MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG TD TF 1500 - Lost smoke - have spark - won't start

Title says it all. I stupidly smoked the white wire that the WS N.22 wiring diagram shows running from fuse block terminal A3 to Coil SW. I had the coil end of the wire connected to the CB side of the coil. As soon as I turned on the ignition switch there was smoke around the fuse box and melted insulation.
I had just installed the John James' Cover Gasket and removed the distributor to help in the installation. While I had the distributor out I decided to reinstall my OEM points plate it place of my pertronix plate.
The points plate has a single (white/black) wire (from the points) that connects to the CB side of the coil. My Pertronix plate has a red (+12v) and black (ground) wire coming from it. The red is to connect to the +12 terminal of the coil and the black is connect to the CB terminal of the coil. I'm so used to connecting up the Pertronix plate that I automatically connected the wire from the points plate to the SW side of the coil. Turning on the key closed a current path from the +12 terminal of the battery through the ammeter to the Control Box A terminal through A1 to terminal A of the ignition switch through the IG terminal to the A3 fuse block to the white white wire (now connected to the SW coil terminal) to the (now closed) points in the distributor to ground. The white insulation on the 16 gauge wire gave up the ghost as white smoke until I could open the battery ground connector.

After kicking myself for my stupidity and thinking about our classic Lucas smoke joke I started tracing the white wires in the aforementioned wire path. I had to pull the dashboard back from its mount to see the end of the white wires behind the switch. It's a heavier gauge than the wires to the coil. No smoke got lost there. I cut the damaged wires out of the harness and replaced 'them' with a new 16 gauge white wire. So, now back to SOP OEM ignition wiring.

Slight problem - Lazarus' engine refuses to start!!
LUCAS' REVENGE!!! I took everything back to OEM SOP. Pulled the distributor, checked the points, removed the valve cover, checked the valves #1 and #2 to be closed when the pulley timing marks are at pointer - set for timing at TDC. Still no go. Hooked up my variable strobe timing light and watched it flash.
fuel pump is happy to click right along. Pulled off the top plate of the air filter and squirted some started fluid into the carburetors. NADA!!

Many hours later I called it quits and came awfully close to seeing if a good dose of Paddy's wouldn't fix it. (I'm out of Edradour.) Called it a night.

Woke up about midnight realizing that, just because my timing light flashes, it doesn't necessarily mean that that there's a spark across the gap of the spark plug in the head. I got of bed at about 01:00 and started writing this in hopes that one of you Tcar folks from down under might see this and tell me where to look next. Hope you don't mind my verbosity on this, but I'm frustrated. My bride of 66 years just shook her head when I got bed at 01:00.

Cheers, Bud Krueger
Bud Krueger

If you are sparking then ignition would seam to be OK. But did you check that when on TDC the rotor is pointing to the correct plug wire for that cylinder and thant you have not got it 180deg out.
B W Wood

B W, I couldn't tell you how many times I've checked that. I've never had a single sputter of ignition. I think that the 180 out configuration would give some raucous sputtering. Thanks for the thought. Bud
Bud Krueger

Is the coil sulking?
Ian Bowers

I think that I may have the answer, though not necessarily the solution. Some 70 years ago I started learning things about electrical stuff. One of them was the term 'relative to' when it comes to voltages. For instance, the 'high' side of our batteries are 12 volts relative to the other terminal of the battery, the grounded one. Look at our car's coil wiring diagram on page N.22 of the W.S. Manual. There are 3 terminals to the coil. One is labeled SW, connected to a switch that goes to the high side of the battery. Another is labeled C.B. (circuit breaker) that is connected to a switch of some kind connected to ground (earth). The third terminal is not labelled. It is the high voltage output of the coil when the current flowing through the portion of the coil connected between SW and CB is suddenly broken. The 'transformer' action between that portion of the coil and the portion that leads to the output creates the high voltage to cause the spark. BUT, that high voltage is 'relative to' the other end of the coil. If the 'other end' isn't the head of the engine there's not going to be a spark across the spark plug.

