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MG TD TF 1500 - Starter Switch Revisited

Last night I had a revelation.

This past season I have been having a difficult time starting the car. It would crank but not fire up. On occasion, as I released the starter pull cable, the car would start. I never bothered to analyze this phenomenon. I now have a theory as to what was happening.

Picture the construction of the starter switch. The plastic base has the two screw terminals riveted to it. (See photos in my prior posting). Each screw terminal has a protruding metallic flange as part of its construction. When you pull on the starter cable a metallic washer presses against the two flanges completing the circuit. I believe that over the years, and through many cycles, the mechanical force of pulling on that washer and thus applying force to the two flanges on the screw terminals has caused a camber to the screw terminals. Heat on the plastic base could make it slightly flexible. During the moment of pulling on the starter cable, that force would cause the screw terminals to camber in and cause a short with the casing. Thus not enough voltage at the spark plugs to start the car. At the instant I would relax the starter cable, the short would stop and contact would permit the car to fire.
I believe that over time it became easier and easier to create a dead short when I pulled on the starter cable.

This theory would explain exactly what was happening recently.

I am confident that when I put the new switch in the car will once again be easy to start just the way it used to be.

I will keep you posted.

The bottom line….. Don’t pull too hard on the starter cable.

Mort
Mort Resnicoff

Mort,

You can easily check your theory before waiting for the new switch. All you need to do is, with the ignition on, manually short the three switch leads together, then see if you have correct starting. If so then that was the problem, if not then you still have your strange starting issue.
However you need to be VERY careful not to touch any metal parts with the hot wire from the battery. It may be safer to get a second person to help you do this.

John
J Scragg

I'm not in a rush. I have to put in the new switch anyway. It should be here in a couple of days.
Mort
Mort Resnicoff

Mort the only observation I have made about starter switches is that my original switch, one that I presume was in the car when it left the factory, was working fine when I stored the car in 1972. However when testing it with an ohmmeter about 8 years ago it was inoperative. I've not been able to explain that. Cheers
Peter TD 5801
P Hehir

Looks like a Lucas part number on the case, together with the rotted rubber makes it very old. Every time you pull the switch, there is an arc/spark with starts burning the copper disk and contacts. Eventually resistance builds and it fails. From sitting, it likely oxidized. I'm amazed that yours worked at all as fried and rotten as it is. George
George Butz III

Another theory to explain the phenomonom.
The battery is weak (or the wiring is).
So when cranking, the voltage drops and it is not high enough to produce a strong ignition spark.
Releasing the swith, the engine can still turn on its own inertia for a bit of an angle, but the voltage on the primary coil rises immediately. If this happens clos to a compression stroke, occasionnaly, it may start.

Laurent.


LC Laurent31

Mort, there is a basic electrical function that might be the answer to the firing up on releasing the switch, it's written as V=L di/dt. It's what creates the spark at the plugs. It says that voltage is created by an inductance (L) proportional to the rate of change of current through it (di/dt). Points opening in the distributor are one example. We all know that the current that flows through the wires into the starter is a significant number of amperes to turn the engine over. This current is flowing into windings in the starter. These windings are coils, i.e., inductances. When the starter switch is released the current drops from this big number to zero in a very short time (BIG I/ little t). If this L di/dt happens to coincide with the distributor rotor aiming at a spark plug, and the polarity is right, I can picture this added voltage causing a spark to occur and causing vroom.
I can think of a lot of arguments against this, and I'd never thought of it before. But, I wish I had an oscilloscope to play with. Bud
Bud Krueger

I can tell you from taking apart solenoids on other starters, that the washer that makes contact gets electrical arc burn across it. These can warp the washer and also cause corrosion so contact is not consistent.
Bruce Cunha

Of course we will never know the exact cause but having fully examined the physical evidence I am convinced it was a mechanical issue. By mechanical I mean that pulling on the starter cable eventually caused enough camber to short out the screw terminal with the casing.

I have replaced the switch and all is well. With the old switch, the cranking RPM's were lethargic and draining. Now, when I pull on the starter cable, I get high RPM's and a nice peppy turnover and a quick start.

Thank you all for your suggestions and thanks again to our intrepid bulletin board manager. Having this forum to discuss issues is worth it's weight in gold.

Mort
Mort Resnicoff

As stated above....to test your theory...just jump the two switch terminals with a large diameter wire...like your jumper cable....with the key on and see if it starts.
If you happen to be running a pertronix, the you can skip that as I have already been there. Same symptoms you describe. Turns over but won't fire until you release the starter.


As your battery ages it can have enough amperage to turn the motor over but the voltage drops below 12 volts when doing so. The Pertronix won't fire with less than 12 volts. Release the starter and the voltage jumps up above 12 volts and the still spinning engine fires and takes off. The solution is to either replace the battery or replace the Pertronix with either points or a Crane ignition system.


Just my two cents.
L E D LaVerne

I hear you LaVerne but I am 100% convinced it was the bad starter switch. My battery is relatively new and was well charged. The terminal on the battery side of the switch was arcing to ground through the case as I pulled on the starter cable. There was was enough voltage to turn the starter lethargically but not enough to start the car.
As I would release the starter the gap between the terminal and the case would increase and the full voltage would go to starting the engine. For just a moment, as I was releasing the pressure on the cable, the gap would widen but the internal spring kept the electrical contact just long enough to start the engine.
Now that the new switch is in, there is no problem starting the engine. I turns over quickly and fires right up.

Just my three cents.

Mort
Mort Resnicoff

This thread was discussed between 01/02/2020 and 05/02/2020

MG TD TF 1500 index

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