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Marty, Mercedes Technician
Category: Mercedes
Satisfied Customers: 54028
Experience:  Bosch Certified Master Tech. Over 35 years experience.
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2004 ML500- Twice this past winter (on extremely cold days)

Customer Question

2004 ML500- Twice this past winter (on extremely cold days) the heater/AC blower motor cut off randomly. I tested the fan directly and it worked fine so I ordered a new resistor/regulator to put in it (because of age I ordered a blower motor as well).
While waiting on the parts it began working fine again and I figured I would do the job when it went out again. It has been fine until the past few weeks (extremely hot days). It randomly shut off a few times in the past couple weeks and finally stopped all
together a few days ago. Having the fan and resistor in the garage, I started my diagnostics over again from scratch. Fan again worked fine on direct 12v power source. I went ahead and unplugged the pigtails from the old resistor and just plugged in the new
one before I uninstalled/installed and fan still did not run (even on highest speed). I checked the voltage coming out of the resistor to the motor and it was around 3.5v so I checked the red wire(?) going into the resistor and it was always about the same
as the exiting voltage. Even though it was getting a little power I checked the fuse under passenger side by the blower and of course it was fine. I went to the relay under the hood and swapped it with a good one and nothing changed. I pulled the relay and
tested the socket pins and the pin from the fan switch tested no power when off and full power at the pin when I turned the fan switch on. I have not pulled the climate control panel because I can't pull the radio yet (read that was step 1). The rear climate
control is working fine. When I have it on auto (controlled with front) it changes with all fan speed settings and off. When I change temp setting on front auto it gets faster the colder I set it and slower the warmer I set it and off when I exceed current
ambient temp. This is all this DIYer knows to do. With all this being said I know the question is ..... why 3.5 v and am I done until I can pull the climate panel? I don't have a schematic and I am having a hard time finding continuity consistently.
Submitted: 2 years ago.
Category: Mercedes
Expert:  Marty replied 2 years ago.
Hi, welcome to Just Answer. My name is***** 12V on the red wire to the regulator comes from the 40 amp fuse F4 in the passenger side foot well fuse box. You need to measure the voltage to that fuse. If you have 12V at the fuse your problem is between the fuse box and the regulator. If you don't have 12V to the fuse you need to trace the voltage back to the battery. Fuse 4, 5 and 6 all get voltage from the same source. If the other fuses have 12V and fuse 4 only has 3.5V your problem is in the fuse box.
Let me know if you have any questions.
Customer: replied 2 years ago.
I checked all three. All tested 13.73V at both points on each fuse (Car running, been testing all day with it off).
Customer: replied 2 years ago.
If F4 shows Front Blower Motor and F5 shows Instrument cluster/ Air conditioning does that mean that it goes from that panel to climate control and then back to the resistor or does it go straight to the resistor. I do not have a schematic to follow on paper but will try to follow around dash.
Expert:  Marty replied 2 years ago.
F4 goes straight to the resistor. If you have 12V at the fuse and 3.5 at the resistor your problem is between the fuse box and the resistor. You might be losing it right at the output of the fuse box. I'd remove the box and check the red wire where it exits the box.
Let me know what you find out.
Customer: replied 2 years ago.
Sorry about the delay, I have been tied up at work but back at it. I had no luck. But I did notice that when the pigtail going into the resistor was plugged in it read low volts on red wire but when I tested the red wire unplugged from the resistor the red wire tested at 12.52V. I did run a jumper wire from positive lug bolt on fuse panel to red wire entering resistor and fan did blow high speed but had no fan speed control from dash. Do not know if that indicates anything or not.
Expert:  Marty replied 2 years ago.
If your 12V is dropping to 3.5V when you plug it in it means that the wiring between the fuse box and the resistor is faulty. You should measure the voltage at the fuse with the wire plugged in. If it stays at 12V the problem is with the wiring. Either that or your resistor is defective and pulling the voltage down but I would expect the fuse to blow. This sounds most like bad wiring between the fuse and the resistor. If you powered the resistor directly you should have been able to control the speed. It sounds like there may be other problems as well, possibly a defective resistor.
Customer: replied 2 years ago.
The voltage stays the same at the fuse whether the pigtail is plugged in or not. I unplugged the new resistor and jump red wire from pigtail into the resistor to pigtail going to fan and fan ran high speed. It looks like you might be right(?), the new regulator might be defective and the premise of my troubleshooting may have been flawed from the start.
Expert:  Marty replied 2 years ago.
I'd get another resistor and try it but if the voltage from the wire leaving the fuse drops to 3.5V when plugged into the regulator there has to be a fault with the wire as well. You should measure 12V at the power lead to the regulator whether it's plugged in or not.
Let me know what you find out.