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MG TD TF 1500 - rea riddle, wrapped in a mystery, inside an enigma

My friend Pieter and I live about 1/4 mile apart and we both have and drive TDs- His TD10879 and mine TD25009. Both are bone stock except for the distributer/ignition systems. Pieter's is a stock distributor with a pertronix module and mine is a CSI distributer from Len. Both cars run great and are driven with spirit regularly. Pieter's voltage regulator seemed to be not working (badly burnt points) so we replaced it with a good VR spare that I had. In the process, we were checking the generator output by putting a VOM across from A1 to earth. With my old analog Tandy VOM we had no problem seeing the voltage do what it should do with a properly working VR. BUT, with three different digital VOMs all we could get on Pieter's car was a flashing screen. on my car both the analog and the digital meters worked perfectly. Pieter decided that the solution would be to put a condenser across the leads of the VOM. We tried a 22uF condenser from a set of points and it didn't work. We tried a big (several hundred uF) condenser from a big motor and the digital meter settled down and gave us the expected readings. The only difference between the two cars is mine (digital works) has the CSI distributor and Pieter's (digital doesn't work) had the Petronix.

Any idea why??? Thanks. Jud
J K Chapin

DVMs get irritated by varying voltage, since they have a short measuring cycle. Therefore, with the big capacitor, the reading gets smoothed and so stabilised.
The VR regulates the voltage output by quickly switching one relay on and off. Since it is a mechanical device, it is not that precise. So eben if both VRs work fine, the switching frequencies of the two VRs may be different and one just interferes stronger with the measurement cycle of the DVM. So it may not have anything to do with the ignitions.
On the other hand, the Ignitions may have similar effects, they generate spikes when they turn on and off that may have an effect on the DVM. The whole setup of the ignition with coil, capacitor, cables, plugs plays a part in that.
To locate the source, you could just swap the two VRs between the cars.
On the other hand, since all works fine, you could just leave it and drive ;-)

Rgds, Mike
Mike Fritsch

I've now found that just like Pieter's TD, my Pertronix equipped otherwise bone stock MGA also makes the DVM flutter. I understand about the DVM's sampling and the voltage regulator's chatter to provide an average of the desired voltage. What I don't understand is why my TD, otherwise identical to the two others except for the ignition systems, doesn't make the DVM flutter. I can't see how the ignition systems could be a factor and suspect that they are just a red herring. Nonetheless, they all run great and lots of driving will continue. We're fortunate to live in a pert of the world where open roadster driving is a year-round activity.

Thanks.

Jud
J K Chapin

Hm, here‘s a theory.
The Pertronix just emulates the points vie a switching device (transistor) triggered by a hall sensor. The primary side of the coil is switched similarly to the classic points and is directly connected to the +voltage generated by the VR. The switching on the primary side of the coil will induce spikes that irritate the meter
The fully electronic ignition possibly decouples the primary side of the coil from the rest of the car, so that the spikes stay „within“ the ignition circuit. Means the ignition could have some internal voltage stabilisation. Or the switching is smoothed electronically to generate a better spark.
Maybe…
Another theory is that the mechanically addressed ignition systems suffer from flutter due to mechanical play of the distributor. The CSI being fully electronic may be more stable and smooth.
Maybe..

Rgds Mike
Mike Fritsch

Just a remark. Ignition condensers are about 0.22 µF. Not surprised that it does nothing to smooth a voltage which impedance source is very low.

Laurent.
LC Laurent31

LC, Oops, what's a couple of decimals points among friends :-) Wow, that really makes the difference between the car capacitor and the motor capacitor significant. Thanks. Jud
J K Chapin

Digital voltmeters have issues on my bone stock TC if I get them too close to the generator. The difference in your cars could be as simple as the plug wires being more shielded on one than the other, a stronger versus weaker spark, etc.
Steve Simmons

Steve, plug wires. That's a difference (perhaps) that I hadn't considered. When I installed the CSI distributor I also installed new wires per instructions. That could well be the difference. Thanks. Jud
J K Chapin

This thread was discussed between 04/09/2024 and 05/09/2024

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