MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG Midget and Sprite Technical - Tacho not working when warm

Hi all,

I have a strange problem with my tachometer. It works great when I start the car in the morning for something like 10 minutes. Once the engine get warm it starts to work in a slow mode, showing rpm with delay and after next few miles it stops working at all, showing last higher rpm, usually sits around 3.5k rpm without any move. I believe it has to do something with temperature, as during summer hot days it starts slowing down significantly faster than these colder days...

Any idea what is the root cause and how to fix it?

Thanks!
Stepan
Stepan Marek

Stepan

When does the tacho return to zero? Is it when you switch the engine off or does it take some time to return to zero?

If the former is the case I’d suspect the electronics if it's the later I'd suspect a mechanical problem, but this is not definitive.

If the problem is mechanical a small drop of very light oil (sewing machine oil) on the shaft of the spindle may be enough to free it up.

If the problem is electrical more investigation will be required.

Bob
R.A Davis

Bob,

it take some time to return back to zero, sometimes even hours, so it could lead to mechanical problem... I'll give it a try this weekend.

Shaft of the spindle - is it accessible from the back of the tacho? Or do I need to take the tacho out and dismantle?

Stepan
Stepan Marek

Stepan

You'll need to remove the tacho but it's very simple, just remove the wires and undo the thumb screws and it pulls straight out, sometimes it's easier to do the other way around if there sufficient slack in the cables.

To gain access to the shaft you'll need to open up the tacho. They're not difficult to open the only challenge you may have is removing the chrome bezel. To remove it you need to turn the bezel about an eighth of a turn until the locking tabs line up with the slots, sometimes they're a bit difficult to turn other times they're easy, you can try putting a rubber band around the bezel to get a better grip.

Once you've got the bezel off there are a couple of screws on the back of the tacho, remove these and you can lift the mechanism out.

Check the freedom of the movement before doing any work by moving the pointer carefully with your finger, if it feels in anyway stiff try a small drop of oil. I'm sure someone will come along and say you shouldn't put oil on these but I don't think a small drop of very light oil will cause you any problems. If you do oil it I would then suggest you exercise the movement by carefully moving the needle to full scale several times.

Bob
R.A Davis

Bob,

thanks. Will give it a try on Sunday.

Stepan
Stepan Marek

Stepan. What model is your car, is it a Frogeye or Mk2 with the mechanical tacho or a later car with the electronic tacho? I ask because I have a similar problem with my tacho. I have a Mk3 '65 Sprite that I converted to negative earth when I fitted an alternator, which involved also converting the tacho. It seems to work OK for about 15 minutes then starts to read a higher rev reading for a given speed, for instance, when cold 3500rpm= 70mph, but after about 15 minutes it's 3500rpm =60mph, so to get the speedo to read 70mph I have to increase revs to 4000.(Ford type 9 'box) I've recently had the speedo reconned and the problem existed before the recon so I'm sure it's not the speedo. Anyone got any ideas?
Thanks.

Bernie.
b higginson

Bernie,
sorry can't help with your tacho (coil perhaps? or igniter head?) but I can direct you to Stepan's 'View vehicle profile' as his is a 1973, 1275
Nigel Atkins

Thanks Nigel, I didn't notice that.

B.
b higginson

Bernie

A quick way to determine if it's the tacho or speedo would be to compare it to a satnav.

Bob
R.A Davis

Bernie,
no problem I often miss bits in posts, I find the 'View vehicle profile' very useful, if yours was up it would confirm if you have a 3.9 diff 1275 which would give about 20 mph per 1,000 revs (which you probably know)

don't know if you saw my thread on Type-9 gearbox oil
Nigel Atkins

This thread was discussed between 13/04/2012 and 16/04/2012

MG Midget and Sprite Technical index

This thread is from the archives. Join the live MG Midget and Sprite Technical BBS now