W124 Heater woes - PeachParts Mercedes-Benz Forum

Addbhp · #1 04-10-2007, 08:34 PM

My heater has stopped working. It 1st started on the weekend, where despite what buttons were pushed nothing worked. Then after about 1 hour it came on out of the blue. At the time it seemed to be on defrost mode again despite what buttons were pushed. Then it worked fine for about three days on auto, where I didn't touch anything except adjusting the temp, afraid it would die on me again. Then sure enough last night it died - and it seemed after going over a bump. The blower motor was new when I purchased the car a month ago (saw receipt), and I looked at the small fuse outside of the fuse box and it looked okay.

I can hear the buttons working in the dash, but have the sense it's electrical but not sure where else to check. I've had the heater control unit out to make sure nothing was loose, but that looked fine. Any suggestion.

1st we tackle this, then I have questions on why the seat heaters don't work, buttons do light up okay, but no warm seats?

softconsult · #2 04-10-2007, 10:05 PM

There is a strip fuse, like a ribbon, in a small black rectangular box about 3/4" x 1 1/4" just in front of the main fuse box. There is a good chance that this may be the problem.
The fuse can look good and still be bad.

The random thing happened to one of my 124's along the way and that was the culprit.
You could try bridging it with a wire to determine good or bad fuse.

Fuses available at the dealer , very cheap.

Steve

Addbhp · #3 04-10-2007, 10:09 PM

I try that since, it would be the least expensive place to start - although it looks fine. Car is a 1990 300e if that matters at all.

Uncle Acky · #4 04-10-2007, 11:08 PM

Best to replace the fuse as mentioned.

Please note that the system will stop working in the event of excessive heat at the blower control unit AND/OR a short circuit. It will resume operation if/when these are no longer factors.

If you still have problems look for a three wire harness and connector with the engine hood fully raised, look directly behind the brake booster near the top of the firewall. You will see the black connector with three wires, Red, Black, and Yellow. Disconnect the connector, it can only go back on one way. Note the wire colours to the respective terminals.

You will need a voltmeter for this test. With the engine running and temp wheel at max, fan speed on low, check voltage at the connector terminals NOT at the wires from the connector to the engine compartment.

You should have 12 volts between the red wire + and the Black (ground) - Terminals.
You should have about 1 volt at the yellow wire terminal + to Black (ground)- terminal. (Note that the fan will not run as it will be diconnected at the firewall)

Now push the Auto fan speed button. You should now have about 1.4 to 4.5 volts at the yellow wire terminal + and ground.

Now push the high fan speed button, you should now have between 6 to 9 volts at the yellow wire terminal + and ground.

If you find these readings your pushbutton assembly is OK and the problem may be the blower regulator or excessive build up of dirt and junk at the intake of the evaperator causing the blower regulator to over heat.

If this is the case you will have to remove the blower and regulator to inspect and clean the exterior of the evaporator coils.

Sorry but I don't have my notes or diagram with me.
Let us know what you find.
Acky

Rick76 · #5 04-11-2007, 05:40 AM

If your regulator and blower (and fuses) check out OK then your pushbutton unit is probably at fault. They are known for intermittent problems. If it is made by Bosch, the solder around the 5 volt regulator integrated circuit cracks from heat. If it is a MK unit (the most common), they develop solder cracks around the switches and temperature adjust potentiometer.

As for your heated seats, what year is your car? Early 124's had the controller module on the floor in the wiring channel - drivers side. They would corrode from the mats getting wet. The controller was later moved to under the back seat. Of course check your fuses first.

brewtoo · #6 04-11-2007, 05:03 PM

First, I assume your blower motor is stopping, right?

I had one with an intermittent problem. Turned out that the three-pronged plug where the blower plugs into the firewall was a bit corroded.

Pulled the plug off and put it back. Worked fine after that.

lee polowczuk · #7 04-12-2007, 09:24 AM

i wiggle my ignition key if my blower motor stops working... go figure.

Addbhp · #8 04-13-2007, 09:21 AM

Well still no heat, last night tried removing and reinstalling the three prong plug along the firewall, and that did nothing, connections looked clean. Then upon closer inspection when looking at the fuse for the third time, did realize it had a hairline crack. Off to dealership this morning, replaced, still no heat.

Tonight I'll buy a voltmeter and test the voltage across the terminals of the plug, but fear I'll be going into where the blower motor is to replace the resistor. I've replaced a blower motor and resistor on a 91 BMW e30 in the dead of winter, but am hoping this one is a little easier to get at. Wish it would warm up here in Toronto to do this work.

brewtoo · #9 04-13-2007, 09:15 PM

When you say no heat, do you mean the blower motor is not running?

If so, there is more than a resistor by the motor, and it's not cheap.

I had the blower stop working on my '90. I took the motor out and the shaft would barely turn it was so tight.

I put some oil on the shaft and turned it a bit by hand to loosen it up. It worked perfectly after that and has for the last seven years now.

lemur · #10 04-17-2007, 03:57 PM

i wiggle my ignition key if my blower motor stops working... go figure.

I had that problem. It got worse until the blower didn't work at all. Turns out that the electrical portion of the ignition switch was the problem (part # M5050-32967). I replaced the wobbly tumbler cylinder while I was at it--I've heard that it's a very good idea to replace these when they start to get too loose, at least on the w123's.