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MG MGB Technical - Overdrive intermittent function

Hi folks, hope you can help with an overdrive problem.
I have just bought my first mgb roadster ('75 RB) and took her for a 100 mile run yesterday in our sunny 'Norn Iron' weather - should have worn a hat as I burnt my baldy napper! Anyway, I noticed at higher speeds/RPM that the overdrive was switching on and off by itself. I slowed down to 40mph engaged it again and all seemed OK until I got to about 75mph and then it started jumping back and forth again.
Driving at 40/50mph the OD stays on with no problems.
Could it be an oil level problem or something more sinister?
Hope you can help.

Thanks,

Nick
Nick

Nick, It could be low oil level (the filler/level is on the side of the gearbox) or an intermittent switch or wiring connection, or the reason of O ring seal suggested here, http://www.mgcars.org.uk/news/news363.html

Rich

Nick,

I had a similar problem with my O/D a while back. Turned out to be the lock-out switch located on the top of the 'box had a loose terminal on it and kept breaking the circuit.

This switch is accessible through the shifter hole if you lower the 'box slightly. It's the same switch as the reverse light unit so I just swapped an old one of those with the defective one and have had no problems since.

Cheers
Simon
Simon Austin

Thanks guys. Don't think it's the 'O' rings as it only does it when OD is engaged (not when I'm in 4th).
I'll get her on a ramp and check the oil level and connections at the weekend. I'll let you know the outcome.
Cheers,
Nick
Nick

Nick

I had similar problems. It was a combination of 3/4 lockout switch and the terminal on the switch in the gear lever. See my post in Thomas Koch's thread about overdrive for the way I rewired the overdrive circuit.
This is much better than the standard wiring arrangement.

Cheers
Ray Taylor 77BGT
RM Taylor

Nick,

Here's one that hasn't been discussed before. Check you batteries for specific gravity level and charging system in general.

Last year, I took a 1000 mile run down the coast. Overdrive did the same thing you're describing. In the bright sunlight, I could not see my ignition warning light slight aglow. The car finally died on the road. When charging/electrical system was fixed to full function, intermittant overdrive issues went away. Perhaps there was not enough power to keep the solinoid engaged?? Not sure, but never had another problem with it. hth

Paul
Paul Hanley

Hi guys.
The OD works when I hold the gear lever to the right and to the rear - the link that Rich suggested earlier states that this is the 3/4 lockout switch. If I can find the thread that Ray was talking about, that sounds like the most likely cure. I'll keep you posted. Nick
Nick

Nick, Heere is the thread Ray is talking about,
http://www2.mgcars.org.uk/cgi-bin/gen5?runprog=mgbbs&mode=thread&access=&subject=71&source=T&thread=2005061716355821650

I don't think wiring is your problem. I think it is the depth adjustment of the 3/4 switch, that was the cause of a very similar problen I had several years ago. To ajdust the switch I unbolted the transmission mount and let the transmission rest on the crossmember and removed the switch and one shim, replaced the switch and the remaing shim to correct the problem. Hope there are some shims under your switch. It's a tight fit and you may have to remove the shifter and the oval plate on the tunnel to reach the switch. If it does turn out to be a wiring problem you should be able to do that with the shifter out and transmission lowered.

Clifton
Clifton Gordon

The switch sometimes comes unscrewed so if you can get to it, and it's very tricky, simply retightening may well do the job. (Make sure you get the correct switch as the reverse light one is identical.)

I've just rebuilt my gbox and overdrive and took a few minutes to study these switches with the box out and a multimeter on the terminals. I bought 2 new Lucas ones (reverse and OD) and checked the 2 fitted which were also both Lucas and of different date codes and designs.

So, 3 types and ages. All 3 worked fine and coincided exactly with the amount, in turns from fully home, that they needed to be fitted. In all cases 1 fibre washer gave the correct pin depression and I used a drop of Loctite to secure them as fibre washers compress with age/heat and cause loosening.

Rich

Hi,

I had the same problem a while back. I had to hold the shift lever back and to the right in order to keep the od from dropping out and shifting me down to 4th. It was especially bad at 70 mph+ . If I hit a high spot in the road then immediately a low spot, the od would disengege then reengage. Maybe the switch needs adjusting as well. However I recently had occasion to remove the leather boot surrounding the shifter, and realized that the boot had been installed in such a way as to cause it to force against the shift lever a bit. It was putting just enough pressure against the shift lever to force the lever forward almost to the point of going into neutral. I guess that the connection between the switch and the business end of the lever inside the box is barely touching. Going over a hump in the road at speed would throw the car upwards and cause the weight of the trans to be neutralized and the gear shift boot would exert just enough forward pressure to cause the od to work intermittently. I rearrangeed the shift boot and the problem went away. I suppose I really should drop the trans and do the proper repairs, but it works well for now. FWIW Alyn
Alyn

Nick. I agree that the switch should be the first thing checked. I just installed a rebuilt overdrive on my daughter's 77. It had been rebuilt at John Twist's "four syncro transmission/overdrive" workshop. John found some bad parts which he had to order so, the box was left with him, he tore it down again and installed the new one way clutch, then tested it, including the switch, before returning it. I tested both switches before mating the box to the rebuilt engine and installing them. In the car, the shifter did not move quite as much as it did outside of the car, the twice tested switch needed a shim removed and, then, all was fine. Check the switch first, as Clifton mentioned, a single shim seems best in actual operation even if it works with two shims off the car. Les
Les Bengtson

Les, you're correct. All the info I found says 1 fibre washer is standard. When I checked (as posted above) it confirmed this.

Two washers will still work but the switch plunger is then only depressed a few thou past its switching point so wear could become an issue. By contrast, 1 washer still allows more movement in the plunger (it is not depressed too far so that it bottoms out) and means the plunger is switched well past the point it needs to be, taking out any wear/tolerance problems.

Rich

This thread was discussed between 13/06/2005 and 26/06/2005

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