MG T-ABC

During the pace lap at Lake Garnett, MGTC 0820 ("Boxley") suddenly lost electrical power. The ignition quit and the fuel pump stopped clicking. Nothing happened when the key was turned. We had to be towed off the course to the pit place.

This outage occurred after a total frame off rebuild. This is the 3rd rebuild after 42 continuous years of pleasurable companionship with "Boxley".
The wiring was re-installed from the earlier (about 1985) Moss wiring harness. This was a still excellent Moss version, and great effort and expense was taken to reweave the cloth covering, but with ballistic thread, in the original colors. The Moss harness has its own schematic and connections that are altered somewhat from the MG factory diagrams. The harness was professionally installed at the time in accordance with Moss instructions.
At the same time, the polarity of the circuitry was changed from Positive ground to Negative ground.
At some unknown time in the past the original Lucas RF91 voltage regulator was changed for an RF95 model. Both have 9 posts so no change in the harness seemed to be needed at the time of its installation.
Before its recent reinstallation, the RF95 voltage regulator was checked as OK.
After rebuild and before reinstallation, the generator (dynamo) may have been switched again in polarity. The D and F wires were connected back to their original posts.
No problem was ever noticed about any deficiency with the new ignition coil. The battery is new and was found to be fully charged at all times even after the outage.

Prior to the outage, the only electrical problems noticed were that the ammeter needle worked backward and the clock did not self-start when connected – it had to be shaken to start, but then it kept time and ticked audibly.

During troubleshooting of this outage, the fuel pump was tested. Initially, when bench tested, it seemed not to work but then suddenly came back to life, with all its original strength. The pump seems to pump equally well whether negative or positive leads are hooked to the main terminal.

Further checking determined that the ignition dashboard warning light does not go out with the key “OFF” (Found a forum posting that this can be a serious sign of a short circuit in the charging circuit – have not yet tested the voltage regulator for short circuit until further information from this forum is obtained.)
TC’s Forever also mentions that if the charge indicator (ignition warning light on the dashboard) stays on when engine is running to check the cutout of the voltage regulator, which is on the right side of the regulator – have not checked this as I cannot get the car to start.
(A forum posting says to check for cutout points action at around 1000 rpm which I have not been able to check as I cannot get the car to start.)

1 Should I take the voltage regulator off for it to be retested?
a. If found to be defective, should I replace with an electronic negative ground RF95?
b. Ditto, if going back to an original RF91 voltage regulator, what are some pointers for enduring its life? Does it require Positive grounding?
2 How to determine whether the generator (dynamo) is properly polarized?
3 What other wiring and electrical pitfalls must I try to avoid?
Yours truly, Robert Griffey and colleague Boxley (MGTC0820)[/youtube][/size]

Hi Robert, if the red lamp stays on when the key is off, then the control box is probably in need of attention. This is a serious fire hazard so disconnect the battery until you're ready to address the situation. I would consider rebuilding it. Chances are, the points in the box are stuck closed so current is free to constantly flow. You can work them by hand to see what happens. The original control boxes are sturdy and reliable, and do not care which polarity you run through them. Electronic versions are polarity-sensitive so if you go that route, be sure to order the correct one and never connect it backwards.

Have you checked for voltage at the pump and for a blown fuse? I've seen pumps stick when they overheated so maybe a power issue related to the control box problem caused this?

Whatever polarity you go with, be sure to polarize the dynamo and control box. Or in the case of electronic control box, just polarize the dynamo.

Steve, Thank you very much because I think the voltage regulator has been shorted out when the polarity was reversed by mistake. Will remove it and try to repair in order to attempt to resolve the problem, rather than try another unit immediately.
Hope to be able to report the results

Almost sounds like it could be something going on with the ignition switch. Are you running points or electronic ignition?

This site has a new article in the resources section all about the TC electrical system. Check it out.
http://www.mg-tabc.org/library/mgtc-ele ... rcuits.pdf

Thank you for the link to another really good article about the TC electrical system. Got to reading it and everything else that could be found. Hope we now have a better understanding of what goes with the components and wiring.

Now a Long interval of winter delayed investigation of some things here, but finally have some answers.
Long and short of it: The generator (dynamo) was polarized for positive ground but operating in a negative grounded system. Had thought the polarity of the dynamo was negative and ran the car that way for awhile with no ill effects. Then apparently this made the fuel pump stop -- therefore the car stopped.
Steps to diagnose started with the fuel pump. This pump is original with no diode. Tested it and found it seemed "locked" -- like the points were together, so bumped it. It started again. Tested the fuel pump with both negative and positive polarity and this seemed not to make any difference in pump performance.
Wondered whether coil polarity had something to do with the car stopping, but found that reversal of the leads on the "CB" and "SW" terminals would not disable the coil, just make it less intense. Anyhow, used another coil just in case. The car would start and run, but the ignition warning light would stay on regardless of how high the RPM would go.
Then removed the voltage regulator (control box) to check it out. Could not see anything wrong with it but cleaned the points and reinstalled it. Still could not get the ignition warning light to go out.

Finally was able to get the car booked into an electrical shop that specializes in older auto electrics. There the fault with dynamo polarity was found. Re-polarized the dynamo to negative earth. But the ignition warning light still would not go out. Testing the voltage regulator control box suggested it had a fault, so elected to replace it.
Found an old R91 control box that could be converted to electronics, so sent it to Hickory North Carolina.
When it was returned and installed, everything worked. !!
Now hope this problem never happens again.

Looking forward to going through some of the pointers that have been shown here and checking more wiring and switch connections in the future