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Intermittent operation of tachometer

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SteveHall64Healey

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My tachometer is working intermittently. It always works at startup, including the first few miles of fully warmed-up driving, but jumps between zero and the current engine speed periodically as I drive. The engine speed indicated when it is working is consistent with throttle position, i.e. when it works it seems to be correct.

Here’s what I’ve done to diagnose. Before I commit to the expense of having the gauge repaired, I would welcome any fresh ideas on the problem:

- examined all wiring under the hood for any loose or damaged electrical connections: all good and all other electrics working normally.
- tested resistance of wire from coil to tachometer and the resistance of that wire to ground with tach disconnected: zero ohms resistance on the length of wire running from coil to tach and infinite resistance from that wire to ground.
- tested resistance between screw posts on tach and ground: zero ohms.

thanks
Steve
 
This is a stretch because I'm working from (always suspect) and vague memory, but I seem to recall that there is a capacitor in the tach that is known to go intermittent or fail outright (capacitors do break down over time). I don't know to test a cap in situ, and de/resoldering components might really mess things up (and up the repair bill). Unless you've done something recently, like switching to electronic ignition or changing the inductive pickup somehow it sounds like you've checked the obvious things (the next step would be to open the case and start checking resistors, etc.).

The tach in my BJ8 will bounce around occasionally when gunning the engine not under load, and I think this started after I installed a Pertronix Ignitor but, again, memory is suspect.
 
My old Petronix ignition failed over the winter and I replaced it with exactly the same type. Prior to replacement, I did get some bouncing just as you’ve described.

The car is positive ground and the tach was serviced by Nisonger two years ago as a positive ground tach.
 
I'm curious as to what 'servicing' can be done to an electronic tach. I've not seen a schematic--anybody have one?--but I believe they're driven by a motor/solenoid of sorts, with circuitry to smooth and amplify the pulses coming from the coil primary. I do know there's a pot (potentiometer) than can be tweaked to calibrate the tach, but you need to drill into the case unless you want to open it up (I've done this). My best guess would be a failing 'motor' or capacitor (note points ignitions have a condenser, which is a capacitor, and they're replaced routinely). I wonder if there's a doc, like Anthony Rhodes' treatise on speedometers, that could help (mechanical tachs are basically speedometers without odometer and tripmeter gearing)?

I'd be tempted to at least open up the case and check any resistors, capacitors and the motor, but I'd want a schematic. There may be a power transistor to drive the motor.
 
I think you’re right about opening up the case. If any components are visibly fried, including the wire loop pickup, then I probably have my answer. The wire from the coil carries a series of voltage pulses, which carry a magnetic field and I believe generates a voltage in the pickup, which is then smoothed and amplified and then displayed. It would be ideal if there is an article similar to the excellent one on speedos that you reference.
 
Unfortunately, internet searches don't seem to be of much help; everybody only wants to talk about digital tachs. You should be able to check the resistance of, er, resistors, and I think you can at least check for a shorted capacitor. Electrolytic capacitors, in particular, break down and leak over time (be sure to keep the polarity straight, lest you induce more problems). If you have a metered power supply, you might be able to apply low voltage to the 'motor'--assuming there is one--to see if it appears to work. While you have the case off, you might want to drill a hole over the pot so you can calibrate later.

EDIT: Found this: https://accutach.com/smiths-tachs

EDIT2: It's the thermistor that I've heard goes bad over time (not a cap or resistor).


 
Hi All,

Steve, the leaking capacitor can be easily replaced by a .22uf 25v capacitor from ebay for under $2.00 US and the tach will need to be calibrated. There are a few articles available at www.healey6.com under the Technolog section under Gauge. Although your issue could be from this failing unit, the normal symptom is a high reading and not a bouncing needle. I would check the line connection and its coil that the tack senses before taking the tack apart.

Good luck,
Ray(64BJ8P1)
 
Steve,

Yes, I did mean the loop but for some reason had a brain freeze and couldn't think of the word...getting old.

How many loops are you running through the tach sensor? I have found that changing the number of loops can change signal strength to the tachometer. My Crane Optical ignition was too sensitive for 2 loops and, after changing to 1, all performed good. If you are running 1 loop through the tach, I would try 2, and vis versa.

Also, have you changed polarity? This would require changing connections in the tach and reversing the loop's coiled direction. If your change was prior to the Petronix failure and working correctly back then, I would only give the change a fast check (which you may have already done).

Ray(64BJ8P1)​
 
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