1991 300CE no spark
 

A friend has a 1991 300CE, so the M104 but with the KE-jet and the billion dollar EZL.

He said last year the car would run, but did not like to restart when hot. If he let it sit an hour it would restart. Now it does not start at all.

I checked for spark and he's not getting any. Out of curiosity I gave the intake a little spray of ether, nothing.

Fuel pump comes on and hes getting fuel at the injectors and the fuel distributor.

So that would make it the coil or the ezl? Is there a way to test the coil or test the EZL?

Really nice rust free 104k car, so he'd like to get it back on the road for summer driving.

Start at the plugs and work your way back. The classic "won't restart when hot" is usually cap and rotor.

Try testing the crank sensor first. That is the only other usual suspect, and it doesn't fail often. Do a search to be sure, but I think it's supposed to read between 800-1200 ohms if it's good.

Otherwise I would give it a full tune up: plugs, wires, cap, rotor. The ezl never goes bad.

If I'm not getting spark at the coil, why would the cap and rotor be suspect.

It will not start at all now, car has been sitting for a year since it got questionable.

Then check the crank sensor. It gives the signal to the coil. You unplug the coax cable at the ezl and check the resistance between the middle prong and the outside wall.

I say change the cap and rotor because it needs it, and it will solve the no restart when hot problem.

The m103 (your particular m104) needs a tip top ignition system to run right. If the car started when it was parked, and now won't start, either the ignition system got condensation and got corroded, or a mouse ate some wires, particularly the crank sensor.

Is this the front crank sensor or the rear? As someone who owns exclusively 60X cars I'm only familiar with the rear crank sensor but I know the gassers have a front one.

The rear crank sensor is the one. The front crank sensor has no effect to the engine, it is only used for testing/diagnosing.

Have had a few in here like that, and it was just the coil.


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I'm curious: how does one test the coil anyway?

If you have the E core OE coil, check for voltage at connection +. Have someone crank engine while you hold the coil lead close to a ground. If you have 12 vdc at + but no spark cranking...it's most likely the coil.

let us know the results.

Here's a link to a new coil....1991 Mercedes Benz 300E Ignition Coil - Replacement REPM504620 - Discount Body Parts


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Sweet, thanks for the advice I'll see if there is 12V at the coil.

This.

The giveaway is that the first symptom was no start when hot.

The rear crank sensor can provide the same symptoms! but I would say 10 times it's the cap and rotor to every 1 time that it's the rear crank sensor.

With new cars needing virtually no tune ups ever, people forget that the m103/ m104 with cis-e needs :

New spark plugs every year
New fuel filter every 2 years
New cap, rotor every 3-5 years,
New wires every 5 years.

2 years and 5k miles ago this car got cap, rotor, plugs, wires, and timing chain at the dealer.

I will have to find time to get out to my friends house and check for voltage at the coil and check the crank sensor for continuity. I looked and M103s and CISM104s use the same coil so I may take a spare with me to try and if it works we will buy a brand new bosch coil.

Coils from various models of Benz's of the era are the same. If you have a spare one off a M103 - then that will do. Even borrow the one of a working car temporarily to see if this is the problem.
Last time I had your problem it was the coil.

Pray like mad that it's not the EZL.

can't be a cap and rotor on a 104.. doesn't have one... sounds more like crank sensor.... the only problem is that it doesn't start now... ezl is a minute chance... coil..not likely, but possible.... i have one of those if you want to test.