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MG MGB Technical - Fusing early D overdrive circuit

I checked the archives and see a 30A fuse mentioned if you want to protect the white overdrive circuit , but I think this was an LH has anyone tried a 30A on a early D? It looks as though the fuel pump is on terminal 3 of the fusebox on the circuit diagram as well, is that right or did I go cross eyed reading the workshop manual diagram? Has anyone successful fused that?
There are only 2 white wires on that terminal and one of them must be the switched volts via the ignition switch. 5 minutes with a meter will find out of course.The circiut shows 2 wires as stadard plus a dotted 3rd wire for the overdrive when fitted.
Stan Best

The white circuit is not fused at all. Cheers - Dave
DW DuBois

The D-type overdrive circuit uses two feeds off the white circuit - one for the manual switch and relay from the fusebox, and the other for the solenoid from a bullet connector in the mass by the master cylinders. It's the relay that operates the solenoid.

The first one is teed off the live unfused side of the fusebox from the ignition switch.

The schematics show the second one coming off a 4-way bullet connector that only has two wires, the other wire being looped into a 6-way bullet connector. This 6-way has five wires in it - one from the ignition switch, one to the tach, one to the fusebox (as above) all in the main harness, one to the fuel pump (rear harness), and one to the overdrive sub-harness.

The dotted white off the fusebox on my Leyland schematics is for the HRW on GTs, not the OD.

To fuse both OD whites (without doing some wiring changes) you would need an in-line with male and female spade connectors where the white joins the dash switch, and another with bullets both ends where the white goes from the main harness into the overdrive sub-harness.

The D-type solenoid has a two-stage system, whereby a low-resistance winding taking about 17 amps pulls the solenoid in, then a 2 amp winding holds it there. A standard 17-amp rated, 35-amp blow fuse is fine for both positions.
Paul Hunt

Clarification - the wiring described above applies to 65 to 67 cars.

For 62 to 64 cars the white for the manual switch comes direct off the ignition switch, and the white for the relay contact through to the solenoid does come off the fusebox.

But despite that the fusing info is the same for both.

It sounds like you have the later car, as without an OD there are only two whites on terminal 3 of that model, an OD makes three.

The earlier model has three whites at the fusebox without an OD, an OD makes four.
Paul Hunt

Thanks Paul, perfect. I got one 30 A in line blade fuse and holder and will need another. My car is the later one, my manual was given to me in about 1970 when a guy I worked with sold his MGB, an Iris Blue pull handle car, and yes he knows what a mistake that was. This accounts for why it doesn't line up with my cars wiring as mine was built Dec 66. I think I will fuse the circuits at 30A, a gross fail will pop the fuse but they should not give nuisance opens with inductive spikes.
Stan Best

Hi Paul, there are only 2 white wires on the T3 of my fusebox, the o/d is factory fit. I removed one and turned the ign on, the other went live and the overdrive was completely dead, these wires are lighter gauge than the ones feeding the solenoid.I replaced the white wire on T3 and removed the heavy gauge White one from the relay contact. Now I could control the relay but the solenoid didn't work, so yes the relay coil is fed from the fuse box I put an inline fuse holder (bayonet style, takes the same 35A glass fuse as the fusebox) between the fusebox and the white wire which presumably goes off to the overdrive switch. I crimped an insulated female spade and swapped it to the top of the 2 terminals, I have had sparks when I have been trying to locate the bonnet stay in the dark and it contacts the live side of the fusebox. I will be "borrowing" some heat shrink on Tuesday for the bare spade which is now "flying" temporarily with some masking tape on it and while I am there will solder the bullets on the heavy current inline holder using a luxury temperature controlled soldering station :-) then that goes onto the point where the heavy gauge white goes into the overdrive loom near the master cylinders and that's it.
Thanks again for the info and this should archive how to do this job.
Stan Best

This thread was discussed between 25/09/2015 and 27/09/2015

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