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MG TD TF 1500 - Tach Reduction Gearbox



MY tach (refurbed about 7 years ago) started acting jumpy lately. I would have to tap it to get it to indicate after starting. Also while running, it would wobble 50-150 RPM at idle, with no discernible change in engine speed. Today, it would fall to 200 RPM at times (with no change in engine speed) at idle, After a drive it quit completely.

We removed the tach reduction gearbox and coupling and my ace mechanic neighbor was pretty convinced after fiddling with it the tach GB was toast. BUT, one thing he did while it was off the car was to slide the brass collar back as far as possible and then tighten the clamp. He did say the clamp was loose. He had to leave, and when I reassembled everything back on the car, the tach worked like a champ. No tapping to get it started, and steady as a rock. Still working now.

So, is this a common occurrence? I mean, cold a loose clamp on the collar cause this? I have my doubts. Or, is just a matter of a short time (maybe on the next start) before the tach GB really IS toast? If so, where is the best place for a quality replacement? Moss, A/S, other?

I don't think the problem is on the generator end or the tach cable end, but I'm not "ejamicated" enough to know for sure.
L Karpman

Larry, first I would disconnect the cable behind the tachometer and make sure inner cable is turning with the engine idling. Maybe put a rag on the end of the cable and see if it stops under load. If it stops, I would check the connection between the gearbox and the generator. If that is OK then look at the connection between the gearbox on the cable side, both inner and outer.
It's gotta be somewhere because it can't be nowhere, as they say. Last on the list is your refurbished tachometer. Could be the inner cable is shot.
Jim Merz

Check your drive cable by unscrewing it from the gear box and attaching a Portable variable speed electric drill to the inner.
Ensure the drill is set to turn in the correct direction and have an assistant observe the tacho as you slowly increase the drill revs. Some drills have a stop on the switch so you can set the drill to run at a constant speed, this enables you to determine if the tacho is maintaining the same indication at constant revolutions of the cable.

Same process is a primitive way to check your speedo calibration if you have a hand held tacho to determine what revolutions the drill is turning at a constant revs setting.

New gearboxes are available from;

http://www.speedograph-richfield.com/html/minature_gearbox.html
G Evans

My tach was bouncing all over the dial. I tested the cable and tach with a drill (if memory serves the drill has to run in the unscrew direction). I drilled out the rivets in the housing, cleaned up the insides and the gears and added fresh white LI lubricant. Not being good with rivets I reassembled it with screws and thumb nuts. The tach has been stable now for two years. I think the issue was that the gears were misaligned and were skipping. Jud

J K Chapin

Installed. Jud

J K Chapin

Thanks for the advice folks. Right now the tach is working fine, but I'll investigate the cable from both ends. Still a suspect too was the connection to the generator by the coupling and collar, which is now tighter. Is there any way though to tell if it is the tach gearbox at fault with any certainty without drilling out the rivets and looking at the gears? Probably not, but thought I'd ask.

L Karpman

This thread was discussed on 31/01/2015

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