BenzWorld Junior Member
Date registered: Mar 2011
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engine surge w/ alternator failure - 280SE 4.5
MY 1973 280SE 4.5 just experienced its third alternator failure in the year since we purchased it.
What appears to happen is as follows: weeks/months of normal, excellent performance; then on a cold start, the engine idle surges from high to low (even with the transmission engaged and the vehicle moving, you can still feel the surge, although it doesn't seem to affect power or movement -- in other words, acceleration overcomes the surge). After five minutes or so the surge evens out and the engine runs normally (at higher idle until it is warmed up and the idle drops as usual).
Now for the third time of this occurring, the generator light has come on within a few minutes to a few hours. This happened with the alternator on the car when we got it, the replacement, and now the replacement for the replacement. The battery tests at 12.15-12.25 v. after the apparent failure. The electric motor shop where we had the alternator replaced a second time tested it and said the alternator was "all over the place," but since we weren't expecting a third failure we didn't seek further information.
It's possible that this is a chicken-egg problem; the alternator could be failing first. The generator light seems to vary in brightness as you rev the engine, so it's possible that it is on faintly prior to the surge happening.
The alternator is a Bosch, and is correct for the car.
Any ideas what could be causing this?