Tacho reading high.

D

dvdocon

New Member
The tacho on my 1972 3500s has been reading high ever since I bought it, I remember being a bit worried that at 50-60mph, the tacho was showing around 4500rpm. This seemed odd, but it kept me from going too fast I guess! I knew from my previous "s" that this was not normal though. Any ideas as to why this is occurring? The needle is steady & there is no delay in giving a reading, nor does it burst into life before the engine is started.
 
dvdocon said:
The tacho on my 1972 3500s has been reading high ever since I bought it, I remember being a bit worried that at 50-60mph, the tacho was showing around 4500rpm. This seemed odd, but it kept me from going too fast I guess! I knew from my previous "s" that this was not normal though. Any ideas as to why this is occurring? The needle is steady & there is no delay in giving a reading, nor does it burst into life before the engine is started.
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You could first try re-calibrating the tacho. On my 1972 3500s I have an RVI type tacho, (which I am converting to RVC as it wont work properly with my electronic ignition), and there is a calibration hole in the back of the unit where, using a suitable screwdriver, you can turn the calibration control.

IMG_1608.jpg

The tacho type of RVI stamped on the face of the dial.

IMG_1626.jpg

The calibration hole is at the top right of the unit in this picture.

IMG_1629.jpg

Internals removed from tacho. The calibration control is the yellow component.

Using an alternate more accurate way of measuring the RPM, calibrate the tacho to match the accurate reading.

Make a note of where the control is set to before changing it, then move it in small steps and determine in which direction you should go. One direction should increase the reading, and the other decrease the reading.

You may have the alternate RVC type tacho on your car, and I would assume there would be a similar calibration control.
 
Thanks for that detailed reply Phil, I will check to see what type I have fitted. I too have electronic ignition, would this also affect the reading? if so, at least I know how to adjust it, I know that Ron (sydneyrover) has put up a list of what readings to expect at certain speeds, so I can use these to calibrate my one.
 
dvdocon said:
Thanks for that detailed reply Phil, I will check to see what type I have fitted. I too have electronic ignition, would this also affect the reading? if so, at least I know how to adjust it, I know that Ron (sydneyrover) has put up a list of what readings to expect at certain speeds, so I can use these to calibrate my one.
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It seems that some electronic ignition systems don't like RVI type tachos, so you may not be able to calibrate it in the way I described, since you have an electronic ignition fitted. A quick google search brings up plenty of examples but here is one:

RVI and Electronic Ignition

Some reports say the tacho either reads wrong, fluctuates or does not work at all :?
 
And of course my tacho is RVI ! it looks identical to your one. I checked it out on the way to work this morning & I'm getting a reading of approx 3100 rpm at 40 mph. I will check out the link & see what that says. My previous 3500s had Lumenition fitted, it was also a 1972 model, & it had no problems with its tacho. I did not fit it, so not sure if anything was done to make the tacho behave itself when it was installed.
 
Just to put one "way out there" possibility:-

Using Ron's (SydneyRoverP6B's) figures from here :-

SydneyRoverP6B said:
In 4th which has a ratio of 1:1, at 30mph...1250rpm, at 50mph...2083rpm, and at 70mph,...2917rpm
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60mph is midway between 50mph and 70mph, so an approximation of the rpm at 60mph would be the mean of 2083rpm and 2917rpm, which is a nice round 2500rpm [but do feel free to check my working out!]

Seeing your tacho reads ~4500rpm between 50-60mph (we'll assume the mid-point again, 55mph) - mid way between 2083rpm (50mph) and 2500rpm (60mph) is 2291.5rpm [again, do feel free to check my working out!]

2 X 2291.5rpm = 4583rpm - approximately what you see on your tacho.

Why 2 X (twice)? A V8 has 2 X (i.e. twice) the number of points/LT pulses for every rpm compared to a 4-pot, so I'm wondering if someone has fitted a 4-pot (2000TC/2200TC) tacho to your car?

One indication (provided they have not fitted the V8-tacho face!) would be to see 1) where the red sector starts, 2) the largest number on the face

4-pot: Red sector starts at 60 (6000rpm), the largest number on the face is 70 (7000rpm) :-




V8: Red sector starts at ~52 (5200rpm), the largest number on the face is 60 (6000rpm) :-
 

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dvdocon said:
Thanks for the idea Darth, just went outside to check, & unfortunately it is the V8 one fitted.
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Ah, OK... Darn it!!!!

I'm sure it is possible to fit a V8 tacho face to the 4-pot tacho... or even fitting the innards of a 4-pot tacho into the belly of a V8 tacho.

Just seems weird your tacho is reading roughly twice what it should :!: :shock:
 
It is weird & a bit annoying, especially when I want to give it some welly & the tacho goes off the scale :shock: I just have to go by the engine tone when deciding to change gear.
 
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