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MG Midget and Sprite Technical - Erratic tacho

When I bought the midget it had been fitted with a freshly reconditioned rev counter. Before long it stopped working but could be made to work by tweaking the connections.

I returned it to the man who had fetteld it and he confirmed that there was a faulty internal connection which he repaired without charge.

It then worked perfectly for a while but now it seems to get fed up after,say,half an hour and will not indicate over 4500 rpm. However the next time I take the car out all seems to be well.

I haven't repeated this procedure enough times to know if the same thing always happens, because some of my trips are not long enough to cause the fault.

Any ideas?
Roger D

Perhaps a little spray of GT85 around the needle shaft.
Nigel Atkins

Roger

It might be a component "drifting" in value as the circuit warms up. The circuit contains a "thermistor" which is, itself, a heat compensating device and may be faulty. Impossible to be certain without internal inspection.
Bill Bretherton

Thanks Bill,

How would I recognise a thermistor and would I be able to check it and replace it if required?

When I first had problems with it I did remove the case to examine the "works" to see if there was a poor connection so I know how to get into it.
Roger D

This schematic might help. It's old so not sure if it applies to your tacho. C2 is the usual culprit but the thermister referred to above is in parallel with R2.
Rob


MG Moneypit

Thanks very much for that but I'm afraid it's a bit above my pay grade. I don't think I could translate that into hardware!
Roger D

Roger
Could you get in touch with the person who reconditioned the tach and find out if/what components were replaced?
Bill Bretherton

Bill,
Yes I can do that, I thought I would try the expertise and knowledge of this forum first!
Roger D

Roger
I should have been clearer! It would be useful to know if any components have been changed so that they can be eliminated as the cause. Also, is it an RVI or RVC tach? - will be printed on dial face at bottom.
Bill Bretherton

Roger,
why not post up a good clear high resolution photo of the back of the face with all its bits show then the photo can be marked up with different coloured arrows and/or text and resubmitted.

And a second photo of the face will have the tacho/part number on it to save any confusion.
Nigel Atkins

I think I've sorted the problem, maybe.

I drove for approx 30 mins yesterday, parked up for about 30 mins and then started back on a 30 min drive home. During the drive home the tacho started playing games again, not reading more than 4500rpm. When I switched off on returning home the needle stuck at 1000rpm instead of returning to zero. I checked all the connections giving them a good tweak and on restarting everything was back to normal and the needle had gone back to zero.

It would seem therefore that the problem is mechanical rather than electrical in so far as it being a faulty connection rather than the failure of an electrical component.

Time will tell.
Roger D

Sounds like Nigel was right about the needle sticking.
Bill Bretherton

Hmm, I'm not quite sure about that. The needle returned to zero as soon as I poked about with the electrical connections and I have had the glass off and gently moved the needle around the entire scale and there didn't appear to be the least resistance or sign of sticking at any point.

I need a few more drives to confirm my prognosis. The weather here has been superb for the last week,dry,cold with blue skies but at this time of year the sun is so low that it has been blinding at times and even though the screen is clean, you realise when the sun is full on it, how pitted and degraded a near 50 year old screen can be.

Nonetheless, great driving conditions.
Roger D

Roger,

What Tacho do you have RV1 or RVC ? (its written on the gauge face)

I have a spare RV1 2340/01A if you are in need of a replacement.

R.
richard b

Richard,

Thanks very much for the offer. The numbers on the face of the gauge say RVI 2401/008 although I believe that those are not correct. I seem to remember Alan Davis of The Gauge-Shop telling me that the internals of my tacho had been replaced and it was no longer a RVI 2401/008 as shown as it had been modified for use with electronic ignition.

I'm not sure which is the correct instrument for a 1973 MkIII. Would your RVI 2340/01A be appropriate?
Roger D

Hi Brian,

Firstly I made a mistake in typing my numbers should be RV1 2430/01A (old age strikes again !) and according to Terry Horlers book this was fitted up to Dec '72 build - when at Car GAN5/128263 it changed to RVC-01AR.

The RVC tach's are usually considered better for use with Electronic ignition but can still be eratic at times and often a dedicated live supply, seperate to the ignition supply is suggested as a cure.

The RV1/2401 is listed as a Positive earth tach fitted until car GAN4 60459, but as you noted it has been modified so who knows - must have been converted to negative at that time I guess.

