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MG MGB Technical - Speedometer woe

While out driving this morning, I noticed the speedo needle was oscillating. A few miles later, there was a noise as if a bat was being tortured behind the dashboard. A few miles after that, the noise stopped - and so did the odometer. The odometer actually ran for half a mile a while later after being parked for a few hours.

The speedo now stopped its oscillating too ... I thought it was OK until I realised I was the fastest car on the dual carriageway doing only 65mph.

Checking speeds with the satnav, a speedo reading of 21 gives a true 30mph, and 31mph gives a true 40mph.

My MGB is 1980 with overdrive, speedo is SN 5234-02 1000

Do I just need a new cable?
Which one?
How on earth do I get at the back of the speedo?
Geoff Everitt

Yes New cable needed. Moss part NO GSD315. When fitting remove inner and lightly grease it full length.

With difficulty. I find it a little easier with steering wheel removed but its still a pain getting ones hand to loosen the nut. If you have thin arms it is possible to gain access thro the cold air vents in the middle of the dash.. Jim
jim soutar

Thanks Jim!
Geoff Everitt

If the speedo is still working, then the cable is too, It's the odometer that is DOA, and the speed error probably is a result of the destruction that killed the odo. On late cars you just turn the instrument a bit and it comes out the front of the dash, where you can disconnect things with ease.

FRM
FR Millmore

Might also be worth checking the drive cog in the gear box.
Michael Beswick

Thanks Gents ... more things to bear in mind.
Geoff Everitt

Hi Geoff,

Taking the tacho out will give you a lot more access to the speedo. Good luck with solving your problem.

willyphixitt
W A Nixson

Sounds like the cable was wound up, which was why the odo moved half a mile when you were stopped i.e. it unwound again, as things cooled down and freed up. As such it could be the cable, or it could be the speedo head partially siezed until there was enough tension in the cable to move it again. My oscillating speedo needle was caused by a dry bearing in the input to the head, the oscillations were relatively slow, in time with the odometer movement which put that bit extra load on it. The cable binding near the speedo could do the same thing. Where the oscillations are more rapid than odo movement it is usually broken strands in the inner catching momentarily.

I've also accessed mine through the air vents, but not a plastic dash car. Once the speedo is out you can pull the inner out, which will allow you to check for broken strands, as well as measure the length. With it in and the speedo off you can watch the end with a rear wheel safely jacked off the ground engine running and in gear to see if it is cable jumping, if not it must be the speedo. If you do need a new cable and get the same length you could get away with feeding the new (oiled) inner through the old sheath, but it isn't that difficult to replace the whole thing, check you can undo the gearbox end first.
P Hunt

Paul -
"Sounds like the cable was wound up, which was why the odo moved half a mile when you were stopped"
That would seem to be something on the order of 500 turns of wound up! I think he meant that he drove again after being parked a while.

FRM
FR Millmore

Hello again

Thanks for your further thoughts.

I'm assuming at some point the odometer "worked" again for a short distance. It had stopped at 134.5 (trip) and 51712 (main). After restarting I noticed it was 135.0 and 51713.

I removed the central air vents and was then able to unscrew the knurled nuts which secured the speedo. I was only able to pull it out a couple of inches, not far enough to disconnect the cable and certainly not far enough to reconnect.

I'll put the car on the ramps to get at the other end of the cable, perhaps tomorrow, perhaps the day after.
Geoff Everitt

I've had the speedo out today.

Before that, I went out for a drive with my SatNav stuck to the speedo glass (so I couldn't see the speedo), a distance of about 20 miles.

After I'd removed the speedo, I noticed that the odometer had moved on to 51733 - 20 miles more than it was yesterday! I put the speedo back in the vain hope that it would work again, but no, speed doesn't start until I'm doing 10mph and odometer is not working.

New speedo required?
Geoff Everitt

Geoff -
If the speedo reads, then the odo should too, proportionate to the speedo reading, else you do need a speedo. If the speedo does not read, or if it reads and the odo matches, you have another problem. Busted angle drive, bad cable, or loose drive gear in the main gearbox.
The loose drive gear is a result of the 3rd motion shaft main nut being loose, and lets the gear slip; this gives slow or no readings, especially once it's hot - it is also another source of the dreaded "clonk". Remove driveshaft and tighten nut.
Bad speedos frequently take out the angle drive and cable if they seize up in failure, so check all.

FRM
FR Millmore

The odometer again appeared to work OK but the speedo was reading 7-10mph slow. I think I need a new speedo.
Geoff Everitt

"500 turns of wound up"

Ooops!
P Hunt

Hi Geoff,

Hopefully this later post may still be of some help to you.

I had weird speedo problems, like very slowly winding up to speed then staying there for a while when stopped.
Turned out to be insufficient end play in the pointer shaft.

Anyway I found an article on the web that was most helpful.

triumph.daveola.com/NOTES/Speedo_Repair.pdf

If you decide to try and fix it yourself there is one hint I can pass on and that is to remove the high beam light fitting before trying to extract the works from the case. It fouls the assembly when tilting it to get the trip reset out.

Herb
Herb Adler

Thanks Herb.

I removed it, sent it away and had it reconditioned.

When removing the speedo, be careful not to slightly dislodge the capillary to the oil pressure gauge. I returned home one day with a circular patch of oil on my jeans over my left shin. Oil had oozed from the fitting and dripped from the back of the dash onto my leg!
Geoff Everitt

I might need to stop reading this bulletin board. The day after I read this thread, I heard a noise like a tortured bat coming from under the dash of my '67 BGT. Never heard it before, so must assume it's a result of reading about it here. Drat, before I could return home, the speedometer quit working.

I have enough experience with the speedometer to be able to remove it while seated comfortably in the driver's seat, so I did that. (And yes, having thin hands helps, but knowing the layout is key.) As soon as I had the speedo out, I checked the cable. Pulled the center out, actually, and verified both ends were square and firmly connected. This left the speedometer and transmission connector to scrutinize, so I started with the speedo.

I had to take it down to component parts, and it turned out the main bearing was seized. After a struggle, I removed the shaft from the bearing. Found some galling on the shaft, removed it with 600-grit sandpaper, and reassembled everything after cleaning and lubricating. Now the speedo bench-tests fine, so I returned it to the dash. Tomorrow I will determine what remains broken -- probably the angle drive or pinion gear. -G.
Glenn G

Glenn-
You must disconnect the car's internet cable before restarting, else it get more ideas.

FRM
FR Millmore

This thread was discussed between 17/01/2010 and 12/02/2010

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