• Hi Guest!
    You can help ensure that British Car Forum (BCF) continues to provide a great place to engage in the British car hobby! If you find BCF a beneficial community, please consider supporting our efforts with a subscription.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

Sanity check please!

Share this page

Thanks Randall.

My DMM only measures resistance at x10, x100 and x1000 ohm range. I'm afraid there's no 20 megohm setting.

I pulled the condenser from the 25D distributor. Checked it ... and got exactly the same "almost infinite" resistance as the MB condensors.

Either I need a megohm range, or a padded cell. Man, I was sure I had found the problem. Good grief.

Tom
 
The way you check condensors (or capacitors) is to use a good old VOM (analog), connect leads in one direction (say, red on wire, black on case) in Ohms.
Watch the meter needle.
Reverse the wires.
As the charge goes back through the meter movement, the needle will kick, and slowly (or rapidly, depending on setting and movement) to zero.
You can do this several times and watch it as it does the charge discharge cycles.
 
So ... are you saying it doesn't matter what resistance level the VOM is set to? You're only watching the needle movement?

TOC said:
The way you check condensors (or capacitors) is to use a good old VOM (analog), connect leads in one direction (say, red on wire, black on case) in Ohms.
Watch the meter needle.
Reverse the wires.
As the charge goes back through the meter movement, the needle will kick, and slowly (or rapidly, depending on setting and movement) to zero.
You can do this several times and watch it as it does the charge discharge cycles.
Click to expand...
 
Did we not chase this back to a wonky ignition switch once before?
 
DrEntropy said:
Did we not chase this back to a wonky ignition switch once before?
Click to expand...

Yep - sure did, when the car first arrived, it wouldn't start. I hot-wired the battery directly to the coil and it started fine. Thought sure that was the problem. But the next day, I discovered the engine would die as soon as I opened the throttle. Turned out the problem was the stud on the distributor (where the ground strap and condenser are attached), was loose. Made contact when you started, but open the throttle, vacuum advance pulled the innards away from the internal stud contact. That was fun to diagnose. Discovered the problem when I went to replace the original condenser (which I thought maybe was dead). I started to turn the nut on the stud, and found the stud actually wiggled loose in the distributor hole.

I removed the old condenser, did the VOM test, and found the condenser was dead - at least according to the test method I used. Still not sure how you actually determine if a condenser (capacitor) is "bad".

That's why, when this problem started up again 30 days ago, never thought it could be ignition again (stupid me). Spent a month troubleshooting the carb. Nothing fixed the problem. Just today I thought what the heck, test the condenser again. As the test gave exactly the same result as when I first did this, I figured this new condenser was also dead.

Are we confused yet? (I sure am.)

:topsy_turvy:
 
Condensors store DC voltage spikes. They pass AC. You generally can never check a condensor (capacitor) by resistance.
Remember earlier when I said to leave the vacuum advance disconnected?
Not only the wires where they fit through the side of the distributor case, but the little ground wire from the breaker plate to the case. Moves every time the advance moves.
Have you tried hotwiring it yet again?
 
I did try this afternoon with vacuum advance line disconnected. No difference. Also sucked on the line and watched the plate move.

Did the hotwire yesterday - no improvement.

T.
 
Okay.
1) propping the points open with a piece of clean cardboard at the point contacts, check the voltage at the primary side of the coil to ground with the key on.
2) ground the distributor side of the primary, check voltage again.
3) should be battery voltage or close to it both times.
4) key off, ohmmeter check the base portion of the points (not the spring/wire side) to the engine block or head.
Should be zero ohms or very close to it.
Report back.
 
TOC said:
As the charge goes back through the meter movement, the needle will kick, and slowly (or rapidly, depending on setting and movement) to zero.
Click to expand...
Shouldn't be to zero, but infinity. Once the capacitor charges to the supply voltage (the flashlight battery inside the VOM), it stops passing current and effectively becomes an open circuit. What range you select will have a big impact on how quickly the needle moves.

This is fine for the paper non-polar caps used in a car ignition, but it's not a good idea to charge electrolytics (eg power supply filter caps) backwards. For them you should know what polarity the meter uses (it doesn't always match the lead color or case markings) and only test the capacitor with the proper polarity. You can short the leads to discharge the cap to repeat the test.

I used to know a method of actually checking the capacitance of non-polar caps, by connecting a meter and a 6vac transformer; but I've forgotten the details offhand. It's so much easier to just substitute a "known good" one (saved from the last tune-up).
 
Ah. Zero current, so infinity. Using a VOM you can charge anything backwards. If AC or RF is up against it, there is more power there than any VOM I've ever seen.
You just watch for the kick of the needle, and drop back down, reverse, same thing.
Won't tell you if it's perfect, but will tell you if it's is not shot.
I used to use a big, whacking tester for them. Dial in the capacitance until the meter zeroed....but that was 45 years ago.
Helps to not have a migraine.
 
Just to be clear, here's what happens when I test the 2200 uF radio condenser.

Analog VOM set to x1000 ohm (reading times 1000).

Condenser on work bench.

