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MG MGB Technical - Tachometer trouble

Hi all
I’m a new member here (about 10mins ago)
I’ve rebuilt my 68 B
All ok except for the tacho which is fine up to 2500 revs, over that the needle just bounces around
Any thoughts
Cheers Pete
PJ Thompson

Try a new set of ignition points, and if that doesn't fix it maybe a condenser
William Revit

Hi William
Thanks for your reply but should have said I’ve fitted a 123 electronic distributor
Regards Pete
PJ Thompson

I helped a friend fit a 123 to his 68 BGT. He had previously converted it to Luminition magnetronic ignition without doing anything to the tacho. The tacho worked fine in both cases. We are pretty certain is still the original RVI type. What program have you selected ? We went for number 2 as being the closest to the original advance curve.
Paul Hollingworth

Where are you picking up the 12v supply for the 123? If it's the coil +ve then try moving it to another ignition switched source such as the white at the fusebox.

The coil +ve supply comes via the tach with the RVI tach prior to 1973, and normally the only current pulses in that circuit are as the points open and close. But with electronic ignition if you connect its 12v supply to the same place you can get additional current signals through the tach which can cause problems.
paulh4

Hi Paul Hollingsworth I think I set the curve on no2 not 100 % sure tho, would need to remove to look.

Paulh I’m gonna go and try that this afternoon
The red and black coming from the 123 go to + & - on the coil
So you’re saying feed the red from another switched source such as white wires on the fuse box

Thanks both for your help
Ill let you know how I get on
Regards Pete
PJ Thompson

That's it.
paulh4

Worth putting your hand up the back of the dash and wiggling the unit and wires. I know the tacho on my B which still has points occasionally plays up especially when not been used for a while and this seems to fix it.
Paul Hollingworth

Hi Paul
Just been in the garage and removed the red wire from the +on the coil and linked it to the white on the fuse box, the needle on the tacho is a lot smoother with a little flutter between 3000 and 4000 revs

BUT did notice I have the wrong gauge fitted RVC 2415/00AF
Should be RVI 2430

Apart from + & - earth I don’t really know what the differences are
I probably should start with the correct gauge
Regards Pete
PJ Thompson

Interesting. That era of tach (1973 on) doesn't normally suffer from problems with electronic ignition as it uses voltage pulses from the coil -ve rather than current pulses through the coil, and the wiring at both tach and coil is different.

That - amended wiring - does lends weight to the other Paul's suggestion that it could be dicky connections somewhere. They could be anywhere in the coil circuit or confined to the tach, but the former would be likely to cause a misfire at the same time.

Out of interest do you have toggle switches or rocker? Do you have face-level air vents in the dash? And does the fusebox have four circuits or only two?
paulh4

Hi Paul
The car has toggle switches, no face level vents and 4 circuits on fuse box.
I brought the car one owner from new but had sat in the garden for 20 years so was pretty rotten
I may still have the original gauge but probably of no use
Regards Pete
PJ Thompson

Ok, I was wondering about the actual year. Four fuses in the fusebox were provided for the 1971 model in August 1970 at chassis number 219001 so there is certainly a question mark over the date. Anything is possible to retro-fit, but the additional fuses were for the parking lights one per side which would be quite a big job involving the main and rear harnesses if the top two fuses have one red/green on the front and four reds on the back. Not that that helps with the tach!

If it's liveable with now then fine, otherwise probably the next step would be to examine the wiring behind the tach and see just what is there, and what wires you have on the coil +ve and -ve.

paulh4

Pete
I had a peculiar fault on my car with some similar symptoms to those you describe when we were on a long distance tour.

I have an MGA with 5-brg MGB engine and so I had to convert to using a compatible MGB electronic tacho.
Mine is an RVI 2415/00AF (4-cyl -ve earth)

I first noticed that the fan belt was slipping, this got worse and the squealing noise got louder. I tightened the belt but this only improved things for a while and then the squealing came back louder until I could smell rubber burning.
We swapped the belt for a new one and things seemed better for a while until our long journey home.
We were driving at a steady 70 mph on the motorway (at about 3500 rpm) when I noticed that the tacho needle was jumping wildly up and down between 0 and 7000 rpm.

This was accompanied by a sound similar to pinking from the engine.

This came and went and so as we were close to home we sort of coasted the car home.

When I looked at the car the next day I found that the Lucas alternator was partially seized up and so it was the bearings that were making the pinking noises.
The taco needle jumping about was I believe due to the voltage spiking up and down when the alternator stopped and started spinning again.

Maybe not exactly the same as your problem but it could be a poor connection to your alternator causing the fluctuations.

Colyn


Colyn Firth

Obviously some interaction from something there, but if the battery is still connected that is supposed to smooth out the spikes from the alternator, which they always generate, and why alternator-equipped cars have the label that says not to run the engine without batteries connected. When there is no output from the alternator the battery will be running everything.
paulh4

Hi Paulh
This car is a bit of a mish mash
the origonal car was totally rotted out
so i started with a new shell 4 sync original was 3 sync
the loom i had made by a company in derby the name has slipped my mind for now
I spoke to a guy named peter and he made a loom for my chassis no slightly customised ie alternator etc
Regards pete
PJ Thompson

Unless your harness started out wired for RVI it's probably better left with the RVC it has now.

As mentioned earlier if you want to try and eliminate the problem altogether you are probably best off looking behind the tach at the wiring.
paulh4

Hi Paul
ill have a look tomorrow, just been watching a clip on you tube, a chap in the states with a similar problem found after lots of checking the fuel pump caused the tacho flutter.
he removed the feed to the pump at the junction under bonnet and fed it direct from the fuse box

Regards pete
PJ Thompson

That sounds like poor connections somewhere, although how the fuel pump and tach are interconnected depends very much on the year and market of the car. It would have been better to find and fix the rear problem rather than just working round it.
paulh4

This thread was discussed between 11/07/2019 and 14/07/2019

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