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MG MGB Technical - Most peculiar


Just replaced the guts of the tacho with a modern IC circuit. Prior to this it wandered about when doing 3500 to 4000 RPM. Strangely enough the speedo wandered at that road speed but never worried about it too much.

I had the tacho out of the dash to adjust the reading with it's new cct and at 4000 it was rock steady, but speedo was jumping from 0 to 10 MPH, in neutral, wheels on ground!

Put tacho back into dash and it jumps around at 3500 to 4000 along with the speedo. Take tacho out and it's steady but speedo still jumps.

Not really expecting anyone to know whats wrong, but it sure is a mystery.

Tony

Tony Oliver

Tony, I agree it's strange. Don't have any suggestions. Are you using points or electronic ignition?

Clifton
Clifton Gordon

Hi

I had a similar problem last summer when I added a new electronic tach and speedometer. With thanks to FR Millmore and Paul Hunt I was able to solve my problem with relays and a complete go through of my wiring system

Both threads are rather lengthy but as you can see I tried pretty well everything there was to try.

I hope it helps

Bruce

http://www2.mgcars.org.uk/cgi-bin/gen5?runprog=mgbbs&access=&mode=archiveth&subject=71&subjectar=71&thread=2007050620412729669

http://www2.mgcars.org.uk/cgi-bin/gen5?runprog=mgbbs&access=20923651500099&mode=archiveth&subject=71&subjectar=71&thread=2007080704230512769
Bruce Mills

With a mechanical speedo there sure be absolutely no connection with whatever the electronic tach is doing, and no movement when the wheels are on the ground and not turning, of course. The only thing I can think of as regards the speedo is vibration, but surely that would be noticeable otherwise. You could remove the cable from the back of the speedo to be absolutely sure it can't be the cable causing it.

With the tach being different out of the dash to in the dash, if it can be regularly reproduced, could be voltage/grounding problems similar to Bruce's. But it could also be jumping in sympathy with the speedo, if there *is* vibration, as it would only be subjected to it when in the dash. The usual cause of tach jumping is ignition LT problems, of course.
Paul Hunt 2

Thanks gents. The car is a '67 model and standard in every way except for the alternator I fitted.

I think it has to be vibration, but sitting in the car with the engine going at 4000 rpm vibration is "everywhere". The tacho behaves (or not) every time it is removed or fitted. I thought of earth problems so earthed the case with a piece of cable.

Bruce, I'll read the threads you listed to see if they shed any light on the problem, but it seems that I am going to either put up with it or try and damp the vibrations in the dash area.

Tony
Tony Oliver

Tony-
What you describe indicates a resonant vibration in the car, which causes the needles to bounce around 4000. Start by checking for ANY mechanical contact betwixt engine and body; it may not be in contact at other than 4000, but will be very close. If the mounts all appear to be in order, and nothing is in contact, I would get worried about an excessive out-of-balance condition in the engine, like a loose flywheel/clutch or something coming apart. This is not normal.

FRM
FR Millmore

Thanks FR. The car performs well. New engine and g'box mounts fitted recently. I will check to see if there is any unwanted connections between engine and body.

Just finished a 200KM trip on various road surfaces with no problems. In the garage it ticks over and revs OK with no unusual vibrations but as you suggest it must be some sort of resonance. I can't feel any harsh vibrations but it is hard to tell with all the noise and vibrations of a healthy B series engine in full song. The motor is certainly not shaking around.

I am going to try damping the area around the instruments with chunks of foam and other insulation type materials. We'll see what happens.

Tony
Tony Oliver

This thread was discussed between 02/01/2008 and 04/01/2008

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