coil problems on '86 W124

lewise · #1 11-14-2002, 08:14 PM

Hi guys,

Ok, heres what happened.

1. 1 week ago my car wouldn't start(1986 300E with the M103.983 motor)

2. Diagnosed the problem to a bum coil. Used a timing light to find a no spark condition on all plugs.

3. Absolutley needed the part ASAP. Local dealer quoted $177 and would have to order, would take 3-4 days.

4. Local Euro Import shop had the correct Bosch coil in stock, price $66 bucks. Just to be sure I take the old coil to the shop, we compare part numbers, find out that Bosch has a "superceded" or updated part, and the one he has in stock is this new part.

5. Take it home, it installs perfectly, all the bolt holes line up, the terminals are in the right place and the right size etc.

6. Car starts and runs GREAT.......for exactly one week

7. Now same conditions found, no spark on any plugs using timing light method.

Does anybody have any ideas?? Could something( and what thing) could be burning out coils? or could it just be a bad/wrong coil.(perhaps this coil wasn't quite up to MB standards) even though that seems hard to beleive since the original coil was Bosch manufacture.

Sure would appreciate some insight from you guys.

Thanks
Evan

joe p · #2 11-14-2002, 08:53 PM

Ok, first off a coil has to have power and ground interuption (signal) to fire. However, the flaw in your testing is simple, you check for fire from the coild at the coil wire, you check the spark plugs wires for cap and/or rotor problems. Take the coil wire off, place the end about 1/4" from metal and have someone turn the motor over for you, if it fires, it aint the coil. :-) If it does not fire, time to look for signal, this can be done with a dvom or a test light by removing the ground lead and chacking for ground interuption.

If you have fire from the coil and none to the plugs either you have a bad cap, rotor or rotor support. If you have no signal for fire to the coil, EZL or crank sensor. However, before you condem anything, make sure you cover the basics, power to the coild, fuses ok, OVP (while it wont stop a 103 from starting it will cause other weird problems.)

Joe

joe p · #3 11-14-2002, 08:54 PM

Oh ....DOHHHHHH!

New parts crap out too. :-)

Joe

lewise · #4 11-15-2002, 01:07 PM

joe,, again tell me how do I check to see if the coil is getting signal? I understand that the crank position sensor feeds in to the ezl, and then it into the coil. I'm pretty sure that theres no juice coming out of the coil wire going to the rotor, since I clamped and inductive timing light around the coil lead, and cranked the motor: no light, I figure it should have lit up continuosly. To check for signal, how do you do that... I'm assuming it one(or both) of the small (small bolt and nut) arrangement on the top of the coil. I see that one side appears to be grounded,,,,,,can you use a test light or DMM to check for voltage across those points????

Thanks
Evan

dpetryk · #5 11-15-2002, 05:21 PM

The EZL ignition module may have a problem. If you have a scope check for signal across the the two terminals of the coil. Not the HV (secondary winding) but the primary windings connections. A voltmeter could also be used.

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