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MG MG Y Type - Ammeter problems

The ammeter on my YB was not registering any charge or discharge. I had another ammeter which came in a spare instrument set with the car. I checked it was working on a headlight bulb discharge bench test and fitted it in the car.

However when I fitted it and connected the battery the car thinks the ignition is switched on, when it's not. I assume this is an issue with the ammeter which, in the car, shows a discharge with the headlights switched on.

Any suggestions gratefully received.
I Anderson

Hi Ian,
does the discharge get bigger when you do turn the ign on ?
John YB0362
JC Jebb

Hi Again Ian,
I think you may have trouble with the cutout, disconnect the wire from terminal A on the cutout and you should lose the discharge, if that the case DISCONNECT battery and with very FINE emerypaper clean the points ( dont over do the cleaning)try blowing away any dust produced by the cleaning, at the same time check the earthing of the cutout terminal E. it goes without saying all wires and terminals should be spotlessly clean.
John YB0362
JC Jebb

It's now behaving as it should, however with the engine running it's not showing any charge, with the ammeter needle flickering erratically on the minus side. The ignition warning light is pathetic but does go out with the engine running.
I Anderson

Hi Ian,
What happens if you rev the engine up email me if you like wih a contact phone number. mgyb@btinternet.com
John YB0362
JC Jebb

John. No change, the needle flickering slightly on the minus side.
I Anderson

Hi Ian,
very basic this is, take cover off cutout and from tickover start to rev engine does cutout close, drop back to tickover should open. I dont know how much you up in the car repair world so i hope you are not offended by this very basic first check
John YB0362
JC Jebb

John. No offence, advice gratefully accepted. No movement on the regulator contacts at all. I bench tested the dynamo after I changed the brushes and got output.
I Anderson

Ian,
Remove cable from D on cutout, connect a multimeter to the D lead and to a GOOD earth on the car start car DO NOT REV TOO MUCH or get anywhere near 20VOLTS if no reading you have a break in the system between the dynamo and cutout
John YB0362
JC Jebb

I have continuity on both cables from the dynamo to the voltage regulator and measured output from the dynamo. Have I got a faulty regulator?
I Anderson

If the control box is original to the car, it may well have become caput. The cut-out contacts on mine (Y5703 1950) first became intermittent, then immobile. The ammeter was always dead or negative with the lights on. Ignition warning light almost always on except at 4000rpm plus. Always seem to be running on the battery.
Fitted a new facsimile control box and all sorted.
Label all the wires as you take them off the old box, and rewire the new box before you bolt it back onto the bulkhead, and you'll save a lot of bother.
I found the Lucas Overseas Technical Courses available on this website to be tricky to get through, but worthwhile, as one understands the mysterious black box under the bonnet about which even the wise give evasive answers.
A J R Felthouse

Many thanks all. I'll try and source a reasonably priced Lucas control box.
I Anderson

Mine is a Holden RF95 Control Box, available on Ebay. Cost was £140 last year, but it might have gone up. You can put your original control box cover on if you like.
A J R Felthouse

hi all,
Although the official YB workshop manual states
RF95 voltage control unit it's a RB106 they put on the car replacement units can be got new for as little as £30 ranging up to a £100 PLUS all copies, with a years cover
should a problem arise.
John YB0362
JC Jebb

I got an RB106 5 post voltage regulator for £38 off Ebay, as fitted to early Land Rovers. However this has not solved the problem. I rechecked the voltage output from the dynamo and whilst I get an output voltage that rises with engine speed it is intermittent rising and falling to as little as 1 volt. Back to the drawing board. I previously fitted new brushes and cleaned up the dynamo armature and commutator surfaces.
I Anderson

Problem solved. I stripped the dynamo down again, cleaned everything, repolarised it and refitted. Now charging. On to the next problem. My fuel gauge is reading full, I've seen the posts on this.
I Anderson

Reactivating, as I found the above discussion interesting; and because I have a question: I fitted an electronic replacement for my original RF95,and all is functioning well, except that if I put on the (halogen) headlamps, the ammeter will sometimes shoot over to full negative, but returns to slightly negative when I tap the glass face sharply. Should I be investigating further? John.
J P Hall

In the absence of any recent comment, I've investigated the innards of a spare ammeter. As pointed out above, the electromagnetic component is a simple set up with very fine adjustment screws available to free up the movement if, I suspect like mine, it has become a bit gummed up after 74 years. I used electric contact spray and a cotton bud first; then a bit of trial-and-error twiddling the adjustment screws securing the 2 halves of the mechanism. Result: free movement throughout the range (bench tested with headlamp current), and no sticking. When time permits, I'll do the same with the ammeter that's in the car.
Hope this is of interest. John.
J P Hall

John--What wattage are your headlights
William Revit

Sorry Willy - been away. They're 35/35 Halogens from Anthony Pearson in Adelaide. Very effective headlights, although I generally run just on the sidelights, as the Y is a daytime shopping trolley. With engine running at idle and headlights on, the ammeter reads a slight negative, but into pos as soon as you rev up; ignition on and headlights on but engine not running, she shows negative 12 amps.It's just the odd flick of the needle to full negative, whereupon it usually sticks, I tap the glass, and it returns to correct reading.
My initial worry was a short in the headlamp wiring somewhere, but now I'm not so sure. Thoughts?
John o'the far north.
J P Hall

With a pair of 35W globes the draw should be around 6A so with ign on and probably side lights as well your 12A sounds about right-----"If" the flick on the Amp meter is only when the lights are on then yes, I agree there must be a short in the wiring/bulb holder there somewhere----Easiest is to do a wriggle test, -get someone to sit in the car watching the meter with the lights on while you go around the wiring and connections etc giving them a good wriggle to see if you can track the fault--my money is on a short in the sockets for the globes, maybe a strand of wire shorting to the earthed socket

Willy 41.165 S
William Revit

That's what I'll do, Willy. Pretty sure my dear wife will be happy to assist. Not.
I can only manage 17.16 S. You win.
J P Hall

This thread was discussed between 15/09/2022 and 03/11/2023

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