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MG MGB Technical - Overdrive

I haven't been able to successfuly locate where the power comes from for the overdrive. There's no power at the gearshift switch when Im in 4th gear. Ive looked at wiring diagrams, but its confusing at best. Can anyone walk me through the procedure for locating the problem please?
Ross Kelly

Ross - Go to Advance Auto Wire http://www.advanceautowire.com/ Click on Stock Schematics, then on MGB. Scroll down to the print of your car. I think you will find these diagrams much easier to follow than the one in the shop manual. Cheers - Dave
David DuBois

I took a look at them already but it doesn't really give me much clue as to where to start looking to trace the wire. If I knew where exactly (physically) its supposed to connect, then I could work from there. I can see in the wiring diagram that one of the white and brown wires that connects to fuse 3 terminal 5 goes off to the overdrive, changes to yellow along the way and then goes to the gearshift switch. So if Im not getting voltage there, technically it would mean there's a break in the circuit along there or a dirty connector. What I need to know though is, does this wire go into a closed loom? Where would the connector be where the white/brown wire changes to yellow? There's also another connector where the supply for the fuel pump lives as well. Are they both supplied from the same feed or is it just a block connector? Where would this block connector be? All of this info is on page 33 (last page) of the wiring schematics.
Ross Kelly

Further study of the wiring diagram reveals that the alternator induction coils and hence the charge light, the fuel pump and the overdrive circuit all get their supply off fuse 3 terminal 5. That block connector is obviously a distribution block of some description. Anyone any ideas where it is? My alternator and fuel pump are both working so that block connector needs to be my starting point.
Ross Kelly

Ross,
I think that four way bullet conector is is alongside the gearbox, I seem to recall fiddling with it before when I had no O/D....

My O/D is off the dashboard switch, but don't the gearstick versions suffer from a broken cable at the base of the stick where the cable has to flex?

Dave.
David Smith

Can this be accessed by removing the gearstick gaiter?
Ross Kelly

The alternator charge light, fuel pump and overdrove are all fed from the ignition switch. That feed also goes on to the fusebox, but that is all. LHD cars differ to RHD around here so you need to take note. For the RHD two white/brown wires come out of the main harness by the fusebox and goes into a 4-way bullet connector. From there one white comes off the bullet conenctor and into the rear harness for the fuel pump, and a white/brown comes out and goes into the gearbox harness for the OD. That white/brown goes up to the switch on the gear lever, then a yellow comes down and goes to the gearbox inhibitor switch, and a yellow/purple from there to the solenoid. That 4-way bullet connector should be your first port of call.

The OD and fuel pump are unfused and both have been known to experience shorts, which can burn out the gearbox, rear and main harnesses. I strongly advise you to fit an in-line fuse where the main, rear and gearbox harnesses join. Halfords sell them, and the bullets, and you will need a 4-way bullet connector as well. Use the tubular type of holder that takes the same fuses as in the fusebox and use the same rating as you (should) have two spares in the fusebox lid. Take the rear white and gearbox white/brown out of the existing 4-way bullet connector, pllish off any corrosion, and put them into the new 4-way connector, then fit the in-line fuse between the two 4-ways. That way you will be using one fuse to protect both the fuel pump and OD circuits. Make sure the bullets are pushed right into the connectors - they push so far then seem to stop, but need another push to get then fully seated. A dab of Vaseline in the connector and on the bullet makes assembly easier as well as protecting against bad connections from dampness.
Paul Hunt 2

Aha! Thanks for that, Paul! I'll take a look this evening and see how I get on.
Ross Kelly

Now that I think of it, there's already a bunch of fuseholders in the engine bay so I think the circuit might already be fused. One of these might be the culprit, but I checked the fuses and theyre all intact.
Ross Kelly

Any ideas where that 4 way bullet connector lives in the car? Is it in the engine bay, under the car, beside the gearbox as suggested or underneath the gearshift gaiter?
Ross Kelly

I now have power going to the switch. The 2 bullet connectors under the gearshift gaiter were pulled out. I reconnected them, tested for power and there was power there. I took it for a drive, and it still doesn't work!! There's power going somewhere cause I checked just there now and theres 800mA flowing, so I can only assume thats the solenoid. Is it likely that I just need to go for a drive at some decent speed and get the revs up in 3rd and 4th with the overdrive on to get the hydraulic pressure up?

