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Rev counter problems

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healeyboz

healeyboz

Jedi Knight
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So, I finally got a cable that will work properly. The cable that I got from VB was too long. When I cut it down, I cut it too short. I ordered another from Moss and it works perfectly. Here is the problem...... the tach reads way too high. I imagine that it is reading double of what it is supposed to. Has anyone had this issue? I am running a 1275 with the 948 generator and a 1/2 inch shorter belt. I can't imagine the belt being the issue,,, is it? I wonder if the rev adapter from the gen to the cable is off of something else?

THoughts?
 
Dunno?? The original reason that I put the shorter belt on was to compensate for a bracket that I stole from a Mini motor.
 
Belt length would have nothing to do with anything.

Can not even think why it would be counting turns in error. Unless you do not have the generator/tac gear box installed somehow. I wonder if there are more than one tac gearboxes out there think it should be a X4 thing, let me check. Yep is marked 4X1 Reduction.
 
My advice would be to "modernize" and go with an electric tach and forget the headaches the mechanical tachs provide!!
 
Headache, what headache, that mec tac is part of the charm.
 
Expensive little piece of unreliable charm if you ask me! Cute, but anachronistic!
 
Heh. Mine is orginal. The big problem is that they need to be taken apart every decade and cleaned and lubed well and given a spot of oil with each oil change through the screw in the top.

When that is not done yep they are unreliable.
 
I have just seen and heard of too many that stripped out gears etc. and wound up not working or working erratically. They are not made of the best metals! Then you have problems with the cable binding etc.! I just like the reliability/accuracy of an elctric tach! It is after all a diagnostic tool, not a decoration!
 
The mechanical tachs can be pretty far off, simply because the pointer slips on the pivot. It's just a press fit. Being off by a factor of two seems like a lot, but I guess it's not impossible. Also, the pointer is delicately balanced--there is a balance weight in the circular, center part of the pointer. Be sure this hasn't been removed somewhere, or it will really behave strangely.

All this is why I built my own electronic tach, using the housing, pointer, and face plate of the original. There's a link to this at the bottom of my sprite page (URL below)
 
But if the pointer slipped on its shaft, wouldn't it be way of when not running too?
 
Sarastro, I would love to go with an alternator upgrade and still keep the original tach. The problem is, yes, I will admit, you write-up on the gauge conversion is way beyond my capabilities! /bcforum/images/%%GRAEMLIN_URL%%/smile.gif
 
Jack,
Where does it say 4:1? It will be hard to see without taking back out, but I guess it needs to come back out anyhow. It is wedged up against the dizzy.
 
It is cast in on one side. Just happened to have mine out so I could get to the oil pressure popoff.
 
I personally fought this problem for yrs, while going through three little trannys. Anyway, went with an electronic tach and couldn't be happier.
 
<div class="ubbcode-block"><div class="ubbcode-header">Quote:] But if the pointer slipped on its shaft, wouldn't it be way of when not running too?[/QUOTE]

Not necessarily. As long as there is still enough spring pressure at zero to keep it on the stop (that little wire it rests against at zero), it will stay at zero.
 
There is no reason you could not take the original face and use it on an electric tach! There are threads explaining the conversion if you do a search!
 
You have several options.

Start by checking the pulley diameters as suggested on the first page of this string. The belt length changing indicates that the pulleys are not the right diameter. If you have a larger crank pulley than the generator was sized for... it will run faster... and so will the tach. BTW, this isn't a good idea for generators anyway. I was driving an early MGB one day when its riveted generator pulley split in two and the half off the front put a lovely ding in the bonnet. Generators weren't built to run "too fast".

You CAN recalibrate this, but it will be difficult. It's easier to calibrate the tach to run faster than to slow it down. Google for Anthony Rhodes PDF document on Repairing Jaeger & Smiths Speedometers. The speedometer is just a mechanical tach with an odometer added. The Rhodes document has some advice on calibration methods.

Lastly, you could indeed convert the mechanical tach to be electronic. I've done this on a couple of tachs. Its usually a bit of a compromise regarding the accuracy but properly done you'll keep the look and after bench calibration you'll know where your tach is accurate and where it has errors.
 
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