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MG TD TF 1500 - Field test - bad condenser

Don't laugh ...

This newbee (ME) is writing a tech tip for our club newsletter. Can you help me?

> What are some simple "side-of-the-road" tests that can identify a bad condenser?

Thanks,

Lonnie
TF7211
LM Cook

Lonnie, it is highly unlikely that you would have the proper instrumentation for test a capacitor (condenser). A standard trouble shooting technique in the technical world is 'replace with known good component'. If a known good capacitor cures the problem you can surmise that the other one is NG. It's always a good idea to carry a known good capacitor in your travel kit, along with a known good rotor. You may never need them, but you may be able to save someone else's day. Bud
Bud Krueger

Thanks, Bud -

A spare breaker plate with condenser and properly gapped points is my #1 suggestion.

I agree - I should probably skip describing a simple, yet non conclusive, test to confirm the bad condenser and just say (as you noted) to replace the condenser. Then do more ignition tests if needed when the driver returns home. I sometimes overburden people with too many unnecessary details.

Thanks,

Lonnie
TF7211
LM Cook

The symptoms of a bad condenser are very much like a bad coil - sudden rough running that feels like the engine has wires crossed, or is running on only 2 cylinders, and often is not able to rev very high.

Condensers are a thing of the automotive past, so they now all seems to be made in a third-world country, of very uncertain quality. I carry a spare, always - even buying NAPA does not always mean they work well, or for very long. I've also seen a coil burn out, then a condenser, then a coil, then a condenser.... I tend to change them both, to be safe.

Tom Lange
MGT Repair
t lange

There really isn't anything.

Some digital multimeters have a scale that measures capacitance, but the kind of thing that makes an ignition capacitor fail probably will not show anything abnormal on that scale. If the capacitor is shorted, it will, but that's an unusual failure. The more common is high series resistance, and that may or may not affect the capacitance reading. It has to do with the way the meter works, and I won't bore you with that.

As others have stated, the only real test is replacement with a known good device. Of course, that means you have to get a part, prove to yourself it's good, and that it will stay good. Not always easy.
S Maas

I have four original distributor plates, with capacitors still in situ, but of unknown status, which I'll test simply by replacing them. This is one of the many tasks I'll address once the car is running. Given the abnormally high percentage of new capacitors that are faulty or are close to failure, coupled with the difficulty in testing them, my tip Lonnie would be to target something else. Cheers
Peter TD 5801
P Hehir

I have mounted my condensors on the outside of the distributer using the screw that mounts the cap clip. Run it that way until you feel safe then move it inside.
On a road trip with a 37 MGVA I had to hang a condensor on a piece of fence wire and twisted the small wire in to the ignition wire until I had tools and completed a 200 mile trip to Jacksonville and back to Clearwater area.
Sandy
Sandy

"I have mounted my condensors on the outside of the distributer using the screw that mounts the cap clip. Run it that way until you feel safe then move it inside. "

Why not just get a standard, none automotive 22microfarad capacitor rated at 600 - 1000 volts and mount it permanently external from the distributor - it will be the last one you will ever need to purchase. See the article by Steve Mass on Bud Kruger's TTalk at: http://www.ttalk.info/CondenserFailure.html Cheers - Dave
DW DuBois

Thanks everyone for your help. I'm really glad that I chose condensers for next month's Tech Tip. Looks like all of us have had experiences with condensers failing. My Tech Tips are geared primarily for newbees like me, but hopefully long-time MG owners can benefit also.

I am making four suggestions in my Tech Tip:
1) Don't change your condenser if it is working OK.
2) Carry a spare breaker plate with a condenser and gapped points to use in case a condenser or points fail. (Of course how can you know that the condenser on the plate is good?)
3) Perhaps adapt a condenser for a popular early-70s American car to fit an MG distributor. Maybe the higher sales volume will allow better quality by the manufacturer (?)
4) Carry an emergency condenser jumper. I've shown it before. Two clips. Attach the wire to the breaker-points terminal on the coil or distributor. Attach the case to ground. Bud had pointed out that it will work if the condenser failed in an open-circuit condition. I believe that Sandy gave me the idea. I've used it twice. Once to jump a bad condenser, and once to prove that the condenser was good. The coil ended up being the problem.

