electrical troubleshooting on MC

[steamfitter99]

Brian, just to add my two cents, I have a 420 that had a ground problem between the point set and the backing plate of the distributor. I fought this problem for a long time, new points would make it run for a while, but then no start. I finally pulled the distibutor and sandblasted the backing plate, this fixed the problem. The problem was that the corrosion between the point set and the backing plate kept it from grounding, no ground, no spark. I would pull the points or run a jumper to a good ground and try that. I would also like to add that I put a new distributor in the 420, the old one had a lot of run out, and it starts like a dream. Just a possibility, Tom.

[shinnery]

Brian

You are right that timing should have no effect with the secondary wire pulled out of distributor and being held near ground. For the tests we are doing you don't need to turn the starter over.

There is something wrong with the voltage readings you are getting. As long as the ignition switch is on you should have -6 volts to ground at the negative side of the coil. The positive side of the coil should have -6 volts when the points are open and 0v when the points are closed.
Your readings are almost like the voltage is coming from the dist,

I would think with a good 6V coil on a 6V system you should get 20-25KV in the secondary circuit and a 12V coil in a 6V system you should get 12-15KV which should be enough to make a spark or even run a little, probably wouldn't work at higher rpm.
I would never have tried using a DVM on the secondary side of the coil due to the possible high voltages.

Did you ever try the idea of running a wire directly from the neg. bat. term. to neg. coil term. ? Then open and close the points?

Bryce

[fishhawk]

well just my 4 cents (inflation you know) don't know how many times i have put in a new condenser and still have a bad condenser. bad condenser no spark! fishhawk

[russ61]

Just something I've run into before,run a piece of very fine sandpaper (or your wifes emery board if oyur feeling lucky)between the points contacts.After setting them with a feeler gage it will sometimes leave a thin film of oil on the points(non-conductive).Good luck!
Russ

[hunter41mag]

Hi,

By your voltage measurements in your second last e-mail I would suggest to check the following two items.
1. Possible coil shorted internally to housing. remove all wires from the coil and measure continuity from each terminal (one at a time) to the coil housing (body).
If you have continuity (resistance of 120 ohms or less) than replace the coil.
2. Check the voltage drop of the wire from key switch to coil. do this by putting your meter on low voltage DC scale and hook one end to the ignition switch terminal and the other to the coil negative wire terminal. than try to start the engine and if you get more than 1/2 volt or more reading on the meter you have a bad wire or connections. You may also need to do this from the battery feed to the ignition switch as it may also be on that side. The meter and its leads basically parrallel the circuit you are checking for a voltage drop. The only way to properly do a voltage drop is by having the circuit operate at its maximum amperage (draw), so that why you need to crank the engine to have the coil operating.

Don

NE PA

440IC

[Stan Disbrow]

Hi,

I guess I'll put in my 2 cents as well.

Keep in mind that you can easily have a breakdown of the internal insulation in the coil windings. This would allow it to pass low voltage resistance checks (it's rare to find a ohmmeter that puts out more than 9 volts) yet not produce a spark from the secondary.

The issue would be one where the high voltage is produced, and then sparks to ground *inside* the coil housing. This is a tough one to troubleshoot.

My method is to pull the coil off the machine and take it into the electronics shop, where I run it from a bench power supply. This way, I know the state of everything but the coil under test. Of course, I realise that you probably don't have an electronics workshop. Most folks dont (just us in the electronics business are fool enough to have one).

You can do the same thing on the machine, though. Run a jumper wire from the battery to the coil, and another from the coil to ground. Hook a known-good spark plug to the secondary terminal (and make a good ground to it as well).

Manually hold the hot wire to the coil, and when you pull it loose, there should be a spark at the plug. If the coil is good, that is. If not, then there will be no spark, and you know what you need to buy.

BTW, if the power to the coil is good, and the coil itself is good, and the points are good, but there's still no spark from the secondary, then you'll know it's a bad condenser you're chasing.

Stan

[BLS]

Stan,

Using your testing method, I have a spark at the plug. The spark is very weak (so weak that I almost missed it).

The condenser was new when I installed the points, I am going to have someone (at work) look at the condenser tomorrow (and test it).

I pulled the dist. Last night to see how it looked inside, I found no corrosion. I will get that back on tomorrow and hopefully have a new condenser (if needed) too.

Again, thanks to all for the help.

