REally bad spark, ........oops

RichJohnson · Post by **RichJohnson** » Mon Jun 20, 2005 11:39 pm

Ok, this is an M37 but same thing as the jeeps. so here goes.
I can barley get my truck to start. It wont run at all. I checked spark and found it to be itnermittant and very weak. It wont jump from the wire 1/16th inch to a head bolt. Cap roter points all look good. I changed the coil out and no change in symptoms.
Here is where I did something worng. I wanted to bypass the capacitor in the bottom of the distributer and try that way becase it was suggested to me so what I did was get the electrical test kit out and put the coil prob into the cap in the distributer and tested for power. Got 25+ volts so I disconnected the power input wire to the distributer to remove it from the rest of the electrical system adn hooked a jumper wire from the hot post on the starter to the probe.
This was a REALLY BAD IDEA. I then thoguht I could use the starter to turn the engine over to test for spark but I didnt realize that the COIL PROBE actually probes the ground side of the coil. As soon as I pulled on the starter lever under the hood the points closed and I had a nice fire under the hood.

So I waited till I could see through the smoke and used some insulated pliers to remove what was left of my test lead then opened back up the distributer. When I pulled the cover quite a bit of smoke came out.

So Im gonna pull the breaker plate and maybee the whole distributer tomorrow or the next day and see what I destroyed.
Needless to say I was trying to get it to run right to drive to the Tehachapi camp out but now it looks like im gonna trailer the M38a1 to go.
Why when you use these things as daily drivers do they always break down right before an event. This its 3d time.
Still didnt figgure out the bad spark problem yet, but I got bigger distributers to fry now.

Post by **wesk** » Tue Jun 21, 2005 9:41 am

Is the weak spark on all 6 plugs? If it is then you can rule out plugs and leads. The new coil SHOULD be good. So that leaves the rotor and cap. A carboned cap will short to ground a lot of spark and sometimes leave you with just a weak one at the plug. The condenser and capacitor can fail and leave you with no spark or weak spark.

RichJohnson · Post by **RichJohnson** » Tue Jun 21, 2005 10:40 am

Yes weak spark at all wires. The cap and rotor looked ok but....I don't know....
I didnt know that a weak capacitor could case a weak spark, I thought the capacitor was just an RF filter.
Should I just replace the cap, points, condensor, and capacitor now, since I had toast last night.

Post by **wesk** » Tue Jun 21, 2005 10:48 pm

If the capacitor suddenly has high internal resistance or an open then no spark. I run into problem RF noise filters a lot in General Aviaition aircraft with piston engines since they are used for both magneto and alternator/generator noise limiting. They can cause intermittent failures that appear just like bad connections and broken wires.

I would agree that a good post fire strategy would be to replace all electrical devices exposed to the heat.

Cacti_Ken · Post by **Cacti_Ken** » Wed Jun 22, 2005 10:52 am

I came across this site about automotive ignition sytem. I think it is a good explanation/ illistration on the subject, however it does require some basic understanding of electromotive force (EMF)

http://ourworld.compuserve.com/homepage ... n/CDI.HTML

Post by **wesk** » Wed Jun 22, 2005 11:04 am

Very good site. But remember the military waterproof ignition uses two capacitors. One in the coil power lead for RF noise filtering. This one if it fails open or becomes very high in resistance will give no spark or weak spark since the electrical power to the coil has to pass thru it. The other capacitor is often called a condenser and is installed parallel with the points to act as an electrical shock absorber. A short in this capacitor will effectively weaken coil output or end it. You can partially evaluate a capacitor with a digital VOM but you need a real capcitance checker to completely evaluate them. Most folks don't have access to these so the answer here is to just replace them. Also keep a spare for each around for troubleshooting. Also remember to check coil resistance both cold and hot. If they are going to fail they will normally show this high resistance when hot. What I do is check the primary and secondary resistance with my VOM while the coil is cool then I heat it with a heat gun untill it is uncomforatble to hold onto and recheck it.

Cacti_Ken · Post by **Cacti_Ken** » Wed Jun 22, 2005 12:21 pm

I don't have the diagram of the military circuit, (edited, if there actually is one,) with me at the moment. I'm going to try and find it, to understand what you are explaining.
I am not trying to discount your information, I'm just trying to understand it.
I didn't know you could pass current through a capacitor. Since the make up of it is two thin metal plates separated by a dielectric material (insulatator) and the plates store the charge applied to them. I can see that if you had a resistor in series with the power to the coil, and this resistance became opened or became higher in resistance than intended, this would cause weak or loss of power to the coil in turn producing a weaker or no spark / voltage output from the coil.

. You haven't mentioned having a resistor in the circuit yet, but have said or used the word resistance in your note. And either component could cause spark problems.

(Edited)
Wes, I have now looked at the parts picture of the distributor and see the capacitors your are talking about. I haven't come across a schematic showing how it is connected in the circuit. I also cannot tell where ignition resistor comes into play or if it is part of the cacacitor assy in the housing of the distributor.

Post by **wesk** » Wed Jun 22, 2005 10:56 pm

Ken,
The filter (capacitor) at the power in point of the distributor would probably be better described as the primary input power connector and the unit includes an internally mounted (inside and integral to the connector) 2 microfarad capacitor. A shorted capacitor would then short primary power to ground.

The unit at the top is the primary power connector. The pin on the right side mates with the female plug from the ignition switch with 24 volts DC. The lead exiting it on the left is the filtered 24 VDC lead to the coil. If the capacitor fails internally you would get a weak or no spark condition. If the pin or wire fails open you get no spark. If the pin or wire is shorted to ground you get no spark. As you can see the 24 VDC power is passed straight thru the capacator.

Cacti_Ken · Post by **Cacti_Ken** » Thu Jun 23, 2005 8:52 am

Thank you Wes, I appreciate your efforts in helping me. Two more things, I can't read the notations in the photo. If you send it to me by email maybe I can read them. Last, Can you hand draw me a basic schematic of the distributor circuit with symbols showing all the components envolved. It isn't necessary to show the rotor and dist. cap. One spark gap from the secondary coil is sufficient. I know you are busy and have other stuff to do so take your time.

RichJohnson · Post by **RichJohnson** » Fri Jun 24, 2005 9:54 am

Wow such a wealth of knoledge Wes.
Well I suspect that from what you say my condensor was bad.
I did have a very weak spark but it was also intermittant and would not fire every time.
As I said I used the distributer probe to test for power and Had 25+ volts on the ground side of the coil. So I would figgure that the input capacitor was still good.

Since the fire I have not opend it up, I just ordered replacements from midwest mil. I gave up on driving the m37 to a camp out and resorted to towing my m38a1 up there.

I think the coil might still be good but I could be way off. Since I input power throught that probe to the ground side it cooked when the points closed, in theory it didnt go through the coil.
Should I just check the resistance on the high side and low side (ground) and see if its still good?