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MG MGB Technical - Overdrive in wiper switch

I am installing an od trans. into my 75b. I think I have an od switch in the wiper stalk as it moves in and out. Also I found an unconnected yellow wire near the fuse box going into the wiring loom. Using a test light I was unable to get power. Key on switch in or out still no power. Shouldnt there be power to that unconnected bullet conection? I suppose the next step is to trace it back to the ignition switch. Any other suggestions? Thanks for any suggestions.
kra Karl

The wiper switch pulled back and fore should control the OD in a 75, and it wqs fitted to all cars regardless of whether they actually had OD or not. The yellow wire coming out of the main harness by the fusebox is indeed the OD wire, this would connect to a yellow or yellow/red wire in the sub-harness that goes down to the gearbox, along with the green and green/brown wires for the reversing lamps.

If there is no voltage on the yellow wire by the fusebox with the *ignition* on and the stalk pulled towards you, you need to investigate the wiring at the switch itself. There should be a white feeding 12v in whenever the ignition is on, and that should be fed onto a yellow wire when OD is selected. There is a multi-way plug where the sub-harness on the switch joins the main harness down the steering column and both white and yellow wires pass through this. There are no other connectors in the yellow wire. The white wire comes from another multi-way plug behind the dash that supplies the ignition warning light, power to that comes from the inertia switch for the fuel pump, and power to that comes from the ignition switch. If the pump and ignition light glow power must be reaching this 2nd multi-way plug at least. Unfortunately if that power isn't reaching the white in the multi-plug by the steering column tracing it back to the 2nd multi-plug behind the dash is not easy, without pulling the dash forwards. If that is the cause of the problem it would be easier to pick up 12v for the column switch from elsewhere, and I'd suggest a green as that is fused, the white and yellow aren't and a short at the OD will burn the harnesses.
Paul Hunt

Paul
That was very helpful and a good explanation. I will attempt to trace these wires. If I can get power thru the white wire should I fuse the yellow wire with an in line fuse in the unconnected area? Or should it be fused farther down the wire going to the
transmission. Thanks again Paul
kra Karl

Paul
The white wire you mentioned is hot. I pulled the white wire and the yellow wire and connected them and qot power to the unconnected wire by the fuse box. I havent been able to get power thru the switch. That is surprising as I dont think this switch was ever used. Are these repairable? Thanks again. Karl
kra Karl

Hi Karl. I've not had one of these apart but I have looked at a dip and turn switch in some detail. These have open contacts which are capable of being cleaned and tweaked if they are out of adjustment, see http://www.mgb-stuff.org.uk/wn_electricsframe.htm and click on 'Indicator/turn switch'.

Ideally the fuse should be as close to the supply as possible, so protect as much *wiring* as possible. In every case bar one on the MGB the fuses are there to protect the wiring from burning up as a result of a short, and not protect the components the wiring feeds. In the case of the earlier dash mounted OD switch it is very easy to add an in-line fuse with male and female spades at the switch (I don't like cutting in to wiring) and this is what I have on my UK 73 roadster. In the case of the column stalk OD switch the first convenient location is where that yellow wire from the main harness connects to the wire in the sub-harness that goes down to the gearbox switches, this time using an in-line with bullets on each end and an extra 2-way bullet connector, and this is what I have done on my 75 V8, see http://www.mgb-stuff.org.uk/wn_gearsframe.htm and click on 'Overdrive Fusing'. The latter also shows my fuel pump fuse, which I also strongly recommend. Both my cars came to me with wiring damage as a result of a short at the pump.
Paul Hunt

Thanks again Paul. A lot of good information on your site. I am amazed at the knowledge and help available on this site. I have learned a lot. I guess you can teach an old dog new tricks. Quess its time to get up close and personal with the wiper/overdrive switch.
kra Karl

further update. After pulling the switch off and disassembling it I found the contacts a bit corroded. cleaned them and all is well. Thanks for all the help.
kra Karl

This thread was discussed between 11/01/2009 and 13/01/2009

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