Brian

[Stan Disbrow]

Hi,

You should be going straight from the coil to the plug, not thru the dist. cap. After re-reading my post, I don't think I made that clear enough.

By so doing, you cut out possible cap and/or alignment (as in timing positioning) issues with the dist.

You can also unhook the coil from the original wiring when you do this. That'll cut out the condenser from the circuit.

If the spark is still weak with only the coil in operation, then you'll have it narrowed down to only two components: the coil and the battery. If the battery voltage is correct, then, well, you have your culprit.

This is the 'secret' to electrical troubleshooting. Try and test each thing in turn until something doesn't work. This way, you can shoot out multiple troubles with a minimum of hair-pulling-out.

BTW, in the Navy, E-techs are called 'spark chasers'. In this case, we really *are* chasing sparks (or lack there of)!

Stan

[shinnery]

Stan

When I was in the tin can navy we were called worse than that. LOL
USS Southerland DDR 743
USS Wilkinsoin DL 5
ETR2 discharged May 1963
(that means I have been out 43 years)

Bryce

[shinnery]

Brian
Did you ever get it started?? Haven't heard any more lately, just wondering.
Bryce

[BLS]

Hi Bryce,

I have not had much time to work on tractors, must have something to do with using my wifes nail file...

I had the condenser tested, (218 micro farads) I don't know what it should be.

I put the coil off this machine on a running machine and it fired right up. I plan on swapping each part until I find something that won't make it run. I know that is the long hard way to trouble shoot, but I seem to be running in circles anyhow.

Thanks for your concern. I will let you know when I have good news.

Brian

[Stan Disbrow]

Hi,

Actually, having a set of known-good parts and playing the old swap game is usually the quickest way to T-shoot these things. It may not reveal everything that is going on, but it usually gets you close enough to not matter.

FWIW, being an electrical engineer, I have an old dwell meter / tachometer that I added a set of sockets onto the front panel to attach one of my field-service oscilloscopes to. That lets me look at the waveforms on both the primary and secondary sides of the coil. This is what I revert to when using the volt-ohm-meter doesn't show me which component is faulty.

Using the o-scope method lets one see exactly which area of the circuit is having an issue, from a bad condenser or points (seen on the primary side) to internal arcs in the coil, leaks in the hi-voltage wires, bad plugs and even detonation inside the cylinders (seen on the secondary side).

This is a home-made version of those old Sun engine analyzers. The reason I bring it up is if you can find one of those old Sun units, it'd be worth a try using it to T-shoot the poor old crawler.

That is, if the change out the parts process fails to locate the fault. I'd still try that one first, as I'm sure it'll be a quicker way for you to proceed.

Stan

[jdjoe_indiana]

A thought, I missed if you said you have a new Dist cap. A few years ago, bought a new one for my JD MT, same Dist I think you have, and the cap was made incorrectly. I can't remember for sure but I think the center coil post inside cap wouldn't touch the rotor. I have three new caps on it until I took and old one from my dad's MT and mine started right up. You never second guess new parts, but now I do!! Joe

[boler76]

this is what i would suggest. Disconnect coil wire that goes to ig sw. disconnect coil wire that goes to side of distributor, take a spark plug and lay it on the metal frame so the nut part touches the frame, run a wire from ig side of coil to neg of battery, take a wire from the distr side of coil and connect it off and on a few times from coil to a ground, you should get a spark at the plug, if all is well my 2 cents Jim

[dean r]

Looks like this post has had allot of good advise for checking the circuit, but I thought I might help.

That 12V coil is actually a 6 volt coil that is only 12V when starting, typically. A typical 12V point coil ignition circuit has a ignition resistor prior to the coil to drop the voltage to the coil to 6V and has a bypass circuit from the starter solenoid direct to the coil supplying full 12V during starting. This keeps the amperage down across the points/plugs which makes them last much longer.

Now I'm not sure about the circuit on this crawler but if it has a similar circuit then the original post of about 2V sounds right and running 3V on a 12V (6V) coil may not get you the spark you need and/or you are missing the full six volts during starting. Running the coil on a seperate engine may or may not provide the information you need if it was a 12V system.

Hope this helps and if you have checked all the other posts suggestions and they are fine then I would replace the coil if it was me. It probubly worked when it was first put on but then as time went on and resistance through the coil went up/down it started to fail.

Take care and good luck, only thing that can be more frustrating then car electronics is computer software problems.