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Tachometer does not register correctly
- Thread starter sjgcpa
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Inside the tach there is a screw that adjusts the tach. Some have taken the tach apart and drilled about a 1/2 inch hole in the back of the case to get at the adjustment screw when the case in intact. Then hook the connections back up and adjustments can be made. This is difficult to do if the tach is mounted in the dash, however.
Question. Are you using a Pertronix? If so, many of us have had difficulties with the tach operating correctly.
Question. Are you using a Pertronix? If so, many of us have had difficulties with the tach operating correctly.
AndrewMawson
Jedi Trainee
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The tachometer relies on the current through the ignition coil passing through a couple of turns on a transformer primary formed by a gismo on the back of the tacho. This inductively couples to the secondary which pulses the electronics.
If you have an electronic replacement for the distributor points such as a Pertronix, the likelyhood is that it runs at a much lower current and hence is insuffiicent to trigger the electronics.
So I expect it NOT to be the source of your problem as you are reporting too high a reading, rather than too low or none at all.
The Tacho works as an integrator - the pulse is normalised and fed into a capacitor network to form an analogue current proportional to the rpm. The value of this current, and hence the calibration, can be tweaked by the pot mentioned in a previous posting.
If you have an electronic replacement for the distributor points such as a Pertronix, the likelyhood is that it runs at a much lower current and hence is insuffiicent to trigger the electronics.
So I expect it NOT to be the source of your problem as you are reporting too high a reading, rather than too low or none at all.
The Tacho works as an integrator - the pulse is normalised and fed into a capacitor network to form an analogue current proportional to the rpm. The value of this current, and hence the calibration, can be tweaked by the pot mentioned in a previous posting.
Dave Richards
Jedi Knight
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sjgcpa said:
I have very similar readings in a bone stock '66 BJ8, I'm going to try the method described above.
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What does it read ay idle? Do you have a dwell & tach instrument to check it against? Make sure the wire is looped correctly on the backside of the tach. Did you happen to change to a much higher performance coil? That will do it too.
There's a way to calibrate the tach with a standard 60htz power cord. I read the article I think about 25 plus years ago in the Pacific Center Healey Club mag. Maybe someone knows this proceedure and can respond to you??
There's a way to calibrate the tach with a standard 60htz power cord. I read the article I think about 25 plus years ago in the Pacific Center Healey Club mag. Maybe someone knows this proceedure and can respond to you??
Ed_K
Jedi Knight
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You will need a reference tachometer, like comes as a part of a car test tool. Check your local car parts store for what is available or search on the web.
... My tach was reading high all the time and it was caused by the old worn out capacitor. I read a tip about this on one of the Healey sites, went to radio shack and bought and installed one, then I could adjust the tach with the pot as described above. If the capacitors are shot in your tach, turning the adjustment pot will not fix the problem. I think these tach problems were discussed on this forum. Try the search feature of the forum. Forum member Doug Lawson is an expert on tachs.
Ed
... My tach was reading high all the time and it was caused by the old worn out capacitor. I read a tip about this on one of the Healey sites, went to radio shack and bought and installed one, then I could adjust the tach with the pot as described above. If the capacitors are shot in your tach, turning the adjustment pot will not fix the problem. I think these tach problems were discussed on this forum. Try the search feature of the forum. Forum member Doug Lawson is an expert on tachs.
Ed
Ed_K
Jedi Knight
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I have never heard of using an AC power cord but that is an excellent idea.
3600 pulses per minute.
Lets see, a 6 cylinder should pulse 6 times in two revolutions or 3 pulses per revolution.
3600/3 = 1200
So if you loop the power cord for your shop light through the metal lug on the back of the tach and turn on the light,
that should be the equivalent to 1200 RPMs.
I just figured this out on the fly so if I am wrong, please jump in Dave, Keoke, or Doug Lawson.
Thanks,
Ed
...Any country that is using 50 hz power would mean that it would be a 1000 RPM reference, instead of a 1200 RPM.
3600 pulses per minute.
Lets see, a 6 cylinder should pulse 6 times in two revolutions or 3 pulses per revolution.
3600/3 = 1200
So if you loop the power cord for your shop light through the metal lug on the back of the tach and turn on the light,
that should be the equivalent to 1200 RPMs.
I just figured this out on the fly so if I am wrong, please jump in Dave, Keoke, or Doug Lawson.
Thanks,
Ed
...Any country that is using 50 hz power would mean that it would be a 1000 RPM reference, instead of a 1200 RPM.
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Hi Ed,
It might work. If - the light bulb draws enough current and - IF - only one of the cord conductors is passed through the pickup. Otherwise, with two wires, the going & the coming currents external fields will cancel.
D
It might work. If - the light bulb draws enough current and - IF - only one of the cord conductors is passed through the pickup. Otherwise, with two wires, the going & the coming currents external fields will cancel.
D
dklawson
Yoda
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I personally don't work on the RVI tachs. I have a few friends who use them and one of them used a wall-wart (AC electronics wall adapter) and a single diode to produce a 1/2 sine wave signal for calibrating the RVI tachs. I'm sorry, since I don't work on them I didn't write down the procedure. I'll email my friend and post back when I have his reply.
dklawson
Yoda
Offline
I wrote my friend about his AC calibration method. He did not record exactly how he bench calibrated his tach.
I'm not comfortable enough with his math or his memory of the method to post it here. However, a member of this forum (MG or Spridget list) posted a similar method within the past year. I'm going to see if I can find it and post a link here.
