— through radio hole. Have to remove speedo ?

Thanks

[mbell]

The tacho is easiest access under under the dash by removing the trim.

The original tacho works by sensing current pulses to the coil, which it can convert in rpm based on there frequency. If you add anything that is also switching (elcronic ign, fuel pumps) to the coil power (white wire) it can add additional current pulse that the tacho picks ups and confuses it.

One solution to this is to power items like electronic ignition and fuel pumps via a relay controlled by the coil power. A relay has consisteant power draw so doesn't interfere with tacho operation.

Other possible cause could be failing tacho or some issue with it being converted to voltage sensing incorrectly.

[billwill]

...
The original tacho works by sensing current pulses to the coil, which it can convert in rpm based on there frequency. If you add anything that is also switching (elcronic ign, fuel pumps) to the coil power (white wire) it can add additional current pulse that the tacho picks ups and confuses it.

...
Other possible cause could be failing tacho or some issue with it being converted to voltage sensing incorrectly.

You don't need to mess about with the fuel pump, just take a fresh wire from the power switched by the ignition switch directly from the switch to the power-feed of the tacho (not the SIGNAL feed), but if it has been converted to voltage sensing the signal might be too big and may need reducing with a couple of resistors.

[billwill]

PS: it would be wise to put a 'crash-detector' switch in the feed to the petrol pump, if it does not already have one, so that the pump will stop if you have a crash and so that it will stop pumping fuel into a possibly burning engine compartment.

[Rickrey]

Thanks all. I’m going to dive into Lotus electrics.

[Rickrey]

Well, here is another weird thing with the tach — yesterday, I was checking my signal lights— ignition on, but engine not running. When I activated the signal light, either side, the tach needle gave large sweeps in time with the signal.
Why ??

[stugilmour]

Usually symptoms like this indicate a bad ground. Perhaps start with a long wire connected to the battery ground lug and try grounding stuff to see if you can get rid of the fault. Suggest perhaps checking out the flasher relay first as it is occurring left and right; the pilot lamp in the speedo is also common. Is it occurring with the four way emergency flashers? IIRC the stock wiring for all Plus 2 models use separate flasher relays for the indicators and the flashers.

Note that if your 50 year old tach has not been rebuilt you may have continuing issues. I chased all kinds of problems until I sent the actual tach in for a complete rebuild and conversion. If you open the tach housing you may find the capacitors well past their best. There are new electronics available for the tach if you are comfortable taking it in yourself.

All the best. HTH.

[mjbeanie]

Just replaced a tired capacitor in my 50 year old tach, in the hopes to cure the erratic behavior you described.
Mine used to sweep full scale when the flipping on a switch with a demanding voltage load, such as wipers or heater. I replace the 2.5 microfarad cap in the RVI Tach with a 2.2 uF, 50V fresh cap from Ebay. IT FIXED IT!!
Needle is now calm and very steady -no matter what the electrical load.

Worth a try for a couple bucks. It was a great and simple fix! I love the easy ones.
Discussed in my thread "Tach pegs at the flip of a switch" Good luck.

[reb53]

Just replaced a tired capacitor in my 50 year old tach, in the hopes to cure the erratic behavior you described.
Mine used to sweep full scale when the flipping on a switch with a demanding voltage load, such as wipers or heater. I replace the 2.5 microfarad cap in the RVI Tach with a 2.2 uF, 50V fresh cap from Ebay. IT FIXED IT!!
Needle is now calm and very steady -no matter what the electrical load.

Worth a try for a couple bucks. It was a great and simple fix! I love the easy ones.
Discussed in my thread "Tach pegs at the flip of a switch" Good luck.

I did the same thing some years ago.
Except my capacitor wasn't a couple of bucks.
I actually found one on an old circuit board I had lying around the workshop, ex a VCR !
I knew that it'd be useful one day...

Ralph.