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I’ve run into an issue a few times in the past week, and am hoping someone will have some info for me that could point me in the correct direction.
Over the past 7-10 days, on 5 occasions, when starting my vehicle, the ac fan doesn’t come on. All of the controls and dials respond, but there is just no function from the motor.
Turning the vehicle off, opening the door, closing, and restarting has resulted in the blower motor powering on, and running just fine.
Today, however, it took several attempts for the motor to respond.
Once home, I took the time to access the blower motor under the hood, and it spins freely by hand, even though it failed to power up when I turned the vehicle onto the ‘on’ position.
Based upon that information, and what I’ve read on the threads I could find, it seems that my issue is going to be in the blower motor regulator.
And, thanks to the magic of the car gods, The A/C stopped today!
there is 12.32 volts going into the regulator, and 0.00 coming out.
After cycling the car on/off a dozen times, the fan motor started running again, and was reading 0.32v coming out at the harness, though I may not have gotten the probes pushed in properly, as I still had the blower plugged into the port.
Turning the car off again killed the blower motor, and I went back to getting 0.00v out of the regulator.
I looked at the diagram for my 2013 and the regulator looks to have 3 inputs and 2 outputs. The inputs are 12v+, 12v - and the control input. Outputs to the motor are 12 V + and ground. This might be different for an older GLK. I used the Alldata diagram for my 2013, but don't have a subscription for older GLKs.
Note that its always good to show the year of your vehicle in your profile so forum members will know what year they are dealing with when you ask a question. I am assuming yours is older than 2013 since you said you don't have ECO start/stop.
Not sure what level the control input is but it's the signal telling the regulator what output to produce.
There are a couple of gotchas to watch out for when troubleshooting transistorized circuits using a voltmeter. Transistorized switches are current devices and may show full voltage even when the switch is turned off if no load is hooked up. This is because of the leakage current inherent in transistor switches and the fact that most modern voltmeters are designed not to load the circuit they are measuring. So beware of trying to make sense of open circuit voltages without a load attached. As long as the motor is hooked up as a load, you should be able to tell what's going on.
The other issue is many times switches and regulators actually switch or modulate the negative lead rather than the positive. This means you will show 12 volts on the positive lead at all times if you measure from ground, but the current in the negative lead is what is changing. Since the blower motor doesn't look like it's tied to a hard ground, that may be the case here. You may need to measure the current flowing to the motor to troubleshoot the regulator.
I looked at the diagram for my 2013 and the regulator looks to have 3 inputs and 2 outputs. The inputs are 12v+, 12v - and the control input. Outputs to the motor are 12 V + and ground. This might be different for an older GLK. I used the Alldata diagram for my 2013, but don't have a subscription for older GLKs.
Note that its always good to show the year of your vehicle in your profile so forum members will know what year they are dealing with when you ask a question. I am assuming yours is older than 2013 since you said you don't have ECO start/stop.
Not sure what level the control input is but it's the signal telling the regulator what output to produce.
There are a couple of gotchas to watch out for when troubleshooting transistorized circuits using a voltmeter. Transistorized switches are current devices and may show full voltage even when the switch is turned off if no load is hooked up. This is because of the leakage current inherent in transistor switches and the fact that most modern voltmeters are designed not to load the circuit they are measuring. So beware of trying to make sense of open circuit voltages without a load attached. As long as the motor is hooked up as a load, you should be able to tell what's going on.
The other issue is many times switches and regulators actually switch or modulate the negative lead rather than the positive. This means you will show 12 volts on the positive lead at all times if you measure from ground, but the current in the negative lead is what is changing. Since the blower motor doesn't look like it's tied to a hard ground, that may be the case here. You may need to measure the current flowing to the motor to troubleshoot the regulator.
Hope this helps a little.
thank you!
it is a 2012, I’ll look into editing my profile so it reflects that.
You seem far and away smarter about this stuff than anything I can find on Google, so to make sure that I read everything correctly, I did re-check my output numbers with the blower motor attached to the resistor.
I don’t know that I checked the correct wires, but I continued to get 12.x volt going the resistor, and 0.00 out of the resistor when the motor wasn’t operating, even with the HVAC settings set to high fan, a/c, and recirculate.
Restarting the vehicle until the blower motor powered on gave me 12.42v out of the resistor to the fan, with the same above settings.
So, with 12+v going into the regulator, whether the fan is functioning or not, and the settings all on (a/c, recirculate, fan on high), but the voltage out of the regulator varying from either 0v, or over 12v, is it safe to say that it’s likely the regulator?
Without a diagram showing your particular regulator, it's hard to be sure that its the problem. My 2013 uses a solid state regulator that depends on a signal coming from the HVAC control to operate and change the speed of the fan. The regulator is just a solid state switch to control the current to the blower motor. With your symptoms, it could be the input signal going to the regulator or the regulator itself that is the problem. The input to the regulator is probably a CanBus signal, which is a variable duty cycle waveform telling the regulator how fast to run the blower motor. That signal is pretty hard to troubleshoot without a scope.
It looks to me by checking parts on Ebay that a 2012 uses the same style regulator as the 2013, but I've been fooled by their compatibilty numbers before.