R.
richard b

I am still having the same problem with the tacho,that is after 20 mins or so it will not indicate over 4,000-4200 rpm.
I have tried contacting Alan Davis of The Gauge Shop by text and email but he doesn't reply. Bit disappointed with that really, it was his repair and although I realise that a diagnosis at a distance is difficult some kind of response would have been welcome.

I have found a firm here in France who specialise in repair and renovation of classic instruments so I think I will give them a try. I am a bit reluctant to send it back to the UK given the post Brexit bollocks.

When I remove the tacho how do I keep the car drivable, what needs to be connected to what?
Roger
Roger D

If it's and RVI type the 2 wires that form the sensing wires should be connected together to complete the ignition circuit.
Rob
MG Moneypit

Roger
It's possible that the thermistor, a temperature compensating component, has been changed or the existing one is faulty. They can cause the reading to change as the instrument heats up.
Bill Bretherton

The tach in the B became erratic, with the needle bouncing to 0 then over 5000 occasionally, then more often. The problem was the pertronix distributor. There was wear in the distributor shaft that was causing irregular firing due to variable clearance between the magnet and the pickup. The engine miss was not noticeable until the problem became chronic with the tach needle jumping from end to end frequently.

Installing the pickup and magnet in a fresh distributor solved the problem and lead to an improved running condition.
Glenn Mallory

Glenn,

Thanks for the input. The thing which makes me doubt that your problem applies to my tacho is that it does work perfectly for a while when the car is first started.

I'm beginning to think that it could be a purely mechanical problem. I went for a drive of about 45 mins a couple of days ago and the problem didn't manifest itself for about half an hour, previously it has happened after 10 or 15 mins. I just wonder if connections at the back of the instrument are working loose after a bit of bouncing about. A bit more research is needed.
Roger D

Just to throw my two penneth in.
My Rev counter was randomly playing up about 18 months ago. I did notice when the Rev counter stopped working so did the fuel gauge - obviously on the same circuit.
I replaced the Fuse box and "touch wood" not had problem since. Mine is a 1500/77 midget. Don't know if the wiring is the same as yours
Nigel Axtell

Thanks Nigel. I'll check that but nothing else fails when the tacho plays up.
Roger D

I've reactivated this thread because, although I still don't have a solution, I have another clue.

I mentioned that the problem seemed to occur after 25-30 minutes running which suggested to me that it could be temperature related. I have noticed in the last few days when the temperature has been in the mid to late thirties that the problem occurs after about five minutes, reinforcing my thoughts about temperature.

Any thoughts on this?
Roger D

I know next to nothing of electrickery but *think* it could very well be related to or exasperated by higher temperatures in the battery and the wires or their insulation or security of being held or flopping about. I don't know how much any weaken electronics (is that the right word?) would be affected by heat within the weather heat range.

If you have poor connections and the wires move with the heat that could make the connections worse and blow what they can operate with.

I try spraying small areas or connections one at a time with electrical contact cleaner and see if it makes any odds if it does possibly the last arrear sprayed (or combination of areas) are the ones to concentrate on.
Nigel Atkins

I have related this before, but I once had a cable fault which was very much heat related. There was a break in the wires inside the otherwise undamaged cable insulation. When cold, the wires made contact but as the temperature rose the plastic insulation became more flexible and the cable sagged under its own weight, pulling the copper wire strands apart. When the temperature dropped the insulation shrank and the wires reconnected so it repaired itself! It did take a bit of tracking down!

(It was the wire from coil to an A+ electronic dizzy so it caused the engine to cut out, but would restart after 20 mins of cooling down)
GuyW

>>... the heat that could make the connections worse and - below - what they can operate with.<<

I've had an intermittent fuse and brake light bulb, sometimes you have to expect the unexpected.

As with Guy's wire experience it's why the wiggle test is used on wire, and for there connectors and more for electronics but also other items heating up with a hairdryer (or heatgun for the macho) and freeze spray for cooling, careful use of both as you don't want to make things worse but damaging more or other items.

I would try a gentle bit of heating on the tacho but probably not freeze spray rather gentler cooling.
Nigel Atkins

This thread was discussed between 10/12/2021 and 22/07/2022

MG Midget and Sprite Technical index

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