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I have a 1985 MB 300DT with 75000 miles on it and a very good engine.
Last winter (I live in Chicago) the car started having difficulty on starting. It would start but it would run very rough and the engine would die if I did not give it some gas (not too much, bust just enough to keep it going),until it was warm enough. Thereafter it would run perfectly. So I thought that one or two glow plugs might be worn out and I changed them all (Beru ones). The problem however persisted unchanged. I bench tested each of the old and the new ones and both sets would glow quite well. I also run further tests and I noticed the folowings:
1) The glow plug light on the dashboard comes on, but it goes off much before I can hear the click of the relay disconnecting.
2) I tested the voltage going to each glow plug (installed) with a voltmeter (positive to the wire terminal,negative to the glow plug tip) and turning the key on I had for each one the following: from an original 9.5V it progressively increased to 10.5 to 10.6, at which point the relay clicked and the voltage dropped to 0.5 or so.
3) I changed the glow plug relay, but that did not solve the problem either, so I tested the voltage carried by the hot wire going to it (Positive to the hot wire, negative to ground; hot wire connected to the relay). With the ignition key off the voltage was that of the battery, 12.5 12.6. Turning the ignition key on it dropped to 9.5V, increasng progressively to 10.5 10.6V when the relay clicked, then it returned to 12.5 12.6.

My interpretation is that there must be another relay (probably the bad one) somewhere so that the plugs do not get enough power to heat sufficiently. I do not know where to look though, neither I know if my guess is correct.

I obviously need some help. Is there anybody out there with any suggestion??

Thank you very much

Bill de Rosa
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