1978 300SD. The headlights and dash lights will fluctuate in brightness when at normal cruise speed. They will sometimes become brighter with acceleration but not always. Sometimes they just change for no apparant reason. Does anyone have any ideas about this?

Jim W.

If the engine rpm is remaining the same and the light output is changing, then the system voltage is climbing and dropping.

You can confirm this with a voltmeter attached to the battery. Run the engine at 2000 rpm and watch the meter. If it fluctuates in voltage, the voltage regulator is the culprit.

I just diagnosed a similar problem with my 84 300D

My voltage would drop to about 10 volts, for about 5 seconds, and then go back to "normal". It would do this once or twice in my 1 hour trip to work.

I first suspected my heater/AC blower motor, as they seem to cause problems as they get old. But mine was OK.

To make a long story short, it was my auxiliary fan that was the cause, or something in the circuit to the fan. This fan is part of the A/C system, and is in front of the radiator. I also noticed that its fuse would be warm too. My other fuses did not get warm.

Once I disconnected it, my voltages are where they should be.

You might check to see if any of your fuses are warm or hot (temperature), as that could be the circuit that is drawing all the power.

Hope this helps.

__________________
1984 300D 398K-daily driver
1998 Honda Accord 102K -wife's daily driver
1991 Volvo 740 289K
2000 Toyota Corolla 143K

My 81 300CD did this every morning upon cold start. I cleaned the ground cable connection on the inner fender, removed both cables and cleaned the posts, no more flickering. On the freeway, I would suspect voltage regulator because it is also the brush holder. Worn brushes, weak pressure on the commutator inside the alternator, so the brushes at higher RPM lift slightly away.

Thank you. Now I know which direction to work toward.

Jim W.