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Pelican Technical Article:

Replacing Your HVAC Resistor

Steve Vernon

Time:

5 hours5 hrs

Tab:

$180 to $210

Talent:

***

Tools:

T20 Torx

Applicable Models:

Mercedes-Benz W124 (1986-95)

Parts Required:

New resistor

Hot Tip:

Be careful not to break the plastic mount when removing Performance Gain: Working HVAC

Performance Gain:

Working HVAC

Complementary Modification:

Clean the blower motor

If you turn on your HVAC system and the blower or fan motor only runs at top speed there is a very good chance you have a bad resistor and need to replace it. If your fan is starting to run erratically with no noises coming from the motor, the resistor may be starting to go bad. Replacing the resistor is a lot of work so give a good cleaning and maintenance to the motor, squirrel cages and wiring while you have everything apart.

You will need to remove the mono-post wiper, motor and the blower motor first.

Please see our articles on these procedures for additional assistance.

Tech article on: Mono-Post Wiper Replacement

Tech article on: Blower Motor Replacement

If your blower or fan motor runs at full speed regardless of the setting you have it on there is a very good chance your resistor is bad.
Figure 1

If your blower or fan motor runs at full speed regardless of the setting you have it on there is a very good chance your resistor is bad. The resistor is located below the blower motor in the cabin air box (red arrow). Please see our article on blower motor replacement for additional assistance on removing the motor.

With the motor removed you can see the electrical connection on the resistor (red arrow).
Figure 2

With the motor removed you can see the electrical connection on the resistor (red arrow). You need to remove the four T20 Torx screws holding the plastic mounting piece for the motor in place (yellow arrows). Use care not to drop them into the bottom of the air box.

Remove the motor bracket from the box (red arrow) and pull the wiring harness out from the grommet in the air box (yellow arrow).
Figure 3

Remove the motor bracket from the box (red arrow) and pull the wiring harness out from the grommet in the air box (yellow arrow).

Use a T20 Torx and remove the three screws holding the resistor plate to the air box (red arrows).
Figure 4

Use a T20 Torx and remove the three screws holding the resistor plate to the air box (red arrows). Also remove the two T20 screws holding the electrical connection to the plate (yellow arrows). There is a paste between the electrical connection and the resistor plate; if you are reusing the resistor do not clean this off. Use caution not to drop any of the screws into the bottom of the air box.

Next remove the three T20 Torx screws holding the plastic piece that holds the resistor plate to the air box (red arrows, two already removed).
Figure 5

Next remove the three T20 Torx screws holding the plastic piece that holds the resistor plate to the air box (red arrows, two already removed).

Lift the plastic mounting piece up (red arrow) and slide the resistor plate out to the left (yellow arrow).
Figure 6

Lift the plastic mounting piece up (red arrow) and slide the resistor plate out to the left (yellow arrow).

It is a very tight fit getting the resistor out but once you get the center cut out past the center post it will come out easy (red arrow).
Figure 7

It is a very tight fit getting the resistor out but once you get the center cut out past the center post it will come out easy (red arrow).

Follow the wiring harness through the grommet in the firewall (red arrow) and disconnect it from the connection behind the brake booster (green arrow).
Figure 8

Follow the wiring harness through the grommet in the firewall (red arrow) and disconnect it from the connection behind the brake booster (green arrow). Installation is the reverse of removal.

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Comments and Suggestions:
Rob Comments: Thanks Nick, Have 94 e320/w124 w104 automatic. No B+ voltage on plug x64 behind brake booster pin 2 red, but pin 3 yel has 1, 1.5 & 10v depending on fan sw. Fan controler works junping 2-5v on yellow & +12v/gnd from battery to red/blk. So just no volts on middle pin for fan or on fuse holderstrut. Wire should enter dash near brake peddle bracket ?. No schematics in Bently bible R.Bently Volvo edition does. Ones on .124-zone. partly match.
October 10, 2016
  Followup from the Pelican Staff: Could be a blown fuse. Found this, see attached. - Nick at Pelican Parts
Rob Comments: What do the 3 blower wires look like inside the dash? I don't see voltage at the big fuse by strut tower either. The mechanic took things apart like dash cover & patched wires under hood. A month later it was towed home and I got it started with a new neutral safety switch 3rd one. Thx.
October 8, 2016
  Followup from the Pelican Staff: What vehicle are you working on? i can check if I have the wiring. - Nick at Pelican Parts  
Robin Comments: Hi,

First post on here, I've just returned to W124's I've had 4 W124s in the past now I have returned with a 1994 E300 diesel. It has climate control which is new to me, rather than the two wheels and fan control it has one wheel five air control buttons and three fan buttons on the right. The Aircon functions fine but the air hot or cold comes out very weakly and only from the far right and left vents. The fan seems to be running on slow mode but given the climate control system I'm not sure how it should crank up to full speed, presumably automatically in auto mode and hitting the top fan button should turn it to max. But all I get is a slow whirr from the fan and a weak flow of air. I can't get anything to come from the central vents at all and the footwell vents don't seem to be doing much either, the screen vent does have air to it but again weak. The fan is not intermittent or noisy, just slow, so it seems like the bushes are probably fine, I'm not sure where the speed is regulated from, and whether there maybe a fault closer to the switch or temperature sensor?

I have been reading what I can find, but I would like to try and diagnose the fault before I dig down to the fan itself or pull the dash apart to get to the control valves. I suspect there is a fault in both areas. But I'd first like to get the fan up to speed and wondered if anyone has any hints on what might be wrong.

Any ideas gratefully received?
September 7, 2016
  Followup from the Pelican Staff: If the vents are not changing when selected, I would assume that is your issue with the warmth on the right side. Blower speed will change depending on the temp, so if a major adjustment is needed, it will increase, decreasing when the temp is reached then maintaining the temp in the cabin. I would use a scan tool to check the temp sensor for the cabin and mode door operation.

- Nick at Pelican Parts
 
DrDan Comments: The K.A.E. 124 2821 21 51A HVAC resister that you sell the plug does not come off of the heatsink that I can tell and is seemly impossible to install with breaking something. Suggestions on how to install or remove plug? Thanks
August 2, 2016
  Followup from the Pelican Staff: I haven't seen one where the plug can't be removed. Give parts a call and double check you got the right one. - Nick at Pelican Parts  
Ed Comments: 1989 w124 230E - the blower motor will only work on positions III and IV. Could that be the resistor? I've tested the current at the fuse and every position seems to be working fine, though only positions 3 and 4 actually make the blower motor work. If it was the resistor, could you confirm me if the resistor for the 230E is the same as the one for the 260E. Mine was built in Germany. Thanks
March 12, 2016
  Followup from the Pelican Staff: Yes it could be the resistor. I am not sure if the part from the other vehicle will fit. Give our parts specialists a call at 1-888-280-7799. They can figure out what part or repair kit you need.
- Nick at Pelican Parts
 
Aaron Comments: The vent door above where the fan sits isn't attached from the other side, if that makes sense. How can I get to the back side or the dash side of that flap? I replaced the blower but it is only blowing cols air and the fan speed controls do not change the fan speed. Is that because that door doesn't open? Pic attached
January 20, 2016
  Followup from the Pelican Staff: It has been a long time, but I recall the heater housing has to be removed from the vehicle. - Nick at Pelican Parts  
fred Comments: 1989 w124 300e gas the heater fan motor blow good for the windshield and floor but not the dash. could you tell what it could be.
December 28, 2015
  Followup from the Pelican Staff: Sounds like the mode door is not working. Could be a vacuum issue, or an issue at the vacuum control. - Nick at Pelican Parts  

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