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MG MGB Technical - Overdrive problem

The overdrive on my 71 roadster has always been a bit tempermental but has now stopped altogether. I have topped up the oil level, it only took half a cup full but has made no difference. I have a suspicion it could be a wiring fault as the tacho. has packed up at the same time and I believe they share the same white wire from the fuse box. Any advice would be welcome. Thanks. Trev
Trevor Harvey

Trevor, my OD was acting up in a similar fashion as yours. It turned out to be a faulty OD solenoid. Also, when I took it apart, I discovered that the plunger was extremely rough. I used a flat file to remove the high spots and the OD now operates better than it did when new. RAY
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Presumably you have tried pulling the gear stick back in 4th to eliminate the common problem of wear with the inhibitor switch and adjustment with shims on the switch?

Before crawling underneath I should check the wiring. The o/d was not fused at the factory but has one been added? Both the tacho (and hence ignition) and the o/d are fed off the white wire from the ignition switch and if the cause was linked I think you would have no ignition. (So tacho is a separate but possibly linked fault). Do you have the o/d switch on the dash RHS? With the ignition on you should get 12v on the "in" terminal and when the switch is operated the "out" terminal. (Other probe to a good earth). There is a mess of bullet connectors in front of the bulkhead below the pedal box, the yellow then yellow /red is overdrive: does this get 12v? Clean up bullets if not. It now gets more difficult as the gearbox inhibitor switch is pretty inaccessible. You now need to select 4th gear with ignition on and o/d switch on. There is usually a bullet hanging, covered in crud on the wire that goes to the solenoid. If you get 12v here you have checked the inhibitor switch as working. If not the inhibitor switch is bad (unusual) or badly adjusted as above. If you have 12v here then to check the solenoid you need to use an ammeter. Do you have one-usually a setting on a multimeter? Solenoid wiring internally is generally not a cause of faults but never say never.

However if the tacho is not working that would point to a fault in the ignition switch or the contacts on the tacho or the tacho itself. However I think in either of these cases the engine would stop. And if the ignition switch also the fuel pump would stop.

I'd fit a fuse in the o/d circuit on reassembly.
Michael Beswick

The white wires at the tach on a 71 are for the ignition. If there was a fault there - or the white circuit generally - the engine wouldn't run.

If you have an ammeter, or a test-lamp, connect it between the OD switch contacts, ignition on, OD switch off, gearbox in 3rd or 4th. The engine doesn't need to be running, and if not disconnect one side of the coil to prevent it overheating if leaving the ignition on for a while. You should see about 1 amp flowing/lamp glowing if the electrical circuit is sound, in which case the problem is mechanical or hydraulic.

If no current flow then check for 12v on both sides of the switch with the switch on. If there then you will have to investigate the gearbox switch. You may be able to get at the contacts by pulling the tunnel carpet back and removing the small panel on one side of the gear lever hole.
paulh4

Thanks for the advice. The o/d is sorted,was a disconected joint.
No luck yet with the tacho. Is there a way of checking if the gauge itself is caput.
Looking at the wiring diagram a green wire goes to a voltage stabilizer. Where would this be and should I get 12 volt through this with the engine running?
The white wire from the coil is showing 12 volts at the tacho and increases to approx 13.4 as the revs increase.

The white wire from the ign. Switch I assume is to feed the ign. light and operates normally.
Ther is 2 more wire terminals on the back of the tacho, one male and one female, nothing is attatched to these.
About10 years ago I fitted a Petronix electronic ignition system and the tacho did not work properly with it.I sent it to someone on the forum who altered it for me and then it was ok but I believe I had to alter the wring a littleto get it to work, trouble is I cannot remember exactly what I altered,
Trevor Harvey

The voltage stabiliser is a small metal can screwed to the bulkhead behind the dashboard in the region of the wiper motor. It is intended to produce a steady 10 volts so the gauges aren't affected by the variable voltages supplied by the battery and alternator.
Mike Howlett

The green wire is simply a kind of ring main that supplies power TO the voltage regulator and lights and so on. ie your tacho is/should be receiving a full 12v whenever the ignition is on. And the voltage regulator has nothing to do with the tacho.

The white wire will have 12v on the connection on the tacho, and then on to the coil-once the ignition is switched on. However on my tacho IIRC the white wires connect to the male/female bullet connectors mounted on the back of the tacho. (they are a slightly odd size). Where does your white wire connect to and from on the tacho?

On my Petronix the black and red wires are connected to the two terminals on the coil. Where is yours connected?
Michael Beswick

So power goes from the ignition switch to the tacho if I am getting 12 volt from the white cable to the coil,does this indicate that the tacho is faulty.
Trevor Harvey

I stand corrected - Michael is quite right. Although the tacho is connected by the green wire to the regulator, it is on the 12v side, not the 10v side. Only the fuel gauge is connected to that by the light green/green wire.

