I second the advice on the grounding connections and especially the sensors. Years ago I had a somewhat similar experience. Not so much the overheating, but chronic (really chronic) ac failures. After replacing all (not kidding) of the ac components over time as each failed, the problem turned out to be the high pressure fan switch. My car didn't overheat, but since the fans intermittently didn't cut in the ac pressure went off the scale causing whatever was the weakest component in the system to fail. Learned alot about ac, but really didn't enjoy the process. Simple, and not too expensive to swap them out, which in this case isn't a bad diagnostic approach given the intermittent nature of the problem. But first clean those ground points. These electrical systems are fussy. It is amazing how many strange problems occur from poor fuse or relay contacts, even when they look ok to the eye.

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Bart

2002 ML500, 2009 CLK350 Cabriolet
93 300D 2.5 Turbo 265,000 Miles (Sold - bad idea)

1990 300E: Where is aux. fan temp. sensor located at?

The problem is... low voltage... as stated before.

Why? The primary suspect is the voltage regulator. It is mounted on the altinator.

It also contains the brushes that the altinator uses to generate electricity. If one of these brushes is worn too low then you will get low voltage... as stated by the mechanic who tested the altinator. Another possibility is that your battery cables and battery teminals are corroded and not letting the altinator recharge the battery. Clean up inside your battery cable connectors and around the battery terminal. Then test to see if problem remains. If so then pull out the voltage regulator from the altinator and have a look at it. If brushes are worn low then replace the regulator.

Also do exactly as a former post suggests regarding cleaning up you grounds. Corroded grounds, like corroded battery cables, cause low voltage.

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Earl



1993 190E 2.3
2000 Toyota 4x4 Tundra

Since the voltage regulator is a separately serviceable unit, it may have not been included with a replacement alternator. Earl has a point, it takes 5 minutes to pull it and look at the brushes.

Here is a possible way to test if the problem originates from the ACC system.
ABS light on and radio off is the result of low voltage. These, along with dimming headlights are the classic symptoms of a deceased voltage regulator.

Duplicate the conditions that gave you ABS light on and radio off. Then turn the ACC off (-O- button). Assuming the ABS light goes off, turn the radio back on, then turn on the headlights -bright- and the rear window defogger. If you again get ABS light on/radio off, your problem is outside the ACC system. Look harder for a poor ground, cracked wiring, possibly replace the voltage regulator.

If you cannot make the ABS light come back on with every non-ACC electrical gadget in the car turned on, concentrate your testing on the ACC subsystem.

If you have a soldering iron, before I replaced the ACC control unit, I would pull it and melt every solder joint on it to take care of any faulty soldered connections. (which is not uncommon as these cars age) Same for the OVP relay.