Intermittent power loss 1975 rb roadster

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Charles Farran
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Re: Intermittent power loss 1975 rb roadster

Post by Charles Farran » Mon Sep 03, 2018 10:59 pm

Looking at the Bentley 1975 wiring diagram, unless you can confirm otherwise i think you only have a starter relay. The white wire from the ignition switch goes to a female bullet which then goes to the fuel pump & the second fuse from the bottom in the fuse box. A lead then goes from the same side of the fuse box (same tab) through the resistor to the coil. As i said before, worth checking the fuse box terminals are clean & also that you haven't got a poor contact at the female bullet mentioned above which is probably on the inner wing where the body harness joins the main harness which is a frequent source for many electrical problems.(Incidentally the other side of the second fuse is where all the green wires meet which power the indicators & brake lights which i think you said have played up , but not necessarily at the same time as your LT problem. It may be worth removing the fuse box & cleaning all the contacts & also check underneath for corrosion. I like many have replaced the fuse box with a new one , which is much simpler as direct replacements are freely available).
Cheers, Charles
1980 Roadster

Vic Butler
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Re: Intermittent power loss 1975 rb roadster

Post by Vic Butler » Tue Sep 04, 2018 7:10 am

Charles on the wiring diagram in my official workshop manual there is no started relay on the rubber bumper B's until the later ones with the forward mounted radiator.
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 » Tue Sep 04, 2018 7:23 am

Good Morning Vic & Charles,
I will try a few more tests before I change the Ballast resistor - when/If I run a new ballast power supply in I will be able to reinstate the original wire back with no damage to loom. I'm sensing you feel the issue will not lie within this ballasted cable.

"The distributor on your car should have the number 41610 as should the LE.
The tag on your carburettors should show FZX 1001." will this show it still has original types fitted?

I know about the problems with those lucas connection points as I had several crumble - I cleaned added some electrical grease and changed all of the 2&4 way connectors. I will check the fuse box as you suggest, if the 2 white leads are single tagged that could be a good point, But I have a feeling the 2 white leads are joined with just 1 male spade terminal. These White leads then supply the power to the green wires, which power the brake lights - indicators - ect.

I'm thinking I might also be able to find a weakness within the ballast cable if I use my Megger, sending a 500v test down the lead could show a break or make an arching noise.

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 » Tue Sep 04, 2018 8:28 am

Paul, the tags on the carburettors will show what is currently fitted. FZX 1001 are the correct ones. The distributor number is 41610 is suitable only for rubber bumper cars because the vacuum advance is taken from the inlet manifold not the air filter side of the carburettor.
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 » Tue Sep 04, 2018 8:41 am

Thanks Vic,
I will check them but I know vac advance is definitley coming from inlet manifold and HIF carbs are fitted.

Thanks again for yours and Charles' knowledge it helped me quite a bit last night with the 80 gt. I had my test meter in the right places and that did have an ignition relay. Fixed a few bad connections then a Spark was born!!
Having stood for 5 years it fired up within a few turns of the key

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 » Tue Sep 04, 2018 9:58 pm

Hi Vic,

At the risk of me being pedantic & if i am wrong i apologise but either yours or my eye sight is going , according to the wiring diagrams when there was only one relay it was a starter relay with the later second one being the ignition relay that was added to take the strain off the rest of the electrics!

As a further comment to Paul some years ago i had a similar problem with the earthing wire within the distributor (as picked up by another responder) & couldn't get my car to run consistently having changed the points & condenser. I was aware that the earthing wire looked decidedly frayed & tried to reattach it, when it had almost broken apart. In a moment of extravagance & being fed up with resetting the points bought a 123 Distributor the following day (i guess some 7 + years ago). The car has run beautifully ignition wise ever since. I know & respect that some purists like to stick with a traditional set up, but other than a different coloured distributor cap it looks the same as a 45D without any maintenance. I have of course kept the old distributor points & condenser so they could be put back on the car if ever needed - i just need to put in a new earthing wire!

Cheers,Charles
1980 Roadster

Vic Butler
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Re: Intermittent power loss 1975 rb roadster

Post by Vic Butler » Tue Sep 04, 2018 10:30 pm

Charles, I can only go by the wiring diagram I have and the one relevant to Paul's car doesn't show an ignition relay. Now I have to admit that I am surprised there isn't one because the relay would switch off the supply to the coil from the ballast resistor source when the engine was being started. As we know wiring diagrams are not always correct. The one specific to my car doesn't show the brake pressure failure warning light.
Unfortunately I don't know anyone near me who has an early rubber bumper B so I can't check.
. Vic
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine

Vic Butler
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Re: Intermittent power loss 1975 rb roadster

Post by Vic Butler » Tue Sep 04, 2018 10:32 pm

Paul, on the offside inner wing in the engine bay of your MG is there a black cylindrical or black square object with wires going to it?
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 » Tue Sep 04, 2018 11:00 pm

Vic,
On all the wiring diagrams (Bentley & Haynes) i have looked at where there is a ballasted system (75 onwards), the green & white wire goes directly from the back of the starter solenoid directly to the coil & not subsequently via any relay (albeit the starter motor/solenoid is activated by the starter relay). I believe that a 12V supply down this wire is only live when the starter motor is cranked . At that point i think both it & the wired ballasted route are actually live in parallel. (As the fuel pump shares the same feed off the ignition switch as the ballasted wire then it (the ballasted wire route) must be live when the ignition switch is in the "on position" & it seems presumably before the ignition relay was added on the later rubber bumper cars like mine).
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 » Wed Sep 05, 2018 7:32 am

Good Morning,

I can confirm there is no Ignition Relay fitted to my B - I know what you mean because I saw the 2 relays under the bonnet of the 1980 B I was looking at on Monday.

I need to confirm the fault I have occurring is not coming from outside the dizzy before considering swapping to electronic dizzy. If the fault does lie within the Fuse box or Ballast wire I would still have the same issue as far as I can work it out.

I plan to renew the earth cable inside the distributor between Condenser & Dizzy Body. (arrived yesterday)

Run a new lead to coil with separate Ballast resistor (arriving Thurs/Fri) - leaving the original in place & re-connectable.

I have a new Fuse box on the way also (Thurs/Fri). So I might change this also, although I don't believe this will help with this particular issue. (The White leads are unfused but supply the switched power source).

So hopefully changing these few parts, will fix my issue.

The Ignition relay from the Bentley diagram appears to have been added 1978 onwards, Any ideas why it was added?

Thanks again

Regards
Paul
1975 MGB Roadster
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