Here's a thought. Is the tacho earthed? It won't work if it isn't. Try running a temporary wire from a good earth to the case of the tacho.
Mike Howlett

Also whilst replacement voltage stabilisers output a steady 10v the originals switch 12v on and off about once per second, averaging about 10v.

The tach needs 12v on a green and earth on a black to work, as well as signal pulses through the two white wires to the coil.

The pulses need to go through the pickup on the tach in a particular direction, but as it was working and just stopped that shouldn't have changed. What you can do is disconnect one of the white wires at the tach, and that should stop the engine. If it doesn't then the coil is getting it's 12v via some other route that is bypassing the tach. Again, that shouldn't have happened if the tach just stopped working with nothing else having been done to the car, but it's best to check and not assume anything.

If all that is as it should be, then it does look like an internal failure. Have you given the face a sharp rap with a knuckle while revving the engine? Some tachs can be prone to sticky needles.
paulh4

Paul,As I said earlier thr rev counter was altered so it would work with the elec. ignition.I have had it apart today and it looks like a small module has been fitted with the white wire that goes to the coil. The bullet conectors on the back of the tacho are not connected to anything they just make a loop inside the tacho.
I have tried giving it a good tap I had to do this previously to make it work but now it seems dead.
The strange thing is.if when the engine is running I turn the switch off on the floor that kills the power from the battery the engine still runs and the needle on the tacho bounces up and down.
Trevor Harvey

The earlier tachos did have an externally mounted loop of wire (as shown in Haynes). The additional "module" makes it difficult to be sure, but presumably it cannot have failed "open" or the engine would stop. It does seem that it is not providing the pulse necessary for the tacho to register.

AFAIK, it is generally not recommended running the engine with the main battery disconnected. The alternator tends to object. So what does the switch on the floor do if the engine still runs?
Michael Beswick

I converted an old tacho once for a Lotus Elan so that it would work with an Aldon Igniter. A small circuit board was put inside the case and the white loop was then redundant. This is the gadget I used. You can download the instructions from there.
https://www.spiyda.com/magento/index.php/smiths-rvi-rvc-conversion-board.html

By the way, I think Michael is right about not running the car with the battery disconnected. Bad for the alternator. You could damage it - maybe the diode pack or rectifier?
Mike Howlett

The switch on the floor cuts power from the battery. I normally switch off the engine by the key, then kill the battery by the switch on the floor. I by mistake done it the other way round and was surprised to see the tacho needle jump up and down after it had not been working.
The battery kill switch was fitted by me 10 years ago because in the past the car had almot caught fire when a short had occurred under the dash. Its a quick way to isolate power, it is also a useful anti theft device.
Trevor Harvey

You don't normally need to modify the early tach to work with electronic ignition, just take the 12v feed to the electronics from the fusebox instead of the coil.

Turning the battery off with a running engine results in high-voltage pulses from the alternator, which can damage electronics and apparently blow incandescent bulbs. However I've done it myself on a couple of occasions and it hasn't done any harm - so far. But some of the electronic ignition suppliers state that jump starting either as a donor or a recipient can damage the units and will negate the guarantee (how would they know) which does imply to me they are a bit suspect. Could also have damaged your tach module.

When I've done it there was no effect on the tach either, which works normally when I have done it. Your module sounds like it converts it to the later design, so having an interaction like that does make me suspect connection problems somewhere. It will still need 12v on a green and an earth to work, as well as one wire from the coil, which will be the terminal the electronic ignition is connected to.

You won't really know if it is tach or connections until you can substitute it with a known good one, preferably of the later type.
paulh4

Paul, The set up you discribe sounds like the one I have in my roadster. I have a green to the tach which I have checked and am getting a good 12 volt with the ign. on. Only one white wire from the tach which goes to the coil, incidentaly the engine starts with this disconnected. So this wire obviously is not giving power to the coil.
I will improve the earth to the tach and refit. If this still does not work, it looks like I will have to find a replacement. Thanks.
Trevor Harvey

Re my last comment. There is also a white wire to the ignition light bulb on the back of the tach.
Trevor Harvey

As the engine stops when you turn the ignition key off, the supply to the coil (and Petronix) must be "ignition switched". The most common circuit that this applies to is the green. So in theory your 12v at the green on the tach could be the 12v source for the coil with an internal connection. It would seem likely the coil and Petronix are from the same 12v source. If you can identify and isolate the 12v source to the coil and Petronix. (Connected it runs, disconnected it won't) you have narrowed it down to the fault being in the tacho. Whether it is the circuit that recognises the pulse or the internal wiring is difficult to say.

