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MG TD TF 1500 - Coil needs replacing?

My 1954 TF will not start. I checked the voltage at the input terminal,SW,to the coil and it is 11.6. The voltage at the output terminal, CB, is only 0.6 volts. I assume that means the coil is shot, but the workshop manual says to disconnect the wire from the terminal before checking. Does that matter? Also, it says 'if the correct reading is given, remake the connection to the coil terminal' meaning it is OK I assume. What is the correct reading? Not 0.6 is it?

Ron
Ron Coates

If the points are closed the voltage should be close to zero.
Is 0.6 low enough or is the coil open?
You should follow the WSM.
They are trying to see if current is flowing through the coil

Jim B
JA Benjamin

Tell us more about how and why it will not start. When did it last run? What have you done to it lately? Does it turn over? Does the fuel pump run?

Tom Lange
MGT Repair
t lange

Quot
"I assume that means the coil is shot"
End Quote;

To be a bit more specific, that terminal connects to the points, the points connect to ground. If the points are closed AND the coil primary is NOT open, you should measure almost 0 volts.
If the coil primary is open, you will also measure almost zero volts.
If the coil is open and you are using a digital meter, you can sometimes measure a small voltage.

(I prefer Analog meters here.)

The WAM says to use a test light, between the coil and the wire that goes to the points, With the points closed, the lamp will light. This indicates the primary is not open, however measuring the actual current would be better.

Another test is to place the high tension lead from the coil quite near to the block. open and close the points with an INSULATED tool. you should see sparking.

Also on the earlier TD's there should be a ground wire from the distributor to the block. Do you have one?

Jim B
JA Benjamin

You have current at the input to the coil so why not fit a temporary wire from the CB terminal to the contact breaker and a new piece of high tension cable from the coil and see if this sparks to earth when the engine is turned over. It may be worth also checking the contact breaker by connecting the distributer trminal to one side of a bulb and the other to the live battery terminal. As you turn the engine over the bulb should glow and turn off repeatedly. I have seen points assembled without a fibre washer and this permantly grounds them.

Please let us know how you get on.

Jan T
J Targosz

I drive the TF fairly regularly and went to drive it expecting it to start. The fuel pump ticked like it always does, but the engine would not fire at all. The ignition light glows. I flattened the battery trying to start. I have charged the battery up again, but have been busy since. I will disconnect the lead from the coil CB to the distributor as the WSM says and check the voltage at the coil. I guess disconnecting the lead takes the issue of points out of the picture, but I am not sure what voltage to expect at the CB terminal.
Ron Coates

the WSM wit the test lamp is checking CURRENT through the coil.
Wit the lead disconnected you will measure 12V or so at the terminal.

Jim B.
JA Benjamin


I completed the check of the low-tension circuit. The test lamp between the coil and the distributor flashes when the points are closed and goes out when open.

There are 12 V at CB on the coil with the lead disconnected. 12 v across the breaker points indicating condenser OK. I conclude that the low tension circuit is in order.

I disconnected the fuel line at the carburetor and turned on the ignition. Fuel was pumped out of the line.

I removed a spark plug and it was pretty black. I cleaned and set the gap and it sparked when grounded when the engine was turned over. I removed the other three plugs and they were all black. I cleaned, set the gaps and replaced them all.

It tried starting it with choke. It turned over more than usual and then fired, started and it chugged with the typical over rich beat. I pushed the choke in and it went to the normal beat. I took it out and I went maybe 5 miles. It ran fine at all speeds and a steady enough idle.

I obviously need to weaken the mixture, but the experience was not very comforting. I really didn't find anything amiss except the plugs being black. I would have thought that the engine would have at least fired even with dirty plugs.

I appreciate all of your comments. This is a great site with very helpful people. Does anyone have any comments?
Ron Coates

This thread was discussed between 14/12/2018 and 15/12/2018

MG TD TF 1500 index

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