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MG MGA - Do I Have The Slowest Running MGA

I have the 1800 5-bearing, 5-speed with 3.9 diff.

Late last week I decided it was time to send the speedo off to be calibrated - currently under-reading by quite a bit.

Not wanting to lay the car up for the next couple of weeks I fitted the redundant mechanical tacho in the dashboard hole - and connected it up to the speedo cable!

Coming to work today I was amused to see that I was doing about 2500 revs per 'hour'!

Steve
Steve Gyles

Should we worry about you Steve???
Pete
Pete Tipping

No, clearly he's having fun, don't disturb him! But now that we know what he was doing, we should ask what the car was doing at the time.
FRM
FR Millmore

I am! 2500 revs per hour was about 30mph! Traffic was a bit slow this morning.

I tell you what, though, driving the car throughout the winter (hood down) hardened me up for the recent burst hot water tank I suffered in the house. 15 days without any central heating and hot water was made just bearable by a relatively warm drive to work each morning in the A! Unfortunately £1700 of my supercharger fund has now had to be diverted to a new stainless steel pressurised tank.

Steve
Steve Gyles

Steve, I wonder what Plod would say if he found out?- I sympathise with you - my 5 speed fund has just been eaten away by a caravan and a Rover clutch!!cheers Cam
Cam Cunningham

You need to get out more or counselling


Reverend Counter (call me Rev)

I can't believe that someone would use the C word in a respectable forum like this. Cam, you should be ashamed.

I loved the Top Gear slot where they used a catapult (the type used for shooting cars in special effects in movies) and played darts with it in a quarry, using old cars as the darts, and a caravan in the middle of the target....
dominic clancy

An interesting aside (for me) of my double RPM dashboard set-up at the moment is that it has highlighted a problem I did not know I had with my Speedo.

The mechanical RPM guage is reading absolutely rock steady, running off the speedo cable, whereas the speedo was oscillating plus/minus a few mph. I had assumed it was probably a jerky inner cable, but this has now shown it to be the instrument. I am contacting the instrument basher to check the speedo out.

What is the usual cause of this? Is it something I could have rectified on my workbench?

Steve
Steve Gyles

Dominic - in my defence - it is a folding caravan - so looks like a trailer when being towed - otherwise I would be hanging my head in shame ( or if I were really bold I would be out on the road seeing how many cars I could get queued up behind me ( which is the game I believe tractor drivers play) - cheers Cam
Cam Cunningham

Source of the oscillating speedo needle is the little ratchet pawl pushing the final odometer drive sprocket around. For the 1450 rev/mile unit the final sprocket has 58 teeth, so the oscillation happens 58 times per mile, of almost once per second at 60 mph.

When the first rotating wheel of the odometer gets a little gummed up the friction causes a heavier than normal load on the drive pawl. This periodic load in turn puts an end load on the input shaft from force being fed through the primary drive worm gear. Periodic end load then causes a pulsating end float of the input shaft. This modulates the space between the spinning magnet and the mating metal disc. That varies the magnetic attraction, in turn causing the needle to pulsate.

To fix this you first need to remove the mechanism from the case, which is a trick in itself. Then clean and lube the odometer drives (both of them) and the odometer wheels (all of them) with a small drop of light oil.

If there is excessive end float on the input shaft, this may be reduced using a single thin strand of copper from a stranded electrical wire. Place the thin copper wire next to the bearing journal plate on the input shaft, and wrap it around the shaft in the shape of a pinched horse shoe (all on one plane with ends NOT overlapping). This will take up the space, serving as a thrust washer to reduce the shaft end float.

Reassembly is the reverse of disassembly (but even more challenging). When you finish you will have a nice steady speedometer needle, and it's time for an ale.

This condition of gummed up odometer wheels may be aggrevated by long term storage and/or minimal driving. The cars like to be driven regularly to stay loose. If you store the car a lot, and ignore the wavy speedo needle, the internal friction and mechanical load may increase with time. This can result in shearing of a few of the teeth from the final drive sprocket, and then the odometer stops working. Shearing of the teeth is most likely to occur when turning multiple wheels, as when it turns up from xx999.9 to xx000.0 Professional repair will likely cost more than $100-USD, so it's a good idea to clean it up before it breaks.
Barney Gaylord

Cam
I thought you were getting too old for camping in a tent!..... At least now with your MGA towing a caravan you have an excuse for not exceeding 50mph!
Rob
R J Collier

Thanks Barney. Just spoken to my instrument basher. He says he checks all this out as part of his recalibration service.

Perhaps I will then be able to hold a steady 100mph instead of every other second!

Steve

PS. Beautiful sports car hood-down weather today. Heavy frost, not a cloud in the sky and not a breath of wind. Had a good inward smile watching all the other hood-up drivers trooping to work in their fogged-up cockpits.

Steve
Steve Gyles

It's probably the dent in the bumper that slows him down...
dominic clancy

Steve

We've got it now... Your "2500 revs per hour" was a mistake then; as later you referred to it as a "mechanical RPM gauge". Sorted!

Last week after lots of speedo cable changing and trying all sorts of different cable lubrication, I decided, as you found, it was the speedo that was causing the oscillations. (When doing approx 40mph I was getting a needle swing from 20 to 60!).

With Barney's assistance (thanks again Barney), I managed to get the thing apart. I found the main bearing itself really quite stiff. I lubricated this with a light oil, along with the worm gears and pawls after cleaning them up. It all freed up nicely. The actual odometer numbers all seemed quite free.

Back in the car next day for a short run and whoopee a pretty steady needle! Can't wait to get on a longer run and see what happens when things get properly wormed and loosened up.

Pete
Pete Tipping

Correction: ...warmed and loosened up.
Pete
Pete Tipping

Barry in Oz

Just got your email. This is the thread with the advice from Barney about correcting the bouncing speedo.

Happy winter in Melbourne, just beginning to warm up here.

Steve
Steve Gyles

Thanks Steve and Barney, I have sent email's to you. The procedure really works. I now have a rock steady speedo. I'm sure I have saved $$$ in speeding fines and saved the cost of a repair by the experts. I owe you both one! Though from Downunder it will probably be difficult to pay Barney!! Steve, I will bring you a bottle of Aussie red in Sept./Oct. Keep your "A" clean - I would like a drive along those country lanes!! Barry
BM Gannon

The speedometer is the least important gauge on the dashboard. It's only for looks. Put aything in there you want. Kilowatt hours, inches of mercury, pounds per square inch, artificial horizon - whatever.
David Breneman

This thread was discussed between 19/03/2007 and 04/04/2007

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