Intermittent power loss 1975 rb roadster

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Vic Butler
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Re: Intermittent power loss 1975 rb roadster

Post by Vic Butler » Wed Sep 05, 2018 8:51 am

Actually Paul there are 2 relays which were fitted to the 1976 onwards rubber bumper B's with the forward mounted radiator.
When the starter is operated the ignition relay which supplies the ballast resistor power to the coil is switched off and the starter relay which supplies 12v to the coil is on. When the engine starts and the key is released the starter relay is switched off and the ignition relay is switched on supplying power to the coil via the ballast resistor. I don't know why it was done but 2 years ago on MOT day the ignition relay failed.
Just off for an MOT in a few minutes.
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine

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Charles Farran
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Re: Intermittent power loss 1975 rb roadster

Post by Charles Farran » Wed Sep 05, 2018 8:58 am

"The ignition relay's purpose is to relieve the ignition switch of the burden of carrying the total current of all the components that require to be switched on & off with the ignition.The decision to add the relay coincided with the introduction of the electric radiator cooling fan that is a particularly high current device. " The above is a comment made in Rick Astley 's book "MGB Electical Systems " which is a useful reference source.He said that the ignition relay was fitted from about mid -1974 onwards. (Whether this was for all markets i am not sure).
Cheers, Charles
1980 Roadster

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Charles Farran
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Re: Intermittent power loss 1975 rb roadster

Post by Charles Farran » Wed Sep 05, 2018 9:05 am

The white green feed from the solenoid is on a tab at the back which is isolated from the starter motor after cranking stops to stop a back feed from the ballasted route otherwise the starter motor/ solenoid would be energised during engine running.
Cheers, Charles
1980 Roadster

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Paul Scott
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Re: Intermittent power loss 1975 rb roadster

Post by Paul Scott » Thu Sep 06, 2018 3:24 pm

I hope the MOT went Ok Vic!!

I have now got my new bits and bobs.

New Earth wire fitted inside Dizzy.

New Fuse box fitted - need to renew some tags.

New tag fitted on condenser feed lead - tried to give this a wiggle with megger in bleep mode. No break found - audible note did not flutter. This could be easily renewed from condenser join to coil.

New tags fitted to original ballast wire & power feed wire.
New Ballast added with fresh lead.

Hope to get finished tomorrow

Regards
Paul
1975 MGB Roadster
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Vic Butler
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Re: Intermittent power loss 1975 rb roadster

Post by Vic Butler » Thu Sep 06, 2018 4:18 pm

Yes thanks Paul. It sailed through with no advisories.
I have now disconnected the vacuum advance and advanced the ignition. Next is to richen the mixture one flat at a time on the adjuster nuts. Road test to follow but probably not until next week.
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine

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Paul Scott
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Re: Intermittent power loss 1975 rb roadster

Post by Paul Scott » Thu Sep 06, 2018 7:07 pm

Glad to hear the car is all good Vic.

I've just replaced vacuum hose with a complete rubber hose. I changed the small rubber ends on the original pipe but it appears the bent one doesnt seal properly. So although it doesn't look correct I'm confident it will be a better fix.

Can you confirm fuses in the fusebox? I have 3 fuses top 2 are 35A bottom one 10A. Is this correct?

Green stuff on the back of the original fusebox. Red spade terminals pretty ropey, so I will replace those tomorrow. Going to change the green spades on to the fusebox too. Then they will all be new.

Thanks again for you time & knowledge.

Regards
Paul
1975 MGB Roadster
Webguru for MGB Register

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Charles Farran
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Re: Intermittent power loss 1975 rb roadster

Post by Charles Farran » Thu Sep 06, 2018 10:33 pm

All fuses in the fuse box should be 17 continuous (35 Amp blow rated) . Also the line fuses underneath. It looks like a previous owner has used a different rated one at the bottom. If the bottom one in the has also been used to power a radio you should also ensurre a separate line fuse of say 10 amp is used on its' own power wire.
If you are looking at the rest of the harness add a line fuse of 5 amp to the power lead cicuit to the overdrive as that is permanently live once the ignition is on .
Cheers, Charles
1980 Roadster

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Charles Farran
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Re: Intermittent power loss 1975 rb roadster

Post by Charles Farran » Thu Sep 06, 2018 11:39 pm

In my previous post i refered to line fuses below the fuse box. As your car is a 1975 model you may not have these as they were gradually added in the last few years of production. (There are 3 on my car).
Cheers, Charles
1980 Roadster

Vic Butler
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Re: Intermittent power loss 1975 rb roadster

Post by Vic Butler » Fri Sep 07, 2018 7:54 am

Paul I 've checked the wiring diagram again in my official workshop manual and your car has 4 fuses rated per Charles' post. Pre 1970 cars had only 2. Why you have 3 with one 10A sounds like someone has been messing around with the electrics.
Your car should have a starter solenoid relay which is located by the fuse box.
Like Charles' car mine has 3 inline fuses. These were fitted to the later rubber bumper cars with the forward mounted radiator.
What is this Bentley diagram which keeps being mentioned?
. Vic
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine

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Charles Farran
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Re: Intermittent power loss 1975 rb roadster

Post by Charles Farran » Fri Sep 07, 2018 8:41 am

The wiring diagrams i have been refeing to are from a series of some 30+ for MGBs MGBV8 & MGC freely available on the internet in one place that include some produced by Bentley ( possibly a coincidence that the same name as a famous car name / manufacturer prior to VW takeover) & Haynes). These are much easier to read than those in the repair manual (which i have as well) as they are in colour. As we are aware there are mistakes / omissions in all of these!
The ones on the Internet can be blown up in size & printed off. ( I keep one in a plastic sleave under the passenger seat for easy reference.
Cheers, Charles
1980 Roadster

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