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MG MGB Technical - Overdrive Fuse Blowing

I have fitted an overdrive switch to my gearlever and it has worked fine for several thousand miles.

I included an inline 5A fuse on the white feeding it -just as well, as it has just started blowing.

I cannot see any bare wires/chafing in the obvious place round the bottom of the gear lever or the wires around the gear box OD switch that I can see with an endoscope poked in from the top of the transmission tunnel

I will try and trace the rest of the wiring but could it be the gearbox OD switch itself? It looks to be a replacement one - the type you can get a spanner on.

Mike

Mike Dixon

Interesting---If it's blowing your inline fuse the short to earth must be from there onwards toward/ending at the od solenoid. Unusual for the rev/od switches to short to earth but anythings possible. If you have a multimeter with an audible short circuit buzzer, you could remove the fuse and connect to the od side of the wiring there and to earth and try wiggling /disconecting things untill you find the culprit. Normally it's best to start from the end of the circuit and work back towards the fuse , so solenoid first then back through the circuit.
If you fitted a patch harness from the original dash od switch to the gearlever , maybe check where that goes through the firewall and where the old end wires are joined together--if it was taped up there then the tape may have fell off and the joint is shorted on the dash or something.
willy
William Revit

The later gear lever harness is very prone to chafing and shorting, fitting a fuse is a very good idea as the circuit from there back to the ignition is unfused.

As Willy says the fault will be between the fuse and the earthed end of solenoid, so it depends where that fuse is fitted. There will be several connection points in that chain so it is a matter of breaking the circuit into sections. Connection points are where the white/brown in the main harness joins the white or white/brown in the gearbox harness at the mass of connectors by the master cylinders, where that joins the bottom of the gear-lever harness, where the top of that joins one of the contacts in the switch, where the other switch contact joins the other wire at the top of the gear-lever harness, where the bottom of that rejoins the gearbox harness at a yellow/red, and where that joins the tail from the solenoid, which could be shorting where it passes into the OD or the OD winding itself could be faulty.

It depends on how sensitive a continuity buzzer (not sure about short circuit buzzers) is in a multi-meter so you may need to use the ohms function. The solenoid measures about 15 ohms, a 5 amp fuse will typically blow at 10 amps, which equates to a resistance of about 1 ohm, a digital meter will hopefully discriminate between those two.



paulh4

Thanks - I will get on with that and report back. I had a chat with the ever helpful Bill at Hardy Engineering and they say it could be the solenoid but eliminate the other possibilities first, given the hassle and cost of fixing that. I hope to get it up on a ramp this week.

I put the fuse on the white close to the connector block near the steering column. It's on a V8 and the yellow was damaged and, as I discovered, seriously bodged under the cowling on the overdrive stalk and cut into the yellow in a similar place - switch side so as not to damage the loom.
Mike Dixon

In fact changing the solenoid is easier than accessing the connections round the gear lever! It can be done without draining the oil. Still the cost though so definitely look elsewhere first.

I put my V8 fuse in the engine compartment where the main and gearbox harnesses connect. I also put the fuel pump fuse in that area where the main and rear harnesses connect (both mine and two or three I have worked on have all had fuel pump shorts damaging rear and main harnesses) and that proved to be rather fortuitous. I had a float sink with fuel flooding out away from home, so cross-connected the fuel pump and OD fuses and I could use the OD switch to turn the pump on for a few seconds to put some fuel in the carbs hopefully without any flooding out. Then when it started to stumble as they ran down gave it a few more seconds. Fortunately I wasn't very far from home!
paulh4

That's a relief - I had visions of gearbox out - not something to be done lightly on a V8?

A clutch change was on the cards for this winter - the clutch engagement point is very high, but I would rather leave it until then and use the car this summer and do it in slower time - we have an event planned using it in a few weeks

Has a quick look on a ramp this morning, nothing obvious, planning on more systematic look at the end of the week.
Mike Dixon

If the clutch starts slipping under acceleration in 4th or OD then it's definitely time for replacement :o)

Yes, big job on a V8 (factory at least) even if you have the space and kit to get both out together which I don't. I took the engine out first which is almost as bad as unlike the 4-cylinder you can't get at the top bell-housing bolts with the engine in its normal position. I removed the engine mounts and mounting plates - a whole lot of extra work - which allowed the engine to sit that bit lower, then with a cranked ring spanner could get at them. The next ones down are bad enough but can be done from underneath with various 3/8" extensions and a UJ or wobble joints. With the front of the car raised enough to get the engine out I slid the gearbox out (for rebuild) from underneath but still had to remove the valance. Too old to do all that now.
paulh4

Not slipping yet but it takes very little pressure to make it slip

A friend who has recently done his warned me of this so I have a ramp and a friendly mechanic booked to do it with my'help' on his ramp. I am going to get the box/OD checked over at the same time by Hardy Engineering. Engine and box out at the same time I think for this one

One there think I have read you H&S section on release bearings the Rimmer one( pic att) is the correct one I take it ? Clive Wheatley's looks a bit different?


Mike Dixon

The one you show is the one that fitted mine. I think Clive Wheatley shows the same one but from a different angle as you can only just see the ears that fit against the release arm on that, but I can just see the groove that retains the spring clip.

Make sure you clean the nose of the 1st motion shaft and inside the pilot bearing and lubricate them and the splines with a specialist grease such as Sachs 4200 080 050. Mine had got sticky (perhaps from copper grease having been used in the past) and if I didn't use the car for a few days it would drag and baulk at 1st and grind in reverse. Moving forwards 2nd or sometimes 3rd was needed before anything would engage but for reverse I'd have to switch off, engage reverse then restart.
paulh4

All done, bit of a struggle getting the engine back on the mounts ( both out together) was a challenge,thank goodness for the ramp and professional help ( me hindering him?). Seemed to be much more of a challenge and took probably twice as long than the B for some reason.

The new engine mounts have made a huge difference ( 4 mm on drivers side) The manifold bolts are well clear of the steering column now and the exhaust manifolds slide on and out without having to lift the engine.

I got the Clutch cover and plate from Clutch precision services in Hengstridge - they have OEM stuff there and they gave me a sachet of appropriate grease. Old one did not look too bad but the engagement point is now much lower down and where I feel it should be.

Elecrical gremlin also sorted - it was something in the inline fuse holder? Very odd and it took a bit of finding. I gather that you cannot hear the solenoid operate? The only way to test it was to hot wire it and drive the car with an assistant operatng it. On reflection I should have cut an access hole in the tunnel when it was all out, but its still do-able if required, to get access to the lock out switch wiring.

Just in time for a quick 1000 miles on the HERO Summer trial, bottom line is I now have a lot more confidence in it for longer trips - fingers crossed

Mike Dixon

Using it with an ammeter wired into the circuit is the only sure way to know if the solenoid is being powered correctly or not.

With the 4-synch OD you probably won't hear the solenoid operating and releasing unlike the 3-synch mechanically it is very different. You _may_ hear it the first time it is operated after being fitted, but since it is only oil pressure from a running engine that pushes it back when released you won't hear anything successively operating and releasing with the engine stopped. However with new O-rings they can have a 'spring return' type of effect so you may just hear something very faintly then.

paulh4

This thread was discussed between 07/06/2026 and 08/07/2026

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