Erratic modern tach

[gjz30075]

Well, I sent my RVI tach ('72 Sprint) to Nisonger for conversion for modern electronic ignition. Reason being, the tach was rather bouncy with higher revs and has been for some time. While my tach is out for modification, I've since fitted a modern tach (Sun Super Tach II) that's found in any big box automotive store. It's simply three connections: green to (-) on the coil, red to switched 12v and black to ground. The new tach is now more erratic than ever most of the time. The times when it is not, the needle is steady and accurate. The symptoms are what is described in this tech tip, down by the question:
http://www.lotuselan.net/publish/bouncing_tach.shtml

Thinking I have a bad tach, I used another of the same brand that worked just fine on an American V8 car. I slid the little switch over to 4 cylinder, wired up per instructions (as mentioned above) and it, too, is erratic. So I try one more tach; one that is an antique Sun Super Tach, about 40 years old and I know worked on this car some time ago, when checking for accuracy of the OEM tach. Again, simple connections: red to switched 12v, green to (-) side of coil and black to ground.

This old tach is the most stable but still give erratic symptoms but not often and not 'wild'. More like the symptons in the tech tip link.

I'm running a Lucas Gold Sport coil, DLB 105, NO ballast, points, alternator from RD, electric fan, standard fuel pump.
The two white wires that go to the tach are now plugged together behind the dash. This thread: elan-f15/lucas-gold-sports-coil-t1689.html seems to point to the alternator.

Any ideas?

Greg Z

[gjz30075]

I'd like to take the alternator out of the equation but obviously still need to run the water pump. Can anyone tell me the length of belt needed to run between the pump and crank?
Thanks,
Greg Z

[memini55]

Greg,

You can certainly run the motor with out the belt for a short period to test the tach, even a short drive in cold weather.
Worth a shot.

Mark

[types26/36]

Can anyone tell me the length of belt needed to run between the pump and crank?
Greg Z

Greg,
I dont know the length but if you get a belt for a TC Europa that belt goes from crank to pump with out an alternator.

[gjz30075]

I wasn't certain how long of a drive was needed to determine success or failure. But I took the belt off as Mark suggested and it didn't take long: tach is still wacky.

Back to the drawing board......

Greg Z

[memini55]

Greg,

I would go back to the very basics with this, here are my thoughts. The tach works on a pulse and its frequency and it sounds like you have
some type of erratic pulse happening. I would look at a couple things in your electrics. The condenser could be faulty and easy to swap and while you are in the distributor a new set of points. Make sure you have a good solid ground path, if you have a good ohm meter measure the ground path which would be from the base plate of the points to the battery ground. If that is all good and still having a wacky tach I would swap the coil. Last one that I would try is to run a wire from the plus of the battery to the battery side of the coil and bypass all the inward
stuff attached. I would think one of these should find a solution for you.

Good luck and you will find the source I am sure.

Mark

[memini55]

Greg,

I thought I would toss out something for everyone also. The last couple I have worked on I did a nice conversion of the stock tach to a circuit that I found on line from Steve Mass. I am providing the web link for anyone that might have interest. Have done this to an Elan and to a Bugeye with good results. Going to build up another for the project S2 in the works.

www.nonlintec.com/sprite/Sprite_Electronic_Tach.pdf

Mark

[bob_rich]

Hi Greg

how old is the Car? does it have an early alternator with an external mechanical regulator box? to eliminate the alternator you could just pull out +12V main feed and field (control) wires so it is not connected at all.

Check the earth like the other posts suggest always a good move. Also you could try feeding just the tacho from a separate +12V battery just to see if that helps.

Is it possible the electronic ignition is produsing multiple pulses? in such a way that does not produce a missfire

Hope this helps best of luck

Bob

[prezoom]

Try taking a length of wire and grounding your tach (s?) directly back to the battery. If that doesn't work, you most likely do not have a grounding problem. If the tachs now work. you can now start chasing grounding problems.

Rob Walker
26-4889

[gjz30075]

Tach fixed! Ground from points base plate was perfect and changed points and condensor anyway. The real problem was the ground location that I used for the tachometer. I was using the bracket the wiper motor mounts to because there are other grounds there that run some lights but apparently not good enough for these tachs. I went straight to the chassis bolt behind the dash for the ground and all is well.

Thanks all for your suggestions.

Greg Z

[collins_dan]

Greg, I just pulled the tach from the S4 for the same reason, steady most of the time, but very bouncy when shifting at high revs. I have pertronix set-up which I power separately from fuse box per Rohan’s suggestion, so it works fine other than the bouncing on shift. I am not real good with electronics and soldering, so was planning to send to Nisonger for conversion. They have fixed speedometer, right angle drive and water temp for me, which have all been good experiences. That said, their reviews on here for tach conversions are not good. You must have yours back by now, how is it? I also read that this type of bouncing can be caused by a bad capacitor, but I wouldn’t know where to begin on replacing it. I could send to Nisonger for rebuild, not conversion. Curious to hear about your more recent experience, or that of others on the forum. Many of the bad reviews are older. Thanks, Dan

[gjz30075]

Dan, tach is still working nicely. It is currently a 'big box automotive' store tach, a SunPro, I think, and
its guts replaced the stock guts. I'm still running points and now a ballast with the correct coil for
a ballasted system.

To answer your question on rebuilds, I can't say the Nisonger converted tach works because I'm using
the SunPro. I've had good luck with my recent rebuilds of instruments using MoMa in New Mexico.

Hopefully others will chime in.

[elancoupe]

I had my tach converted by West Valley instruments in California . It came with wiring instructions, which were very simple . It works like a real tach now, no jumping around. It has worked with two different electronic ignitions, and needed no changes when I switched from positive to negative ground.

[collins_dan]

Thanks, Greg and Mike. I was just checking out some of the YouTube videos from http://www.spiyda.com/ which Phil mentioned. I am now wondering if my tach is actually broken. It reads very accurately and only bounces for a second or two after I shift. After looking at the videos, I am wondering if the problem is my expectations. Do tachs of this era all bounce a bit after a shift? My main point of reference is my modern mini. When I shift it, the tach drops a very little bit, then is almost immediately accurate, vs the second or two of bouncing around with the smith’s RVI tach. Just curious as I hate to fix things that aren’t broken. Thanks, Dan
PS. The spiyda conversion is very cool. My only worry would be how to calibrate it after the conversion.

[mbell]

I've done the spiyda conversion on mine.

I initially calibrated it using them audio from tablet method. Which got it done in the ball park. I've since re done it using a cheap generic tacho to use as reference.

For me when calibrating it i had to choose the rpm where I wanted it to be accurate. I couldn't get it to be accurate across the rev range it so had to choose the accurate point, I went for ~3k rpm. So it over reads at idle and under reads at higher revs.

I have mine connected on a low voltage signal from a pertronix rev limiter. Not sure if it might be more accurate on a high voltage signal direct from the coil.