Now I have to figure out (and solve) how the coil lost it's reference to the engine block. (And,not lose any more smoke.) Bud (once upon a time Senior Principle ElectroOptics Engineer)
Bud Krueger

Ian, you've got me on another of those terms that have different meanings as as one crosses our oceans. I've never run into 'sulking' as a technical term. Primarily applicable to teen-aged offspring. But, my previous posting may be along that line. Bud
Bud Krueger

Bud, you are right! Sulking is the perogative of teenagers unable to do just whatever they want to do at that moment. As a result it is difficult to understand what or why they refuse any constructive interaction with reasonable adults!
Ian Bowers

The coil is connected to ground via the ground side of the points (I avoid negative and positive since I don‘t know how your car is wired).
The engine block has an earth strap that connects it to ground. Unlikely either one of those is missing.
When you pull a spark plug connector cap from the plug you should see it spark, either via the upper end of the spark plug or directly to the engine if it is close enough. If it does not do that, there is no spark strong enough to fire the engine.

Maybe the coil was fried internally from the short. Try another one.

Rgds Mike
Mike Fritsch

Bud, Usually when this unfortunate short to earth happens, the flat spring part of the ign. points gets fried as well leaving the spring useless at returning the moving arm of the points back in contact with the stationary contact, giving the occasional very weak spark or none at all. The ign.coil should be ok still as the short wasn't through the coil at all.

willy
William Revit

Bud,

I don't know if the Pertronix plate includes a capacitor, but the standard OEM plate will definitely have one.

If this capacitor has failed (with an internal short circuit), the points will be sort-circuited, allowing a continuous high current to flow through the system as you have described. The current level could be high enough to heat-up/melt the white wire if it is thin.
As an example, using Ohms law, with 12volts applied across a 3ohm coil load, the current would be 4amps, continuous.

If alternatively, the capacitor has failed (with an internal open-circuit) the ignition system will operate normally but the spark will be very weak, so much so that it may not fire the mixture. However, the voltage may still be high enough to activate your timing light.

Check the capacitor!

David
David Padgett

BINGO!! Cured itself, you might say. In one of the myriad of configurations that I tried, the engine just fired up. I had just clamped #1 plug to the battery ground terminal to see if it would spark. Instead, the engine growled at me. Never did see if it sparked. Looking at what I had done showed that I gave the coil cable a good shove into the terminal. My ancient Lucas SP12 sports coil has a push-in connector set deeply into the top. It depends upon the rubber cable cup to keep it in place. If that internal connector isn't making connection the spark can jump the gap far enough to trigger the xenon lamp in the timing light. Back in the '60's I used to develop Xenon lamp flashes to emulate the flash of nuclear devices.

Time for a screw-in connector coil. Anybody know a reasonably priced good one?
Thanks for all of the suggestions from you guys on the other sides of the world. Bud
Bud Krueger

Glad to hear.

Well I use the Moss repo one. But it is far from cheap.

https://mossmotors.com/143-210-coil-screw-type-h-t-connection
Christopher Couper

Bud,

You shouldn't just rely on the rubber cap having to keep the cable in place in the HV internal connector.

The end of the cable should be a reasonably tight fit into the connector without needing the assistance of the rubber cap. If the cable end is loose, even falling out of it's own accord, try 'fattening' it up by distorting/squeezing the end with pliers or fitting a metal clip of some sort around the end, or wrapping the end with some fine wire.

David Padgett

You're right, David. I even have a small tie-wrap around the rubber. I've always liked the performance of the ancient Sports Coil. I've been making my own ignition wire sets for quite a while. The Pertronix LS systems require resistive cabling to avoid the RFI problems. Hadn't thought of the fine wire wrap.
Chris, I think that pricey Moss one is more for show than for performance. But, thanks for the lead.

Now I have to start getting Lazarus ready for the July 4th parade. (Only missed 3 since '95.) Unfortunately, my passenger, the Commandant of the Marine Corps League, is 6'4" long and I'm only an even 6'.

Thanks again folks, Bud
Bud Krueger

This thread was discussed between 10/06/2024 and 12/06/2024

MG TD TF 1500 index

This thread is from the archive. The Live MG TD TF 1500 BBS is active now.