Crazy Tachometer Behaviour

macdonge

macdonge

Senior Member
I'm trying, without much luck so far, to get a sensible tachometer reading on a '39 Merc 239 running on a test stand. It has a Bubba-built crab distributor with points and a Bosch blue (internal resistor) coil. The plug wires are solid core.

Am on my second tach - currently an Equus Performance and, prior to that, an el-cheapo (Trisco) - but neither one has been able to give me rational readings. I've tried the Equus in every possible configuration of inductive vs. hardwire hookup but still no joy. The reading keeps jumping all over the place. Sometimes it looks good but the next second can be two or three times what the expected rpm's should be.

Do you suppose the issue is that these new-tech tachs can't handle the 'noise' from the solid core plug wires (or the generator) ?

Thoughts?
 
macdonge said:
I'm trying, without much luck so far, to get a sensible tachometer reading on a '39 Merc 239 running on a test stand. It has a Bubba-built crab distributor with points and a Bosch blue (internal resistor) coil. The plug wires are solid core.

Am on my second tach - currently an Equus Performance and, prior to that, an el-cheapo (Trisco) - but neither one has been able to give me rational readings. I've tried the Equus in every possible configuration of inductive vs. hardwire hookup but still no joy. The reading keeps jumping all over the place. Sometimes it looks good but the next second can be two or three times what the expected rpm's should be.

Do you suppose the issue is that these new-tech tachs can't handle the 'noise' from the solid core plug wires (or the generator) ?

Thoughts?
Click to expand...

I am assuming that you are running 12 volts negative ground. When we developed our 6 volt positive ground tachometer driver for modern tachs, we had to put more circuitry in it to take care of the voltage spikes etc from the "traditional" ignition systems than we did to change the polarity and boost the voltage on both the logic and signal circuits.

I believe there are suppression units out there specifically designed for this purpose. I don't know if they work, though.
 
Thanks Tubman. Yes, it's a 12 volt system w/ negative ground. I'm not all that concerned about the issue, since I've enough experience to judge by ear approximately what rpm's it running at. Still, it would be nice to know a little more accurately than my 66 year old ears are telling me! That, and the fact that I spent some money on these 'modern' devices to tell me!
 
If you are only using it for test stand purposes you may want to consider using a hand held no contact laser tachometer.
 
I bought an Autometer tach that had your symptoms with a pertronix unit and spiral core wires. AM tech said it was RF interference and a shielded pick up wire would stop it. It didn't, so I put points back in. Still did it. Summit gave me my money back.

It's just the way it is these days.
 
Thanks 51504bat! I'll give the diode a shot and report back.

51 MERC-CT - I have a handheld unit but it's tricky to use as well. Besides that, it ties up your hands and prevents or slows down doing other stuff in parallel (like idle mixture & speed adjustments).
 
I have a 49 mercury with a bubba chevy dist and petronix and a blue bosch coil. I have a total of 3 new bosch coils . One of the coils makes the tach read nice and steady. The other 2 do not. I also ran a direct ground wire from the coil to the battery. I also run resistor plug wires.
 
I have an older Autometer tach. that I've used on many cars and is now on my '48 F-1. I have an older Mallory dual point distributor and a generator. All working fine together. Maybe I'm just lucky!
 
The little AutoMeter tach in my coupe started acting up a few months ago. Like it would read accurately above 1500 rpm, below that it was erratic and often sit at zero. Found nothing wrong with connections and decided to ignore it for the time being.

Then I had issues with missing at moderate to low rpm. The problem turned out to be two of the spiral wound spark plug wires had gone open circuit at their connectors to the crab distributor. When I fixed those connections, the missing cleared up, and surprisingly so did the tachometer problems. I have no theory as to why, other than the bad spark plug connections may have been causing very high voltage spikes out of the coil for those cylinders - perhaps enough to interfere with the tack pickup wire.
 
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