Weak Spark from Distributor - PeachParts Mercedes-Benz Forum

karshack · #1 02-19-2016, 01:09 PM

Hi All,

My problem is that I got no spark from only 1 of the plug sockets(?) on the distributor cap. All other plug sockets give me a vibrant and bright spark light as tested with a spark plug tester. This came to my attention when I could not get any rpm reading on my digital timing light/tachometer from the #1 spark plug wire at all, no matter where on the wire I placed the induction clamp. The clamp received rpm signals from all of the other five plug wires. And again, using the spark plug tester, all other distributor cap plug sockets produced strong and vibrant sparks.

So, I got a new distributor cap. Even put in a new spark plug at #1 cylinder. Ran an resistance test on the cap's contacts with 2 different voltmeters. All readings were equal (no, or negligible resistance - I forget the numbers). And the inside the cap contacts were all stable; not loose, no wiggle. And no evidence of any cracks, or other physical damage to the distributor cap

This time the #1 cap socket sends a weak spark. And I get an intermittent rpm reading from the #1 cap socket. All other sockets are spitting out strong bright sparks.

All this is with a newly installed Pertronix Ignitor I - Model 1846, so there are no breaker points issues involved here. The car starts and runs - with a lot of ignition advance. I don't want to rev the engine to the factory recommended 4,500 RPM (w/o vacuum adv) until I can figure out this spark problem.

When I get home I can post some pictures that show the difference in the spark.

Any ideas would be appreciated. Thanks

Frank Reiner · #2 02-19-2016, 01:21 PM

Suggest you try swapping plug wires between #1 & #2. Does the problem move with the wire?

karshack · #3 02-19-2016, 11:15 PM

Hi Frank, I'll give that a try. I did check the 1 & 6 wires for abnormal resistance. I forget the figure, but they were appropriate.

I am running Accel custom 7mm wires. Just a little over a year old. A real pain to assemble. I had considered going back to the solid core Beru wires - but Petronix warns against solids and I don't want to ruin this before O can get it going good. I foresee another Saturday with the car.

Graham · #4 02-20-2016, 09:34 AM

Quote:

Originally Posted by karshack

I am running Accel custom 7mm wires. Just a little over a year old. A real pain to assemble. I had considered going back to the solid core Beru wires - but Petronix warns against solids and I don't want to ruin this before O can get it going good. I foresee another Saturday with the car.

I do see that warning in the Pertronix catalog. However, many of us have been using the Ignitor I for years with the standard solid core Mercedes wires. The warning appears to apply to both the Ignitor I and II. Not sure if that was intended. In reading the Ignitor II installation instructions, they warn about copper wires, but the warning is not there for the Ignitor I.

When the Ignitor II came out, Pertronix advised not to use it with solid core wires because of possible damage to the electronic components that were added on the new model. As a result, MB owners have in general stayed with the Ignitor I.

It might be worthwhile asking Pertronix about this.

I don't know anything about Accel wires, but if they are graphite cored and/or have high resistance compared with the stock wires, that could affect your car's performance.

Mike D · #5 02-20-2016, 09:56 AM

A fouled or defective plug will also show a "no spark" condition using an induction style test.

karshack · #6 02-22-2016, 08:41 AM

Here are the pix of the spark plug tester exhibiting the weak spark situation with Cyl #1 versus #6. The tester was placed between the distributor cap socket for the respective wire, and the plug wire. Only #1 shows a weak spark condition, all others are just as bright and lively as #6.

karshack · #7 02-22-2016, 08:56 AM

Frank,

I tried a version of your suggestion. I swapped the installed Accel wires with the original Beru wires for (1) distributor to coil, and (2) distributor to Cyl #1. Sadly no change.

The Accels are the Superstock Radio Supression Core 8mm wires (Model 4040). Compatible with both points and electronic ignitions.

karshack · #8 02-22-2016, 09:09 AM

Graham,

As I referenced in the response to Frank, changing the wires did not make a difference. I only changed those 2 wires so as to not give the Pertronix any headaches. The Accel wires are high resistance: Distrib/Coil = 5,000 ohms; Dist/#1 = 5,400 ohms. That compared to the respective Beru resistance figures of 0 (zero) ohms, and 1,080 ohms. And I removed the resistor from the head of the Beru plug wire, clean the ends, and it tested at ~1,080 ohms.

I did attempt to use the Beru plug wire without the resistor. Maybe I should have done.

Frank Reiner · #9 02-22-2016, 09:14 AM

ks:

Give a thought to checking the valve lash adjustments, and then the cranking compression pressures.
A very low pressure condition will require much less voltage to initiate the arc; hence the low light in the tester.

Also try interchanging plugs with that in #1; does the problem move with the plug?

karshack · #10 02-22-2016, 09:33 AM

Mike,

I took your suggestion to look at the plug situation. Prior to installing the Pertronix, I was using Champion N9YC plugs all around. The #1 plug was gapped to 0.030" and showing normal wear.

After your suggestion, I tried these plugs at #1:
1) new Champion N9YC gapped to 0.045" = no improvement;
2) (hotter) new Champion N11YC gapped to .030"; then gapped to 0.045" = no improvement;
3) now going backwards/colder, new Champion N6Y gapped to .030"; then gapped to 0.045" = no improvement

I've re-installed the new Champion N9YC at 0.030".

Next, revisiting the distributor cap/rotor situation..

karshack · #11 02-22-2016, 09:50 AM

Quote:

Originally Posted by Frank Reiner

ks:

Give a thought to checking the valve lash adjustments, and then the cranking compression pressures.
A very low pressure condition will require much less voltage to initiate the arc; hence the low light in the tester.

Also try interchanging plugs with that in #1; does the problem move with the plug?

Hi Frank,
Thanks for the suggestions. I had not considered the valve adjustment issue. However I did do a compression test last weekend while trying to rest my brain over this spark issue. Performed both a dry and wet test, and in both instances #1 compression was slightly higher than the others.

The Service Manual states that the range should be 10-11 atm, with deviation between cylinders not more that 1.5. Cyl #1 looks like it is running a little high. I did not re-test this weekend, so I don't have new numbers. I used a recording Motometer and took a pic of both cards.

I am doubting the wire since swapping them out for the Beru's and not getting any change.

I'm off to work but will try to post more when I get home. Thanks again.

Frank Reiner · #12 02-22-2016, 01:43 PM

ks:

We might also note that the #1 cyl/position in the dist cap is associated with a specific magnet/reluctor tab in the Pertronix rotor.

optimusprime · #13 02-22-2016, 03:25 PM

Wrong type of plug leads and also you must have none resistor plugs only. And check engine earth strap from engine to body .Located from gearbox bell housing , to body ..

karshack · #14 02-24-2016, 01:12 AM

Frank,

Your suggestion about indexing the magnetic ring to the #1 cylinder distributor contact mark on the distributor base solved my weak spark problem.

I won't go into it in length here because I've already written twice; but I don't see any of those posts.

I just want to make sure that all of you who have pitched in with suggestions know that I am grateful. And, I want anyone who is following this thread to know that there has been a successful resolution of the problem.

Thanks again.