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MG TD TF 1500 - Ignition System Problem

I am having a problem with my ignition system. Engine will not start I have a pertronix complete distributor (Not a conversion) and a Pertronix coil.

The symptoms and a temporary solution are as follows:
1. No spark at spark plugs
2. Ran a jumper from the battery to the positive side of coil. Engine would NOT START.
3. Temporarily hooked up a standard OEM test coil, known to be good, using jumper cables. Jumper from positive to positive and negative to negative and kept the high tension cable and all other wires attached to the Pertronix coil. ENGINE STARTED.
4. Disconnected the standard OEM test coil and the ENGINE STARTED,and continuous to start for, at least, several days if not longer.
5. Problem reappears.

I ran some tests before I connected the test coil while the ignition system was in the MALFUNCTION mode.
1a. Battery voltage: 12.4 volts
2a. Voltage at the positive terminal on the coil with all wires connected: 12.2 Volts
3a. Voltage at the positive terminal on the coil with the wire to the distributor disconnected and a jumper cable from this terminal to ground: 12.1 Volts
4a. Voltage at the positive terminal on the coil with the wire to the distributor disconnected and a jumper cable from this terminal to ground and CRANKING the engine: 10.5 Volts
5a. Disconnected all test wires and test coil and reconnected all normally connected wires. ENGINE WOULD NOT START.
6a. Then connected the standard test coil as described in #4 above and the ENGINE STARTED.

At first I thought that the problem was in the ignition switch (see my post earlier today)but now I don't think so based on the test voltages. I now suspect that I have a defective coil that somehow temporarily repairs itself when a good coil is temporarily connected and then disconnected.

This is a puzzler. Any and all comments would be greatly appreciated.

Frank Grimaldi

Frank Grimaldi

Frank, I had the same problem with a new Pertronix coil and it was a bad coil. I called Pertronix and got back a note saying that their coils do not fail. I beg to differ. Put my old coil back on and no problems. I have a how to check coils from Pertronix and if you want it send me your email and I will forward to you.
Tom Maine (TD8105)

Frank, you have curious. I didn't realize that Pertronix made complete distributors for T-cars. Where did you get it? Bud
Bud Krueger (TD10855)

Tom Maine - I would also be grateful for a copy of the Pertronix coil test info.

Thanks. Tom IN Maine...
t lange

frank, did your car ever run on this pertronix set up or is that when the problem started? it would help me if you gave the background info to your problem. regards, tom
tom peterson

Try finding out the coil resistance. Hard to imagine they didn't pair the Hall effect device with their own coil, but maybe they intended it to have a resistance wire or ballast resistor. I elected to go with ballast resistor. Seems they wanted about 4 ohms, with only a few Ohm variation. I called and asked about using an ancient coil and the technician said they needed something like 1 1/2 Ohms minimum.

Cheap ballast resistor & chrome coil I picked up off ebay has about 2.1 Ohms for each, so total of 4.2 or so. Ballast resistor sheds half of the heat that a 4 Ohm coil would generate, and the resistor can be bypassed with a wire from the starter for hotter start.

When you paralleled the 2 coils, you cut the resistance and maybe made the distributor happier.
Jim Northrup

Frank, did you adapt one of their 23D, 25D or 45D distributors? Positive or negative ground? What did you do with the vacuum advance?

Jim, last I knew Pertronix specified using a coil of not less than 3 ohms. But I haven't bought a new one for some time now. Use of a ballast resistor would require some auxiliary circuitry, such as a relay, or a different ignition switch. Bud
Bud Krueger (TD10855)

Answers for the above questions...

1. Distributor: Ignitor - model PNX-D176600
Coil: Flame Thrower - model PNX-40611
Purchased from Summit Racing Equip. $160, $32
respectively.
2. When the original coil and distributor were replaced the car ran fine. The problem developed later.
3. The coil resistance was 3.2 ohms both before (in malfunction mode) and after the test coil was hook up.
4. The distributor was a complete and new Pertronix distributor that replaced the original Lucas distributor. It was not a kit that replaced the points. The distributor has vacuum advance.

