w108 ignition at resistor issue - PeachParts Mercedes-Benz Forum

tivoklr · #1 09-01-2007, 03:08 PM

I have a 1970 w108 108.016, inline 6, I got running again after some downtime yesterday. After checking the points, I found that they were way out of adjustment and weren't opening enough, ala no spark.

I decided to set the points this morning, and I try to start the car and it shoots fire out the carbs. I figure the timing is way off since my points adjustment was just by eye yesterday. I set the points with the feeler gauge and reassemble and no start, no backfire, sounds like no spark.

Now I have a new problem, but it's been kind of ongoing and has made troubleshooting the ignition issue very difficult as having multiple simultaneous failures, blah blah...

There are 2 resistors in my car, sometimes they seem to get voltage and other times they don't. This is all with the key in the #2 position. Biggest problem, the resistor nearest the coil nor produces no volts in the run or the start position. Even if I jump it, I can't get the car to start.

I'm pretty sure there is some kind of circuit that is supposed to give the car 12v at the coil while cranking then drops it back down to the 3 or 4 volts seen normally at my coil, but I have not witnessed this via a meter.

I'm thinking there must be some kind of ignition relay(s) that can be replaced in this situation.

The haynes is pretty vague on the subject.

Any help, please?

Enjoy the holiday weekend.

tivoklr · #2 09-01-2007, 04:18 PM

I'm now thinking ignition control module. Is there any way to bypass this unit to test this theory since these aren't exactly cheap to replace?

Jeff

bill4247 · #3 09-01-2007, 04:50 PM

If you don't get a voltage drop between the + side of the coil and ground with the ignition on and the points open or blocked you have a break between the battery and ignition switch or between the switch and the coil. That would not involve the switching module. I think it should be 12V.

tivoklr · #4 09-01-2007, 05:37 PM

But at this point I have no voltage at the + side of the coil when the ignition is on. I have not blocked the points open to test this so if this is an essential component of this test please forgive my ignorance in advance.

I thought I should have voltage at the + side of the coil, 3-4 volts while running and 12 while cranking. Am I wrong here?

bill4247 · #5 09-01-2007, 05:55 PM

Yes you're right but they're all wired differently. I'm thinking the ICM may be wired between the ignition switch and the + side of the coil so you're ICM could be bad if you're getting no voltage to the coil. Do you have a wiring diagram?

cth350 · #6 09-01-2007, 07:27 PM

same circuit as the 4.5 with the ignition module and two balasts. One is bypassed by the starter. The gap is supposed to be pretty tight, the thickness of a paper match give or take.

Remember, once you change the point gap, you've also moved the ignition firing point, so you have to retime the engine. A good reference point on most mercedes is spark right at TDC when the starter is turning the motor (about 200 rpm). But, that's just to get the car running. Adjust it according to the book after that.

If you don't have the book, invest the 30$ in the CD. It's the best investment you can make for the care and upkeep of a mercedes.

-CTH

ctaylor738 · #7 09-02-2007, 11:22 AM

It's very rare for the control module to go TU. But if it does, go with a Pertronix and blue coil for a total cost of about $100.

tivoklr · #8 09-02-2007, 04:51 PM

That's what I thought, the module was pretty tough, but my problems are so intermittent and difficult to reproduce, (i.e., voltage at coil sometimes and not others)

If I get the pertronix and install it, it will bypass the control module completely, correct?

I'm fine with eliminating parts of the electronics of this car to get it to run...

I want to drive it again so bad I can't even tell you, so please keep steering me down the path.

Thanks!

p.s. I do have the wiring diagram and will be consulting it to check and see how the icm is integrated into the ignition circuit, although I'd like to eliminate all the bs that is down there with the petronix.

p.p.s. I just went out to test the voltage at the + side of the coil, and I get 3V when the car key is in the #2 position, before I try to crank the car, then at position #3 (cranking starter) the reading on the meter drops right to 0 and then stays there even after returning the key to the #2 (running) position. If I turn the key to off, returning it then to #2 results in a return of 3V to the + side of the coil. The - side of the coil is just a small brown wire that runs directly to chassis ground.

ggenovez · #9 09-03-2007, 10:07 AM

hey Tivoklr,

are you using a dwell meter or eyeballing it or with gauges? The only true way to measure is with a dwell meter.

The pertronix will replace your points and condensor and will now have a electronic sensor for ignition. You will still need a coil.

tivoklr · #10 09-03-2007, 05:27 PM

I'm using a feeler gauge but I'm pretty sure the pertronix is gonna happen as it's cheaper to implement than to replace the ignition unit...

Sometime next week. I have a new blue coil, I think it'll work but I may just get the distributor unit and the flamethrower coil.

We shall see.

mzsmbs · #11 11-17-2007, 06:25 PM

you can completely bypass the ignition module. that's how my car has been wired for a long time.

fahrt · #12 10-31-2009, 01:45 AM

Is anyone able to follow up on this thread with detailed instructions on testing the ignition control module that is used on these early '70s cars with points ignition? Are there ohm readings that can be taken on the unit to determine its condition?
Or, maybe someone knows if there are printed or online instructions available?
The MBZ CD manual has nothing about the ICM.
Neither does the big blue book.
Also, what is the method for changing the ignition wiring to bypass the ignition control unit? Which wires go where? Colors? Ballast or no ballast, etc?
Thanks!