Lonnie
TF7211

LM Cook

On point 2 Lonnie if you test it in your car & the car runs OK then you can be confident it will be a serviceable spare. Having a plate to swap complete with gapped points & a good condenser is a top tip. Cheers
Peter TD 5801
P Hehir

A test for a condenser that is not conclusive is:

1. Fully discharge the component by shorting both terminations together, this will fully discharge the device.
2. Select the resistance scale on a high quality multimeter.
3. Place the multimeter lead probes on the condenser terminations and observe the meter readings, there should be an increasing resistance reading until a value of infinity is reached. Ensure your fingers are not in contact with the uninsulated sections of the probes when you disconnect other wise you will receive a whack.

As stated this is not a conclusive test as the voltage used are not comparable with those in an ignition circuit.
G Evans

Hi Dave,
You should fix your replies typo, you didn't mean 22 microfarad, you meant 0.22 microfarad, as you know. Just in case folks don't read Steve Mass's referenced article, an excellent one.
(big difference in size, use, cost, etc.)
Al
54 TF "Emma"
A W Parker

Had some ignition problems with another of my cars, a RR SC1. It started well and run very smoothly unloaded and at low rev. but when under load it misfired and gave very little power.

Conclusion, with a faulty condenser it was misfiring and shooting under load and with a faulty coil it was only misfiring under load. The symptoms can certainly differ a lot, but this might be worth considering if you have a misfiring engine.
YS Strom

A.W. You are correct, I forgot to put the decimal point in front of the 22. Unfortunately, I cannot edit my post, it's having been to long since I wrote it, so this will have to suffice for a correction. Cheers - Dave
DW DuBois

YS is correct, but another cause is too lean a mixture. Under power, the engine can misfire mimicking a bad condenser. I changed condensors 3 times in a TD before the problem was correctly diagnosed.
Lew3

YS

A couple of weeks ago, I thought that the '78 Plymouth Arrow condenser that I use in my distributor had gone bad, based on sudden loss of power although it didn't backfire. I jumped the condenser with my "emergency" condenser. No change. So I figured that my $19 no-name coil was the problem.

Just to be safe, I replaced both. Runs fine now.

Thanks again to everyone for participating in this thread.

Lonnie
TF7211
LM Cook

Lonnie, condensers are funny beasts. If one fails by totally losing its properties, i.e., becomes an open circuit, you can jumper it with a good one and all will be well. However, there are other failure modes stretching down to one becoming a short circuit. Jumping a good one across it will do nothing to alleviate the problem. You have to remove the faulty one from the circuit. You can't just jumper a good one across one being tested. Bud
Bud Krueger

Bud -

Yes they are confusing animals. Too many variables to cover in a simple Tech Tip. I should remove my emergency condenser from my Tech Tip to avoid the problem and stay with the proven road-side fix of changing breaker plate assembly.

Thanks,

Lonnie
TF7211
LM Cook

The simple solution is, if you suspect a failed ignition capacitor, and want to connect another externally, just disconnect the old one and connect the new one. Then, all should be well, as long as the replacement is OK.

The ohmmeter test of a capacitor is fine for determining whether it's open-circuited or shorted, but won't identify the main failure mode, which is high series resistance.
S Maas

Thanks. I read some of your posts and information about condensers. I probably stole some of your words. For the "Tech Tips" column, I want to write a simple tip to rapidly get a car running when it's broken down on the side of the road because of a bad condenser. A spare breaker plate is the best way. Don't want go into shop tests.

Lonnie
TF7211
LM Cook

If you are doing a routine tune-up and are not having any obvious problems with the ignition, I would say that you should change the condenser and points. That way you have a known good condenser as a spare. About the only field test is to check for resistance with a meter while tugging and pushing on the lead wire. If fiddling with the lead causes the meter to jump, there is a problem. True for new ones, too.

I don't like the way the capacitor is mounted in the piece of circuit board in Steve's article. Maybe with some different viewpoints we can come up with a better way of doing it.
N Tesla

This thread was discussed between 25/09/2015 and 05/10/2015

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