The white wire to the ignition warning bulb exits ?yellow/brown to the alternator.
(FWIW Ignition on, car not started, so no alternator charge = bulb on. Car running, alternator charging= bulb not on as Battery output is equalled by alternator input. Car running but alternator failed= bulb on)
Michael Beswick

Edit failed:

The most likely outward connection for the coil supply from the tacho would be the white wire. However as you said the car runs with this wire disconnected it seems unlikely this is the 12v source.
Michael Beswick

White circuit is ignition - coil, fuel pump, etc.

Green circuit is ignition switch controlled ancillaries - wipers, heater fan, instruments, etc.

Brown circuit is 12v permanent power - fuse box, alternator, lights etc - comes off the main battery lead at the solenoid.

Purple circuit is also 12V permanent - interior lights, cigarette lighter.

If your tach has been altered to the later type of operation for electronic ignition then the trigger wire to the tach should go to the ground side of the coil - not the power side.

If you have a 123 dizzy then I think they have a separate wire to trigger the tach.
Chris at Octarine Services

"So in theory your 12v at the green on the tach could be the 12v source for the coil with an internal connection."

I don't think this is likely. If the engine continues to run with the white between coil and tach disconnected, then it's more likely to be the normal arrangement for 73 and later i.e. green at the tach is the 12v supply to its electronics, the white at the tach is the signal pulses for the tach from the coil -ve, and the coil +ve will be getting an ignition switched supply from elsewhere - probably the white at the fusebox.

This white comes direct from the ignition switch, and goes through the fusebox to the green circuits. This was the arrangement on chrome bumper cars (later rubber bumper cars differ) regardless of whether they had the earlier tach or the later.
paulh4

On further investigation I have found another white wire connected to the positive side of the coil that comes from the loom and I assume from the ignition switch. This must be why the engine still runs with white wire from the tach disconected.
I am checking all connections then reassembling,if the tach still does not work I can only think it has had it.
Michael your Petronix sounds the same as mine, there is red and black wire that is connected to the two terminals on the coil. Have you been happy with yours. Fitted mine 10 years ago. The engine runs smoother and has been trouble free.
Thanks for your input and all the other replies. Trev
Trevor Harvey

Depending on what was "modified" there is a lot on the web about repairing Smith's tachs.

Yup- My petronix is fine: It is hooked up an Amythyst programmable ignition. As I have a D43 dizzy I needed to make a tiny collar to lift the rotor arm up 15 thou. I bought mine many years ago from USA when the £/$ rate was very favourable! Seem to remember it was actually dispatched from Guildford Surrey!
Michael Beswick

Michael, If after I have hooked it up and re installed it I find it is still not working, I will check out the info on line, Probably wont be able to understand the advice as I am not very electricaly minded,as you probably realise. Trev
Trevor Harvey

"another white wire connected to the positive side of the coil that comes from the loom and I assume from the ignition switch"

Probably wired the same as for the later tachs i.e. a white from the ignition switch daisy-chained off the front of the fusebox to the coil +ve, then two black/whites from the coil -ve, one to the points and the other to the tach trigger terminal. As yours has been modified, and there is a white on the tach trigger terminal, that is probably the original original white that went from the tach to the coil +ve. If so, it now needs to be on the coil -ve for the tach to work, not the coil +ve, which should only have one white wire.

If it has two whites on the coil +ve, then one probably needs to move to the coil -ve. This would be the one that does NOT have 12v with the ignition on (remove both wires from the coil +ve to test).

Either that, or if a new wire was run in from the coil -ve to the tach, and white was used, you could well have three whites at the coil - easy to get mixed up which should be on the +ve and which on the -ve.
paulh4

Thats interesting Paul, Iam almost sure the white from the tach is connected the pos. On the coil. I will check tomorrow. It has been that way for a long while and has worked in the past,never very well but it has worked.
I have aquired a replacement tach so if it still does not work, wondered whether to alter the wiring to the way that most mgbs appear to be wired. ie ign. tach,through the loop to the coil.
It seems strange that others have fitted electronic ignition systems, like pertronix and have not had to alter the tach as I had to. Could it be because mine was wired differently or because the white to the coil was on the wrong terminal.
Trevor Harvey

You don't normally have to modify the tach, as any electronic trigger simply replicates what the points do i.e. switch current through the coil on and off. As far as the tach is concerned the current pulse is exactly the same.

The problem that electronic ignition modules cause with the earlier tach is that if the module is powered off the coil +ve, as many instructions state, the current drawn by the electronics is also going through the tach, which affects what the tach sees. The fix for that is to move the 12v wire for the module off the coil +ve and onto some other ignition switched point such as the white at the fusebox.

It's possible that voltage fluctuations on the coil +ve as the trigger switched on and off has been just enough to trigger the tach, but not reliably.

