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Premium Member
560SEC, 190E, Frontier, 240Z
1,329 Posts
Discussion Starter #1
A few questions for the smart people as my search didn't show me much.

1988 560SEC (US Federal)
1) No heat from the vents.
A/C works great, all ducts work properly, fan blows normally but no heat, just ambient air.
Doesn't matter if driving or idle.
Once it a while it works... then doesn't.


2) Heated seats flash
The other night my heated seat was on Low and working normally.
My passengers was on high.
Stopped at a light for a few minutes and noticed the passenger side seat switch lights were flashing and no warmth.

Played with all the switches and by then I was driving and all was returned to normal.

I did have just about every switch and light on including fog lights.

Low volts output from alternator?

3) What causes that BATTERY light to come on anyways?

1993 300TE, 1995 E320 Wagon
1,895 Posts
On the heat issue-- #1 suspect is monovalve. Inexpensive and easy DIY.

On the heated seats-- I think it's related to the battery light question. My guess is that your alternator (probably just the voltage regulator) is just on the edge. The brushes wear down over time, and at low revs (at a light), your voltage supply will drop (esp. if the lights are on). I think this might cause the heaters to go off, and there might be a default flash for a heater failure... but I have no idea how this would happen. I'd do a check of your alternator voltage output at idle. Should be 14+ V or so. The VR is also inexpensive (like $30. $65 at the dealer), and easy to swap out-- a bit fiddly with the screws, but doable.

The battery light would go on at a low voltage signal from the alternator. Does it glow very slightly even at speed?

Voltage Regulator parts for 1990 Mercedes - Bma Auto Parts

1986/1990 W126
14,138 Posts
Yup the manual says the seat heater lights will flash if there is too high a drain for them to work Same goes for the rear window heater.
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