1950 MK VI Dynamo Ground Cable?

JGwynne · Joined: 20 Sep 2021 Posts: 21 Location: Florida, USA

Currently troubleshooting my 1950 MK VI that abruptly shut off after driving over a really bumpy portion of road. I found a cable attached to the back of the dynamo and the other end isn't connected to anything. Wondering if this should even be here, and if so, what is it supposed to be connected to? (Image attached)

Also, I measured the continuity between the positive and negative terminals for the battery (without the battery connected) expecting it to be 0 and it read 38ohms. This is concerning, is it not? Seems like this could be a symptom of whatever got knocked loose and caused the car to shut off when it hit the bumps. Thoughts?

Thank you

Martin Webster · Posted: Sun Oct 22, 2023 10:06 pm Post subject:

J,
The larger terminal in the centre of the image is the armature connection and the smaller one on the far right is the field connection.
The lead from the armature connection goes to the voltage regulator and should connect to terminal “D” and the field connection goes to terminal “F”. The connection you have highlighted is the earth connection which should be attached to the chassis. The fact that the earth became disconnected whilst you were driving would only stop the dynamo operating normally but shouldn’t stop the engine. Perhaps an ignition connection from the coil or distributor has a discontinuity which needs investigating.

Martin

John Murch · Posted: Mon Oct 23, 2023 7:24 am Post subject:

Regarding the battery situation, would this not suggest that it is an ageing battery with a build up at the bottom of the plates resulting in the continuity you measured. Perhaps an early sign that a replacement is due?

Martin Webster · Posted: Mon Oct 23, 2023 6:09 pm Post subject:

I agree John, it’s almost certainly the battery. The dynamo possibly lost its earth connection with the rough ride and if the battery was on its last legs then there was nothing to power the ignition.

Martin

JGwynne · Joined: 20 Sep 2021 Posts: 21 Location: Florida, USA

JGwynne · Joined: 20 Sep 2021 Posts: 21 Location: Florida, USA

[quote="Martin Webster"]I agree John, it’s almost certainly the battery. The dynamo possibly lost its earth connection with the rough ride and if the battery was on its last legs then there was nothing to power the ignition.

Would the car suddenly die like that though? I suppose the connection could've came loose before the bumps and it shutting off during the bumps was a coincidence. But it's not like I hit some bumps, then later when I tried to start it, it wouldn't. It died when I hit the bumps. I guess maybe something else came loose. The continuity between the negative and earth cables is suspicious to me. Think I'm going to investigate that further by taking the fuses out one by one and narrowing that down.
Thank you for the help.

John Murch · Posted: Tue Oct 24, 2023 5:26 am Post subject:

Apologies, I misunderstood where you had measured the resistance.
I suppose that where the earth has come adrift with the dynamo cable there was another wire going to the same point which is now also loose and caused to failure to proceed, but can't quite think how that results in a resistance as described,
taking the fuses out one by one seems an excellent start.

Christopher Carnley · Posted: Tue Oct 24, 2023 11:47 am Post subject:

Do you know a mobile, peregrinating, auto electrician?

JGwynne · Joined: 20 Sep 2021 Posts: 21 Location: Florida, USA

Martin Webster · Posted: Thu Oct 26, 2023 3:53 pm Post subject:

The charging socket is to the right of the steering column on the instrument panel. It can be used for charging the battery using a two pin socket which is connected to a small charger. It avoids having to lift the panel under the seat to connect the terminals and is very convenient. It also powers the under bonnet torch when the cable is unwound.

John Robins · Posted: Sat Oct 28, 2023 6:00 pm Post subject:

In addition, do I recall correctly that some change was made to the dynamo earthing connection in the (vain) hope of reducing radiator sludging up?

Cheers all
John Robins

Geoff Allan · Posted: Fri Nov 10, 2023 10:43 am Post subject: mk vi dynamo earth

Your 38 ohms measured across the battery leads would lead to a current of 0.3 amps being drawn and is unlikely to fry anything. (ohms law) Have you checked wires to the distributor and the capacitor connections as these could lead to a sudden loss of sparks. Regarding the radiator earthing Rolls tried various ways of doing this to prevent sludge in the radiator, but the only sure way is to reverse the battery polarity and make the car negative earth. The information of how to do this is available on line from various web sites and it is a relatively easy operation.

Geoff Allan · Posted: Fri Nov 10, 2023 10:46 am Post subject: mk vi dynamo earth

Your 38 ohms measured across the battery leads would lead to a current of 0.3 amps being drawn and is unlikely to fry anything. (ohms law) Have you checked wires to the distributor and the capacitor connections as these could lead to a sudden loss of sparks. Regarding the radiator earthing Rolls tried various ways of doing this to prevent sludge in the radiator, but the only sure way is to reverse the battery polarity and make the car negative earth. The information of how to do this is available on line from various web sites and it is a relatively easy operation.

DuckweedDave · Joined: 14 Sep 2023 Posts: 1 Location: FL, USA

Did you get your electrical problem sorted? I'm up in Gainesville and may be able to help. Also I have a MKVI (There can't be many of us here in FL!) I will be converting it into a special so I'll have loads of parts available if you need them.