300CE weak spark - PeachParts Mercedes-Benz Forum

DieselPaul · #1 11-02-2016, 05:05 PM

300CE I bought from a friend had no spark. Turned out somehow the wire to the crank position sensor on the rear had been cut. Replaced the sensor (at considerable effort) and now I have spark. But it looks really quite weak if I set a plug on the valve cover and crank the engine over.

A spritz of ether into the throttle body will not kick the engine over.

I hear the fuel pump running but I wanted to see if it was a lack of fuel issue, it apparently is not.

The plugs don't look worn, but they were fairly black, I know several people tried to get this car running for him so I don't know all of the cranking fouled the plugs out. I ordered a set of non-resistor plugs for it.

I have 2 EZLs (yes the hard to find ones specific to this car) and both allow the engine to spark, but weak, and the car won't hit on starting fluid or whatever the fuel distributor is, or isn't doing.

Pull the valve cover and with the cams at the timing marks, #1 is at TDC. So valve timing is in the neighborhood of being right, chain is nice and tight so I don't suspect anything as jumped and the engine turns freely, I just wanted to verify.

Anyone had a weak spark experience on an M103 or an M104.980? This is the 90-92 M104 so while its an M104, it uses a distributor and a single coil like an M103, not the semi-coil-on-plug setup of the later HFM M104s.

My next step was to borrow a coil from a friends M103.

Frank Reiner · #2 11-02-2016, 05:40 PM

DP:

Simple first checks:
Coil resistance, primary & secondary, pri.~.2-.4ohms, sec.~10-13kohms

If OK, hook up a secondary wire, coil to one removed spark plug. Then crank engine, better spark?

Check voltage at coil terminal 15 while cranking: above 9V.

porkface · #3 11-03-2016, 09:34 AM

replace the coil. resistance checks are futile. look at the mounting bracket, see any white-out? that's spark leaving the coil through the side and not the coil wire. seen it 100 times. had 1 that acted like rpm cutoff at 45 mph or so. POSITIVE it was fuel until i scoped the ignition. the spark would just pause causing the stumble. very common in that design coil. good luck, chuck.

Diseasel300 · #4 11-03-2016, 09:57 AM

Since this is a distributor system, have you checked/replaced cap and rotor? I'm surprised nobody's asked this yet.

Ferdman · #5 11-03-2016, 10:15 AM

DieselPaul, as you may know with that engine it is extremely important to maintain the ignition system, otherwise the EZL will fail. As mentioned check the distributor cap, distributor rotor and also the ignition wires. Based on the weak spark, it seems the coil may need replacement. I run Bosch F8DC4 spark plugs in that engine.

DieselPaul · #6 11-03-2016, 11:35 AM

I have a set of the proper plugs from the dealer and I need to put them in. However I cracked a spark plug boot end when I was pulling the plugs, have a new one coming in from the dealer tomorrow. So I haven't touched the car since I made this post.

I will try to come up with a new coil in a short period of time.

I pulled the cap and rotor the other night, no obvious cracks in the body. There was a fair bit of buildup on the rotor and the pins, I scraped them off with a flat screw driver. They don't look worn away. Cap and rotor for this car is around $200 so I would like to at least get it running poorly before I throw that kind of money at a guess.

I would imagine you guys are right on track with a weak coil.

For one reason or another the PO paid the dealer to do a timing chain on this car at around 90k. So it has a nice set of Bosch dealership wires on it and F8DC4 plugs in it, but the plugs are dark from people cranking on it and it not firing. So I think the wires are probably solid but the plugs will get replaced tomorrow when I have the one new plug wire boot in.

Called the local parts suppliers $90-180 for a coil and they don't keep them in stock, or $30 shipped online with Monday delivery. It was worth $90 for instant gratification, but its the same wait for the more money.

97 SL320 · #7 11-03-2016, 07:43 PM

Are you checking for spark at the plug wires of the coil wire? I'd check both and compare.

I've seen rotors burn through on way too many different engines so I'd check that if spark is strong at the coil.

Stretch · #8 11-04-2016, 05:38 AM

Word of warning - that's also in the FSM - checking the spark plugs against the block increases the likelihood of buggering the ignition module. You are better off checking the HT side with a strobe light that clamps on the outer sheaf of the leads.

As for checking the coil there's a whole (tedious) heap of procedure in the FSM for this that is worth wading through...

porkface · #9 11-04-2016, 07:47 AM

bosch wires from the dealer have been a headache for me in the past. never again, now it's beru or karlyn for wires. but make sure the cap and rotor are bosch. other brands don't do well. good luck, chuck.

DieselPaul · #10 11-08-2016, 08:26 PM

So I just moved my shop so I am going through a bad case of "what box did that get put in" I cannot find my multi-meter, it was harbor freight unit anyway, may have been tossed in the move, so I have not done any voltage tests.

However I have a test light. Should I have 12v at both terminals on the coil? I thought one of them was the "negative"

New coil, new Mercedes OEM plugs (yes they sell the correct non resistor ones), and still nothing really. I sprayed some ether in the intake and it hits lightly a time or three, but nothing really.

I have two used .980 EZLs and both behave exactly the same way. One is unknown from the junkyard, the other is the one out of this car, which noone has seen run in 3 years.

I have read a few old forum posts of guys putting M102 and M103 EZLs in M104.980 cars. They say they will start and run, but won't rev past like 4,000rpms. That would however be enough to diagnose dead EZLs and give me the confidence to drop $500 on a used unit on eBay. A friend has a spare M103 EZL or two that I am going to go borrow and try.

I am currently looking up the 124 FSM to see if Mercedes sheds any light on the mystery that is the M104.980.

Interesting FSM pages on what the EZL is and what it does, but I don't see in here how to test it.

http://www.w124-zone.com/downloads/MB%20CD/W124/w124CD1/Program/Engine/LHIS/15-0020.pdf

97 SL320 · #11 11-08-2016, 09:55 PM

The "positive" side will have 12V from the ignition switch.

The "negative" side will have 12 V - the resistance of the primary winding if the " contact points" ( EZL ) are not grounding the neg side of the coil.

When the "contact points" are grounding the neg side of the coil, you won't see any voltage.

If you put the test light on the neg of coil and other part of test light on chassis ground, you should see blinking while cranking.

Another test I'd do is measure crank sensor resistance at the EZL plug then measure AC voltage of the crank sensor while cranking the engine. I'm pretty sure the crank sensor on the EZL is a coil / magnet that makes AC ( later non EZL is AC output ) . If it is a Hall effect transistor type, the sensor needs a power source that could be less than 12V, ground, then one wire is an on / off DC output voltage. These specs should be in the manual.

DieselPaul · #12 11-08-2016, 10:33 PM

Cool thanks for the insights. It actually started and ran for a few seconds tonight. Then I went to check and see if I was getting fuel... lines were bone dry. Put 5 gallons of fuel in it, fuel system primed and a hose under the car burst.

Autoparts store is closed to buy some tubing tonight, but we will call it progress of some kind.

DieselPaul · #13 12-28-2016, 11:23 AM

Distributor rotor has a hole/crack in it. Someone else figured it out. Car is running great now. Got a new rotor from the dealer and all is well.

Figured I'd update this thread to maybe help someone else down the road.