heat blows before engine is warmed up...

[Prime]

Anybody got an easy diagnosis on this? The temp sensor works fine - the temp and fan regulate properly once the engine is warm, including the fan going to minimum speed when the cabin reaches the set temp. But when the car is first started in cool weather, the fan blows hard even though the engine isn't yet warmed up enough to provide warm air. It didn't always behave this way. What happened?

Of course I can set the fan to minimum speed manually, but I want it to work the way it is designed.

 

[gsxr]

Yeah, I don't like that either, but all four of my W124 V8 cars do the same thing. Only my 1980's vintage W124's keep the fan low until heat is available. I suspect MB had customers complaining that the ACC "wasn't working" so they "fixed" it on later models. Anyone else notice the same thing?

 

[J. M. van Swaay]

If you have a resistor installed to to make the aux fans come on at lower temp, (ie Jim F's cool harness) that will cause the auto blower to come on before the coolant warms up....... Ask me how I know. Not 100% sure on the V8 124's, but I believe there is more than one temp sensor. One feeds the ECU, the other triggers aux fans and auto blower. If the one that triggers aux fans and blower is off spec, (or if you have a short or other wiring issues) that could also cause your problem.

I agree that the auto blower is a nice function on cool mornings......

J. M. van Swaay

 

[gsxr]

There are three engine temp senors:

1) Four-pin sensor which feeds 2 separate signals the LH and EZL modules
2) Two-pin sensor which triggers the twin electric fans when the engine gets very hot
3) One-pin sensor which sends a signal to the instrument cluster temp gauge

The three different sensors are not interconnected in any way, they are all separate. All are located at the front of the intake manifold in the coolant passage connecting the cylinder heads. The JimF mod should be telling the HVAC system the engine is *colder* than it really is, so I would be very surprised if this was indeed the source. My wife's new '95 E420 doesn't have this mod so I'll have to keep an eye on how the blower acts after a cold start this winter.

 

[Rudy]

same thing here
when colder in the morning hot air blows inside, although engine is cold.
I used to close the center vents, but its still PITA.

Rudy

 

[Prime]

I don't know, Dave - I think he's on to something here. My car never had this problem before - it always waited until the engine was warm before turning up the blower. UNTIL this autumn. Guess what mod I did during the summer? I think maybe I'll go snip the leg from the resistor and see what happens.

 

[gsxr]

Hmmm. If that is indeed the cause... guess what mod is coming off my cars? LOL!

 

[J. M. van Swaay]

The aux fan mod works because the resistor makes the controller think the coolant is hotter than it really is. IE, the controller is faked into thinking the coolant is 110C when it is really 100C. When the coolant is cold the same thing happens. The controller thinks it's warmer than it really is. If the controller thinks it's warm (but really isn't) it will start the auto blower even though the coolant is still cold.

The FSM gives the specs for the temp sensor, to check the temp response of the sensor, place it in hot/ice water bath, hook up an ohm meter........ (my high school chemistry/physics teacher would be so proud)

One my wife's E320 wagon, the auto blower function just stopped working a few winters ago. Turned out to be a bad sensor.

I have the resistors installed in all of my 124's but I also included a small switch to activate/deactivate. In the summer, I leave the switch in the active (resistor in the circuit) position. In the winter, I leave the switch in the inactive (resistor isolated from circuit) position.

J. M. van Swaay

 

[gsxr]

I b'lieve you are correct, JM. Thanks for the explanation - makes perfect sense. I will try this on my cars and see what happens. I have mine set up so the resistor can be unplugged if desired, so I'll just pop it out and see what the blower does... will report back with results.

UPDATE: I tried this before driving to work this AM (40F ambients). As expected, JM is 100% correct. Removing the "CoolHarness­™" cures the problem.

JM, you da MAN!

 

[IBNR]

JM and/or Dave, I'd love to know how you hooked up a switch or removable resistor. Pics? Instructions? Thanks!

 

[gsxr]

JM and/or Dave, I'd love to know how you hooked up a switch or removable resistor. Pics? Instructions? Thanks!

I simply duplicated JimF's CoolHarness­™, as shown below... this plugs in between the engine harness and temp sensor, and will unplug just as easily. More info is at this link.

Adding a switch in the passenger compartment would be considerably more complicated. I'm sorely tempted to just leave the resistor out permanently, as I've not been impressed with how the setup functions in summertime... on hot days, the electric fans never shut off, even when driving on the freeway; as the temp never gets low enough (<90°C or so). This will vary depending on your local climate though.

 

[Prime]

Yep - I also removed the resistor and the problem is gone. A big thanks to JM!