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Old 05-19-2001, 08:29 AM
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Join Date: May 2001
Location: Grand Marais, Minnesota
Posts: 8
I just bought my first MB, a '72 280 SE 4.5. It's been sitting all winter and has a dead miss in the 4th cyl on the right bank. It ran fine when parked. Compression is good, both valves are working, spark is good, injector is NOT working. I replaced the injector and still have the same problem. Any ideas? I read in the service records that it had the "trigger points" replaced when it had a similar problem. What are these and where are they located? Please help as I'm quite stuck. thanks, Jeff
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Old 05-19-2001, 10:11 AM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
The injection system (D-Jetronic Bosch) uses trigger points in the base of the distributor to time the injector firing. There are 4 sets of contacts and the injectors are fired in pairs in the firing order; cyl 1 and 5 would be first. If I recall the order is 15486372. That makes cyl 4 fire with cylinder 8 (which I know to be true).

If cylinder 8 injector is receiving proper signal then it is unlikely that trigger points are the problem.

Just for info sake; the injectors are fired twenty times by a full throttle activation. If the key is on but engine NOT running one can activated the throttle and listen to each firing (twenty clicks to full throttle). If one turns the engine by hand (socket and ratchet actually on crank bolt), each pair (and each trigger contact set) of injectors can be openned in this fashion.

If one has a scope much can be evaluated using the above technique and by pulling a vacuum on the intake manifold pressure sensor (MAP sensor in modern terms). By watching the changing injector pulse width (due to vacuum changes) the sensor can be evaluated without running the car.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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