Be careful switching whites around, if you move the 12v supply to the coil -ve thinking it is the tach wire, you may well blow the ignition module and burn the wiring. Disconnect what you think is the tach wire, and the engine should still run. If it stops the engine, then that is the 12v supply wire which must remain on the coil +ve. Unfortunately having been modified this way there is no way of knowing exactly what the modifyer did.
paulh4

Paul, I have removed the white wire that comes from the tach that was connected to the pos. on the coil,it comes from the module that was fitted inside the tach. Removing this,engine still runs. Connected it to the neg on the coil, when the engine was fired up the needle on the tach jumps and then settles still again, so it is receiving something. Is this the wire you suggest to move to another ign. powered point such as the white on the fuse box.
Trev
Trevor Harvey

That wire from the tach should be on the coil -ve. It looks like the tach module might be faulty, time to try your replacement ... but does it say RVI or RVC on the dial?

The wire that would normally be moved from the coil +ve to the fusebox would be from the ignition module, but only on cars with the earlier tach wiring. With your tach having been modified to the later type it shouldn't be necessary.
paulh4

Both the original and the replacement tachs are RV1
I have found the white live to the coil it comes direct from the ignition switch.
As I said earlier the wiring had at sometime had a fire under the dash. It is possible it was rewired differently after that. It looks a bodged job.If I use the replacement tach, can you advise the best way to rewire this. ie removing this live and running a new one through the looped terminals on the tach.
Trevor Harvey

The white wire from the ignition switch to the tach, does it have to go on the male or female connection of the tach or does it not matter. I have given up on the old tach and fitting the replacement. Thanks to all who have helped with my questions. Trev
Trevor Harvey

"In" to ignition-male. "Out" to coil-female
Michael Beswick

Indeed, the white from the ignition switch to the tach should have a female bullet on it, so a loose wire can't short to earth. For the same reason the tach output terminal to the coil is a female.

What is the RVI reference number? As long as it isn't 2410 it should be the correct polarity for your car. A UK 71 would have been 2430 originally, but if the pick-up is inside the case then it's almost certainly OK.
paulh4

The replacement tach is in, wired from the ign. switch and on to the coil. Problem now is the engine will not start, not a sign of life.
I am getting 12 volt from this new wire with the ign. on, I have tried it on both sides of the coil but no joy.
I have a Petronix elec. ignition fitted, a black wire from this is on the neg side of the coil and a red on the pos. as per the instructions. No other wire goes from the low tension side of the coil to the distributor. The white live 12 volt wire to the coil, what side of the coil should this be attached to, neg. or pos.
Trevor Harvey

The 12v feed from the tach should be to the coil pos, with the Red Petronix lead. The neg has the black Petronix wire. However if you have swapped them round on the terminals on the coil without changing the leads of the Petronix, you have effectively wired the Petronix "backwards". I have Petronix; it says to connect red to + and black to -, but no mention of the effect of connecting the other way round. Other electronic modules threaten disaster when connected backwards.

I think I would try to check the Petronix by disconnecting the wire from your new tach. leave the petronix red to + black to - on the coil. Add a temporary lead from the fuse box 3rd down to the + on the coil. When you turn the ignition on there should be 12v on the 3rd fuse down (and hence to the coil and Petronix). Turning the key to start and it should run, but obviously no tach. Does it?
Michael Beswick

If you connected the 12v wire from the ignition switch that comes via the tach to the coil -ve, while the Pertronix was connected to the coil, you may well have blown the Pertronix, as effectively you are asking the Pertronix to short out the battery with all that implies.

OTOH there may be some other problem.

You will have to go back to points and condenser to properly diagnose exactly what is wrong and get the engine running, and the tach, then you can try the Pertronix again.

With an RVI tach the red wire from the Pertronix will have to go to the front of the third fuse down on the fusebox - which is also white - and not the coil +ve. This may seem pointless (ho ho) but electronic ignition frequently upsets the tach readings unless connected this way. With the Pertronix red connected to the coil +ve the tach is seeing electronics pulses as well as coil pulses, which is why it gets confused.
paulh4

It appears I have blown the pertronix, I wired the power supply from the fuse box as Michael suggested and the engine still would not fire. I have bought another system from Simonbbc and am awaiting delivery. Hope this sorts out the problem.
Trevor Harvey

Trevor, I've run a Simonbbc Powerspark Electronic Ignition Kit on my B for 6 years now. It's easy to fit and just needed the ignition timing reset with a strobe. It's never missed a beat. Much cheaper than the Pertronix (looks basically the same). I keep a spare kit in my boot, just in case (and points as well), but have never had a problem.

Andy
Andy Robinson

This thread was discussed between 21/02/2017 and 15/03/2017

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