Does anybody know the relationship between the hertz from the tach generator and the engine rpm? For example if the output of the tach generator is 100 hertz what is the engine rpm?
I have a Series 2 that does not have the tach generator but I think that what drives your dashboard tach gauge is the voltage coming from the generator and voltage would be proportional to rpm of the generator.
Dennis
Paging @daverawle, please pick up any courtesy phone…
According to what I have from Dave’s tach mods, 100Hz=3000 Crank rpm.
The S1 Tach generator should output 1V/100rpm so at 5,000rpm you should see 50V on the outputs. Unfortunately the magnets in the generator become weak with age which reduces the output voltage significantly which results in the rev counter under reading by as much as 30%. The repro tach generators are not calibrated so they are pretty useless. they also seem to fall apart with use. Best solution is to use a microprocessor board to use frequency rather than amplitude. The Hz to rpm calculation will depend on the board calibration but as Andrew says it will likely be 3,000 rpm at 100Hz, assuming 2 pulses per crank revolution…
None are available at the moment although I have asked Marek if he would consider producing some. If you are able to make a circuit board this schematic designed by Dave Rawle will work:
Full details on installation and calibration can be found here: http://forum.etypeuk.com/viewtopic.php?f=9&t=292
David
Dave,
That is exactly the information I need. Thanks so much. Now I can see if my problem is with the generator or the tach. The tach is reading low.
Thanks again,
Pete
David,
I guess I need to clarify a bit. I have changed my generator to an alternator and -ve ground. The tach generator I was referring to runs off the end of the cam shaft. Today my mechanic measured 30v at the tach generator off the end of the cam shaft and 600rpm at idle on the tach gauge. This seems a bit low as my mechanic says he thinks the revs should be about 900 for the speed of the engine at current idle.
Pete
Correction it was 30hz from the tach generator on end of the camshaft and 600rpm on the tach gauge on the instrument panel.
Firstly we need to decide if you are measuring volts or Hertz. You can do both from the generator but let us stick with volts as that is what Jaguar intended. At the end of the day the tachometer is just a voltmeter and measures the volts output by the tach generator at various rpm’s. As designed it outputs 1volt for every 100rpm of the camshaft. To test it put a voltmeter on the tach generator outputs, start the car and hold the revs at a steady 2,000 rpm. The voltmeter should register 20 volts. If it is showing 15 volts you know your tach is 25% down on actual.
From what you are saying 30hz should equal 1,500 rpm not 600rpm so your tach is under reading by a high margin.
David
David,
I believe you are correct about the 1V per 100 RPM at the camshaft.
In fact, this should translate to 25V for 5000 crankshaft RPM.
I’ve verified that with a 24VAC transformer that brought the needle to 4800 RPM on the tachometer.
Best regards,
John G.
Good info. But can someone clarify whether voltage from the tach generator is AC or DC? I suspect my tach reads low and would like to test the voltage as described by David.
Thanks
Bazjag
The tach generator produces AC. A voltmeter set to AC will register the correct output voltage. The generator produces a sine wave for each revolution of the crank, going positive and then negative in layman’s terms. The upgrade board works on frequency rather than amplitude so ignores voltage and just counts the pulses, two per revolution, a far more accurate solution.
I know Ray L made his own frequency board and Dave Rawle made a batch of them a few years back. I have one in my car and it is highly accurate once calibrated via a PC. I am surprised no one else has produced them as they are pretty much essential if you are a spirited driver.
David
I have confirmed that my S1 tach reads low, and that the tach generator is the issue. Trying to decide which path to follow:
- Send tach out to be converted to run off the distributor by on to the rebuilders
- Send tach and tach generator out to be rebuilt to original by one of the rebuilders.
- Buy a new tach generator and see if I get a good one- many seem to say I may not.
- Keep everything as is and just mentally adjust the numbers.- cheap and easy!
- I read the information above to convert the system to use the original tach generator, but add a board in the tach to read hertz rather than voltage. I like this concept- is anyone building these boards and if so at what cost? I may attempt myself, but would like to see what is available.
Number 5 is my specific question for the group.
Tom
so, I guess from this discussion, rebuilding the tach get with fresh magnets is not done ?
There is a conversion kit available on Ebay UK: Smiths RVI - RVC Tachometer conversion board + calibration cable -566- | eBay
I believe the one you are referring to is for the S2 and/or S3 tachometer.
Tom
Actually, that is what I would like to do. I posted a thread sometime ago inquiring for some help to do so, got some thoughts, but my attempts were unsuccessful. I do know some of the vendors will rebuild them, which I believe can involve remagnetizing them. But then for the cost of that would I be better off to get the conversion? That is part of the decision I am working on.
Tom