EDIT: I found the two threads I was remembering. Neither apply. Both posts involved people using a signal generator, not 60Hz power for the calibration signal. I'll do some more digging and post back later.
I'm not comfortable enough with his math or his memory of the method to post it here. However, a member of this forum (MG or Spridget list) posted a similar method within the past year. I'm going to see if I can find it and post a link here.
EDIT: I found the two threads I was remembering. Neither apply. Both posts involved people using a signal generator, not 60Hz power for the calibration signal. I'll do some more digging and post back later.
dklawson
Yoda
Offline
I can't leave things like this alone. Fortunately, our computer system crashed at work this a.m. which gave me the opportunity to google a bit and ask some questions.
My friend's method makes more sense to me now but it may not be good enough for a 6-cylinder tach calibration. My friend took a 12-15 VAC wallwart and cut off its output plug. He attached a signal diode and a 3 ohm resistor in series with one of the output wires. He then connected the resistor and the other wallwart wire to the induction loop on the back of the RVI tach. He plugged in the wallwart and this gave him 60 Hz x 60 seconds/minute = 3600 pulses per minute. The diode cut off the "negative" portion of the sine wave so this gave him a 50% on/off duty cycle for the pulses. His 4-cylinder tach was thus receiving the equivalent pulses to 1800 RPM.
Unfortunately, for a 6-cylinder engine this isn't really good enough. The 3600 pulses per minute from the wallwart would equate to only 1200 RPM. I guess that's OK if you only let your engine idle.
A co-worker of mine had a suggestion for this but I haven't tested it nor do I have an RVI tach to test it on. He suggested putting a full-wave bridge rectifier across the wallwart output instead of the diode and 3 ohm resistor. Instead of cutting off the "negative" sine wave pulses this would invert them, basically doubling the pulse stream to 7200 pulses per minute. If this works it would allow a 6-cylinder RVI tach to display 2400 RPM. I'm not sure about this though as the "off" time between these pulses would not be very long at all. The tach may not respond. I expect some additional signal processing would be in order. I was thinking that either a Schmidt Trigger or an opto-isolator would be needed so the on/off threshold could be adjusted. Regardless of which component is used, it would probably have to switch a transistor on and off to supply sufficient current to the tachometer's induction coil.
Dave, did you say you were an EE? What do you think?
My friend's method makes more sense to me now but it may not be good enough for a 6-cylinder tach calibration. My friend took a 12-15 VAC wallwart and cut off its output plug. He attached a signal diode and a 3 ohm resistor in series with one of the output wires. He then connected the resistor and the other wallwart wire to the induction loop on the back of the RVI tach. He plugged in the wallwart and this gave him 60 Hz x 60 seconds/minute = 3600 pulses per minute. The diode cut off the "negative" portion of the sine wave so this gave him a 50% on/off duty cycle for the pulses. His 4-cylinder tach was thus receiving the equivalent pulses to 1800 RPM.
Unfortunately, for a 6-cylinder engine this isn't really good enough. The 3600 pulses per minute from the wallwart would equate to only 1200 RPM. I guess that's OK if you only let your engine idle.
A co-worker of mine had a suggestion for this but I haven't tested it nor do I have an RVI tach to test it on. He suggested putting a full-wave bridge rectifier across the wallwart output instead of the diode and 3 ohm resistor. Instead of cutting off the "negative" sine wave pulses this would invert them, basically doubling the pulse stream to 7200 pulses per minute. If this works it would allow a 6-cylinder RVI tach to display 2400 RPM. I'm not sure about this though as the "off" time between these pulses would not be very long at all. The tach may not respond. I expect some additional signal processing would be in order. I was thinking that either a Schmidt Trigger or an opto-isolator would be needed so the on/off threshold could be adjusted. Regardless of which component is used, it would probably have to switch a transistor on and off to supply sufficient current to the tachometer's induction coil.
Dave, did you say you were an EE? What do you think?
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Hi Doug,
I honestly don't know.
The particular tach design was never noted for accuracy or linearity. ie - Probably best to calibrate it at mid scale & accept the errors at high & low rpm.
Calibration methods range from comparing to another tach, bypassing the tach input transformer & applying a square wave generator directly to the input circuit, to an elaborate computer driven test circuit.
I suspect that the less adventurous should settle for comparing to a test tach. I have repaired a couple of these "nightmares" & used this method. As I said, the tachs are not very accurate at best.
Here are a couple of web sites that give some clues.
https://www.classictiger.com/techtips/motach.html
https://www.lotus-cortina.com/electric/convert.htm
It's possible that one of the good instrument repair shops would have calibrating equipment & could do the job.
Makes me glad that my car has an old fashioned mechanical tach which responds to conventional adjustment methods.
D
I honestly don't know.
The particular tach design was never noted for accuracy or linearity. ie - Probably best to calibrate it at mid scale & accept the errors at high & low rpm.
Calibration methods range from comparing to another tach, bypassing the tach input transformer & applying a square wave generator directly to the input circuit, to an elaborate computer driven test circuit.
I suspect that the less adventurous should settle for comparing to a test tach. I have repaired a couple of these "nightmares" & used this method. As I said, the tachs are not very accurate at best.
Here are a couple of web sites that give some clues.
https://www.classictiger.com/techtips/motach.html
https://www.lotus-cortina.com/electric/convert.htm
It's possible that one of the good instrument repair shops would have calibrating equipment & could do the job.
Makes me glad that my car has an old fashioned mechanical tach which responds to conventional adjustment methods.
D