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MG MGB Technical - New wiring harness

BGT 1969

I will hopefully be soon approaching, well within the bext 12 months, be putting new wiring into the car. I have researched replacing with a stock loom or using tne bespoke upgraded versions available from whilst I like the idea of the relays to take the majority of the load the price including postage is getting close to double the price with current exchange rates.

If I was to go down the route of using a replacement stock loom what other relays would you recommend in addition to the headlights?



Tony Brough

Horn, heated rear window, electric radiator fan if you have one, and brake lights. These are the high load items that immediately come to mind. Although having said that many fan kits come with a relay included.

I have the Advance Auto harness in my BGT V8 and it is a superb quality item. The cables are much better quality than the standard loom, although the colours don't have the same apearance as the original, if originality is important to you.

Mike Howlett

I'm planning to use relays for headlamps, horns and brake lights.

Edit: Mike beat me to it!
Dave O'Neill 2

As Mike says HRW is advisable, certainly with the standard harness, as it is trying to take 14 amps with the engine running. The factory did fit an HRW relay to 4-cylinder cars from the start of rubber bumpers until the 1977 model year, when the provision of an ignition relay took over that role as well as powering various other stuff. Before that it is powered with all the other green circuit stuff off the fusebox, which results in current near the max for the fuse, and pulls the voltage down for everything on that circuit.

Testing my 75 V8 HRW (powered from the green circuit) I found I was only getting about 7v at the screen with the engine running. Amazingly the twin fans were also powered off the green circuit originally, even though they had a relay, then I think there was a retrospective modification to power them off the brown via the relay.

Powering my HRW off the brown via a fused relay boosted the voltage to 10v and noticeably improved the clearing rate. My relay was in the engine compartment, but you could fit it at the rear of the car powered direct off the battery and boost performance even more. You can also use a standard relay and in-line fuse, or if in the engine compartment power it off the purple circuit which is already fused.

Make SURE the relay is wired so that it can only operate with the ignition on, which if you use the original HRW switch circuit should be the case.

Thanks for the replies and opinions.

Mike,does the Advanced Auto harness cover all the ares you mention or did it require any additions?
Tony Brough

Tony, if you call up the instruction sheets from their web site the only thing missing is a relay for a heated rear window. I guess this is because the GT wasn't sold in the USA after 1967. My car is a V8 converted GT with fuel injection. Since the injection has it's own fuel pump relay, I used the fuel pump relay on the Advance Auto kit to run the HRW. Other than that I used it as supplied.
Mike Howlett

According to Clausager GTs were sold in North America until the start of the 1975 model year, i.e. including the first rubber bumpers. May have always been optional though, it became standard on UK cars from 1972.

Oops, Paul is quite right. Where did I get 1967 from?
Mike Howlett

Mike - still dizzy after West Ham won the '66 World Cup ?? Yours, John.
J P Hall

I have bitten thw bullet and ordered the Advance Auto harness. From Mikes picture it looks like that is winter sorted.....
Tony Brough

This thread was discussed between 12/06/2017 and 19/07/2017

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