Meter's negative lead to condenser case. Positive lead to condenser wire.

Meter needle zaps over to zero, then slowly moves back to (nearly) infinity.

So ... is that condenser good?

Here's what happens when I test the Mercedes distributor condenser AND the TR condenser.

Same method, but the needle just barely moves from infinity. Certainly doesn't zap over to zero and slowly return.

The TR seems to run fine.

So now i don't know how to interpret the test results.

Decisions ... decisions ...

Thanks for hanging in there with me.
Tom
 
Dave - Points open, ignition on. But I'm a dolt. In #2, you say "ground the distributor side of the primary". The coil has three terminals: positive goes to wiring harness, negative goes to distributor body, center HT wire goes to dist. cap. I tried running a wire from the negative terminal to the block, but get a spark.

Is that what you wanted? Or did you want me to ground a different coil wire?

Test #1: voltage is 12v.
Test #4: ignition off, point base to block, resistance is zero ohms.
EDIT: I repeated #2, but removed the cardboard from between the points. Voltage is 12v.

Thanks.
Tom


TOC said:
Okay.
1) propping the points open with a piece of clean cardboard at the point contacts, check the voltage at the primary side of the coil to ground with the key on.
2) ground the distributor side of the primary, check voltage again.
3) should be battery voltage or close to it both times.
4) key off, ohmmeter check the base portion of the points (not the spring/wire side) to the engine block or head.
Should be zero ohms or very close to it.
Report back.
Click to expand...
 
The primary side has studs and nuts. Secondary has the wire to the dist. cap.
12V with points open and grounded means you aren't losing voltage under load from a bad connection somewhere or a faulty ignition switch.
The amount of capacitance and the type of capacitor will give you different readings on your condensor.
If it matches the TR, it's probably good.
We do have spark, right?
Page 4, have to keep going back and checking...
 
NutmegCT said:
Just to be clear, here's what happens when I test the 2200 uF radio condenser.
Click to expand...
Note that 2200 microfarad value. The points condenser is only about 0.2 uF. That means, for a given current, it will charge some 10000 times faster.

Meter needle zaps over to zero, then slowly moves back to (nearly) infinity.

So ... is that condenser good?
Click to expand...
Well, it at least passes this simple test. But you still don't know how much capacitance it has (old electrolytics tend to lose capacitance as the electrolyte dries out), or how much leakage at rated voltage.
Here's what happens when I test the Mercedes distributor condenser AND the TR condenser.

Same method, but the needle just barely moves from infinity. Certainly doesn't zap over to zero and slowly return.

The TR seems to run fine.

So now i don't know how to interpret the test results.
Click to expand...
If the needle kick is about the same, then I'd say it has passed this simple test. As above, that is no guarantee that it is good, only that you can't tell it is bad with this test.
 
TOC said:
We do have spark, right?
Click to expand...

Note that it is entirely possible to "have spark" but not enough to start the engine. It takes more oomph to fire a plug when it is under compression than in free air.

Many years ago, just driving down the road, the TR3A I had then just quit like the key was off. I spent a very frustrating hour on the side of the road trying to find the problem. I could see the spark jumping a (temporarily removed) plug gap; but it just wouldn't run. A new condensor fixed it right up.
 
I'm sure hoping it's a failed condenser. Will get the new one this afternoon. I could put the TR condenser in place, but as I don't know the capacitance values, I'm afraid to try.

I definitely have spark. Note that the first time I had a similar problem (cranked, fired, but died when throttle was opened), back in November, I also had spark then. Replaced the condenser, and it ran fine. That new condenser has been in place since December.

Now a similar starting problem has occured - and I'm hoping a new condenser will take care of it. But of course ... if the new condenser fixes the problem, then why are my condensers failing so frequently?

One step at a time (I keep reminding myself).

Thanks.
Tom
 
NutmegCT said:
One step at a time (I keep reminding myself).
Click to expand...

Good advice, you should take it! :eagerness:

If it's any consolation, a few years ago we went through 6 different "rebuilt" alternators on the wife's Camry over the space of about a year. Finally found a good one, it's been working fine ever since (although I think my daughter may have traded it in on a new Focus a few months ago). Of course I checked, tightened and replaced all sort of other things along the way; but with the last dead alternator I just whipped the old one off (had gotten really good at it by then) and the new one back on. It really was just bad alternators.
 
Thanks Randall.

Should I mention ... the MB condenser costs $45. :eek:

As I don't know the capacitance, I got it because it's direct from MB, and supposedly matched to the distributor.

If today's (non-MB) condenser replacement fixes the problem, I'm sure as heck getting my $45 back from MB.

T.
 
BTW, practically every car with points uses around 0.2 uF. And if it's not quite right, the worst you will get is more rapid wear of the points. You can even tell if it's too high or too low by which way the points erode (but I forget which is which offhand).
 
While waiting for the magic "new condenser" to arrive, I put the TR condenser in its place.

No change. Doesn't fire at all.

grumble grumble

Edit: just realized the TR is positive ground; the MB negative ground. Hope I didn't kill the TR condenser.
 
Back
Top