5. To Tom Maine: Please send me the "How to check procedure" for the Pertronix coil.

Send to: frank.grimaldi@earthlink.net
Frank Grimaldi

The ballast resistor requires no modifications to the wiring- it is just in series with the hot wire to the coil. It can be added in or taken out in minutes. It can mount anywhere, preferably where there's wide open air, rather than packed in tight with other goodies.

For a couple of bucks, a ballast resistor might be a wise investment if 3 ohms is the magic number. The coil will run cooler.

On the other hand, the pair of coils dropped the resistance down probably in the 1 - 2 volt range. Might be pickup could fail early with the high current at low resistance in the long run.

I got chrome coil/bracket & resistor off ebay for $17 plus shipping.
http://cgi.ebay.com/ebaymotors/60-000-VOLT-CHEVY-GM-CHROME-COIL-W-BRACKETS-59-VALUE-/310246740754?pt=Motors_Car_Truck_Parts_Accessories&hash=item483c259312

Jim Northrup

Thanks, Frank. That distributor (PNX-D17600) is what they supply for the BMC A & B series engines. This is where the 23D, 25D or 45D distributors are found. I'm not sure what is necessary to adapt those distributors to an XPAG. But, obviously it can be done. The PNX-40611 coil is a 40kv coil with the desired 3 ohm primary resistance. I'm guessing that their instructions are to connect the red wire to the hot (+) side of the coil and the black wire to the (-) side of the coil.

Jim, I beg to differ with you. Ballast resistors are used to increase the primary circuit resistance when an engine is running. They are shorted out when starting the engine (to get a hotter spark). This is usually done via a relay, or a separate terminal on the ignition switch. Check the ignition circuit of a mid-70's MGB for an example. Bud
Bud Krueger (TD10855)

I looked at wiring diagram for MGB, no ballast resistor in this diagram.

For the hot start, one only needs to run a wire from starter motor (or solenoid terminal to starter) to the hot side of the coil bypassing the resistor. When the starter is engaged, the coil will get all the voltage there is, (which is considerably reduced by the starter drain).

I didn't bother with that hot start wire since it seems to start right up with no problem with the Pertronix & new coil, and that's one more wire that could some day short the coil and strand us.

The ballast resistor itself is just a thin wire. One of the features of the ballast resistor and the resistance wire that replaced it, is they passed a bit more current cold for hotter spark and as they warmed up while running, the current to the coil would drop.

Jim Northrup

"I looked at wiring diagram for MGB, no ballast resistor in this diagram."
75-on, "resistive cable"

"For the hot start, one only needs to run a wire from starter motor (or solenoid terminal to starter) to the hot side of the coil bypassing the resistor. When the starter is engaged, the coil will get all the voltage there is, (which is considerably reduced by the starter drain)."
NO! NO! NO! >>

"I didn't bother with that hot start wire since it seems to start right up with no problem with the Pertronix & new coil, and that's one more wire that could some day short the coil and strand us."
>> Because that will short the IGN W circuit (and everything before it!) through the starter motor. AKA. you will be trying to run the starter off the IGN W, which will let all the smoke out.

FRM
FR Millmore

Hey Guys,

I need to apologize. My engine is a 3-main MGB fitted to a MGTD. The engine is not a MGTD. When I first posted, in another post, I was addressing the ignition switch which is a TD. When the question of the coil came up I went to the T Series posts without thinking.

Frank Grimaldi
Frank Grimaldi

Frank, I'd venture a guess that your problem is in the connector on the black wire that connects to the low side of your coil. From the view of the distributor in the on-line Pertronix catalog it appears that the leads are open-ended and you have to install your own connectors that go onto the coil. Not sure what type of connector you used, but you might want to take closer look at it (recrimp, solder, ?) Intermittent failures can be a pain to find and are quite often from poor joints. Just a suggestion. Happy hunting. Bud
Bud Krueger (TD10855)

This thread was discussed between 05/09/2010 and 07/09/2010

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