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Old 01-23-2005, 09:02 AM
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Join Date: Jan 2005
Location: Cedar Bluff, AL
Posts: 3
Help 86 300E ignition problem

Ignition begins to fail after 5 to 10 minutes of running but it isn't a total failure. It will idle fine but when you try to load the engine or try accelerate it will die. Testing so far is as follows. We have determined that the temp sensor is working correctly, the throttle position switch is working and the vacuum line ti ig module is connected. We have tried putting a timing light on and watching it when the problem occurs and it goes completely dark or not firing when you advance the throttle suddenly. If you advance the throttle very slow with no load the engine will rev up and run fine at higher RPM's. We tried the timing light on both the coil and #1 to make sure it was not a problem with the coil. The firing stops on the coil lead before the distributer to rule out some problem with the dist. The ignition module is not getting hot nor is the coil. Without expensive diagnostic equipment we are stumped or at least led to believe the ignition module is bad.
One other problem is that once it fails, you can't get it started until it is left for several hours. When trying to start, it appears to be firing, but erratic. The timing light indicates it is firing but the engine belches, kicks back, etc like it is firing out of sequence. I would have suspected the hall effect sender but it runs so well at higher RPM's it should be okay. It (the crank position sensor) measures about 780 ohms.

Any help really appreciated.


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Old 01-23-2005, 11:42 AM
Registered User
Join Date: Aug 2001
Location: Evansville, Indiana
Posts: 8,150
Could be a number of things, but re-check the crank position senor (the one on the back of the engine, the front one is diagnostics only, isn't used by the computer).

Most likely the sensor is bad or the ignition computer is toast, although your problem actually sounds like a fuel delivery problem to me instead!

If you are lucky, this is a bad overvoltage protection relay (behind the plastic cover behind the battery) -- if you still have the one with the red single fuse on top, replace it, it's very likely toast by now.

1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!
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Old 01-23-2005, 12:08 PM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
If you are correct in this being an ignition problem, I would suggest a visual inspection of the rotor finger and the dist cap center electrode. Through your description of test technique I would really suspect a bad coil.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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Old 01-23-2005, 07:08 PM
Registered User
Join Date: Jan 2005
Location: Cedar Bluff, AL
Posts: 3
Ignition problem

Thanks - I had figured out that the front sensor only goes to the diag plug and only tested the resistance of the rear position sensor. I wasn't sure what kind of position sensor it was but was still a possibility. I "assumed" it was less of a chance when we found out it fires fine at higher RPM when the duty cycle is higher even after warming up. My first suspicion was fuel delivery too but immediately gave it a shot of ether and it still wouldn't fire, then pretty much ruled it out when I saw the timing light completely go dark when reving.
This machine does have the original overvoltage relay but it seems to also be okay as far as diodes (Zeners?), fuse and coil. I am not familiar with what this device does other than to protect the electronics/electrics against a runaway alternator but still may be a possibility.

Stevebfl - I had pulled and inspected the distributor and pretty much zeroed in on the coil or module after seeing the timing light go completely dark when on #1 and ALSO the coil lead. The puzzling part is that the spark is strong at higher RPM's when the load doubles or triples on the coil.

Thanks to you both for the help - thanks for taking the time to read and suggest possible solutions.

These parts are too expensive to try troubleshooting by trial and error, so if I can't find the part or if it is the module it looks like an after market ignition like Crane or some other may be cheaper than any one of the Bosch components. It appears that even the Bosch crank position sensor costs more than an aftermarket ignition system.

Again, thanks for the help. Every input helps.

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Old 01-23-2005, 10:24 PM
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Join Date: Dec 2004
Posts: 14
I had the same problem on my '89 2.6

Hi, read your post of similar problem I had with my '89 190E 2.6 w/ 170K miles; long story short, it ended up to be the crank position sensor that fixed the problem even though my old sensor measured at 900 ohms (within specs).
I have shotgunned my misfortune by replacing parts one by one; OVP relay, fuel pump relay, EZL module, ignition coil, and finally the crank position sensor.
I swapped the old parts in after crank position sensor replacement to be sure it was a good fix, it's still running as of today (6 weeks after).
I reinstalled all the new parts since I can't return electrical item, but I have a hunch you may have the same problem as I. Just my thought.

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