To give the background to this problem, when I bought the car, the overdrive was stuck on. After driving a bit, it started working normally. It engaged and disengaged as it should. Other times it would stick off and not turn on. I imagine this was more to do with the loose connection though, but obviously I cant be sure. Anyway I went to change the oil in the gearbox and it didnt look all that bad. I tried removing the filter for the overdrive unit based on the good sir haynes's instructions, and ended up removing what I can only assume was in fact the solenoid instead. It was a bastard to get back in, and when it came out a ball bearing ball dropped out along with it. I stored the ball away safely and replaced the solenoid, but now it looks like everythings gone a bit pear shaped. I don't even know if I replaced the solenoid correctly, but I can only assume it does work considering its drawing current. So where do I go from here?
Ross Kelly

Correction - I was in a hurry. It is the ignition relay that feeds the fusebox, fuel pump, overdrive and ignition warning light, but what I said otherwise remains the same. The ignition switch just feeds the ignition relay and the coil (via the ballast).

For completeness the 4-way connector should be with a mass (or mess) of others by the fusebox, you have already found the two at the base of the gear lever.

800mA is the correct current for the later OD solenoid, so the problems are either hydraulic or mechanical. You should be able to detect OD switching in and out by 20mph, even though you wouldn't normally use it at those speeds of course.

Did you remove the small square plate with the writing on or the round hex plug? The former is the solenoid and the latter the relief and low-pressure valve assembly.

If you removed the square plate and the solenoid is the solenoid ball still stored away safely? If so that will be the (a?) cause of it not working now. As long as the solenoid is in contact with the wire and the body of the OD it will draw current, the moving part could be stuck in the body, or there could be parts missing between the moving part and the valve, there should be 3 O-rings in that area at least.

When removing the round hex plug there are a whole mess of parts in there, possibly as many as 13. I've just changed the oil and cleaned the filters on both my cars and documented it at http://www.mgb-stuff.org.uk/wn_gearsframe.htm and click on 'oil change'. The manuals show a ball and a cylindrical plug but neither of those came out on either of my cars, but what *did* come out was different on one to the other even though they are both black label ODs.

The 'stuck on when you bought it' bit should give concern, if it was electrical i.e. a gear lever switch or wiring problem that should easily be corrected, but if it was something else inside the OD then not only does it indicate there were major problems inside the unit, but if you (or anyone else) reversed at any speed it will have done even more.

If you have replaced the ball and everything else correctly then the next step is to do a pressure test. These needs a special adapter that replaces the relief valve plug and a gauge capable of reading up to 450psi, not something you will find everywhere. With the rear wheels off the ground, the front wheels chocked, and the car *securely* supported, engage top gear and overdrive and at 30mph you should get between 400 and 420psi, and nothing with the OD disengaged.
Paul Hunt 2

Yep it was the square plate with the writing on it and the wire going into it. It was a solenoid. Right so that ball bearing was important. There's my problem so! I have it in a small bag in my tool box. I need to replace that before it will work. When I say it was stuck on. It would come on in 3rd and 4th gear irrespective of whether or not I had the switch on, but it didn't come on in 1st, 2nd or reverse. So where does that ball go? The reason I didn't refit it is because I couldn't figure out where it came from or where it was supposed to go.
Ross Kelly

Oh and another thing... I removed the solenoid when the gearbox was drained of its oil. If I remove it with the box full of oil, will the oil pour out?
Ross Kelly

That's a good question! I don't think you will get much, I'm sure I've read of others removing them without draining the oil, but have a big enough container to hand just in case! Also try to get the rear of the car as high as possible to move as much oil as possible to the front. I haven't had my solenoids out but the manual shows the solenoid plunger coming out of the body of the solenoid coil once the plate has been removed, with the solenoid coil still in place. It shows a small O-ring sitting on top of the plunger, then the ball sitting on top of that. There is a medium sized O-ring round the solenoid valve body, which sits on top of the solenoid coil, and a large O-ring on top of the solenoid coil.

Coming on everytime you selected 3rd or 4th is a much better situation, just indicates the manual switch was failing to open or its wires were shorted together in some way. If they were disconnected at the two 2-way bullet connectors, it's possible the switch is faulty *open*-circuit, or shorting to ground, and the PO had connected the gearbox harness wire directly to the lockout switch wire so bypassing the manual switch.

I do urge you to fit that in-line fuse I mentioned above.
Paul Hunt 2

I don't suppose you have a diagram of this by any chance, do you? I think I get what you mean but Id like to be sure.

There is an inline fuse already. I checked.
Ross Kelly

Attached, items 53 to 63. I haven't bothered labelling the parts as you should be able to recognise them from what you have.

Late model MGBs had several in-line fuses from the factory, are you *sure* is in the white and white/brown wires between the main and gearbox/rear harnesses?


Paul Hunt 2

Nice one, Paul. Thanks again. So its item 59 I need to worry about. I'll double check the situation with fuses.
Ross Kelly

This thread was discussed between 06/04/2008 and 08/04/2008

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