Can you change the speed of the fan when it does operate, or its it strictly on/off?
Your best solution might be to see if you can find a used regulator at a junk yard. Looks like new regulators are between $75 - 125 bucks and you can't be sure that's the problem.
There is one more possibility. Make sure you can spin the blower motor easily. If the blower is starting to sieze, your regulator might not have enough current to get it started. When the car sits, it cools down and might not need as much juice to get it going.
If it is the input signal, you are going to have to get a scan with Mercedes diagnostic equipment.
For thse interested, here is a link to a Youtube video talking about how the HVAC blower regulator works. Its a little more complex than the old resistor network.
ZZGLK,
When the motor is functioning properly, it will work in all fan speeds, and is not stuck in only one particular setting.
My initial thought was maybe the bearing was going out in the motor. However, the fan moves easily by hand, and will complete 1/8-1/4 rotation when given a spin by gripping the center nut. I’m unsure just how much spin there should be, so I may have to stop by AAP to spin a replacement.
FCPEuro indicates it’s the same regulator from 2011-2015, so it should be the same in my 2012 as in your 2013.
Sadly, not too many junk yards around here, so I’d still have to order it online, or drive out to Orlando.
Given the $69 price for a replacement from FCP, and the information you’ve provided (as to the fact it may be the regulator, or the input into the regulator), I think this is where I will start.
This next part might be super confusing, so bear with me:
Are you familiar with any possible issues around jumping the fan? If I have the fan not start up, would running temporary jump wires from the positive input before the regulator to the positive output past the regulator, and doing the same with the ground help pinpoint if it’s the regulator? That would, I think, tell me if the issue in the regulator, or if the signal is bad before entering the regulator, yes?
These types of regulators often have heat sinks to help dissipate large amounts of heat they can create. Heat is the enemy of electronics. In a circuit like this and with these symptoms I would always consider the regulator the primary suspect. Not a guarantee but I think it's likely to be the culprit.
Originally Posted by LadyS
ZZGLK,
When the motor is functioning properly, it will work in all fan speeds, and is not stuck in only one particular setting.
My initial thought was maybe the bearing was going out in the motor. However, the fan moves easily by hand, and will complete 1/8-1/4 rotation when given a spin by gripping the center nut. I’m unsure just how much spin there should be, so I may have to stop by AAP to spin a replacement.
FCPEuro indicates it’s the same regulator from 2011-2015, so it should be the same in my 2012 as in your 2013.
Sadly, not too many junk yards around here, so I’d still have to order it online, or drive out to Orlando.
Given the $69 price for a replacement from FCP, and the information you’ve provided (as to the fact it may be the regulator, or the input into the regulator), I think this is where I will start.
This next part might be super confusing, so bear with me:
Are you familiar with any possible issues around jumping the fan? If I have the fan not start up, would running temporary jump wires from the positive input before the regulator to the positive output past the regulator, and doing the same with the ground help pinpoint if it’s the regulator? That would, I think, tell me if the issue in the regulator, or if the signal is bad before entering the regulator, yes?
MBKLUE,
It doesn’t seem to have a significant heat sink, but it does mount directly into the HVAC system, so it’s exposed to a lot of moving air, and had a series of small, metal bumps on the backside.
So, I think it’s settled. A new regulator it shall be!
Seems like a good location for it to be installed if cold air is blowing across it in the system but not so great if it's heated air in winter. It's definitely a speed regulator in that location and not a temperature sensor?
I do hope that solves the problem. Worst case basis is you'll have a new speed regulator installed. All electronic components will fail at some point. If the old is still good perhaps you'll be able to sell it and at least get a bit of money for it.
Originally Posted by LadyS
MBKLUE,
It doesn’t seem to have a significant heat sink, but it does mount directly into the HVAC system, so it’s exposed to a lot of moving air, and had a series of small, metal bumps on the backside.
So, I think it’s settled. A new regulator it shall be!
Seems like a good location for it to be installed if cold air is blowing across it in the system but not so great if it's heated air in winter. It's definitely a speed regulator in that location and not a temperature sensor?
I do hope that solves the problem. Worst case basis is you'll have a new speed regulator installed. All electronic components will fail at some point. If the old is still good perhaps you'll be able to sell it and at least get a bit of money for it.
Perhaps it does double duty? It’s mounted under the glove compartment, just before the air filter. There’s even a foam gasket, I assume to cut down on air leaks. I think where it’s positioned is before the air goes past the heat exchanger, so it’s cabin temperature air.
photos attached of my old one for added clarification.
[QUOTE=LadyS;8072923]ZZGLK,
When the motor is functioning properly, it will work in all fan speeds, and is not stuck in only one particular setting.
My initial thought was maybe the bearing was going out in the motor. However, the fan moves easily by hand, and will complete 1/8-1/4 rotation when given a spin by gripping the center nut. I’m unsure just how much spin there should be, so I may have to stop by AAP to spin a replacement.
FCPEuro indicates it’s the same regulator from 2011-2015, so it should be the same in my 2012 as in your 2013.
Sadly, not too many junk yards around here, so I’d still have to order it online, or drive out to Orlando.
Given the $69 price for a replacement from FCP, and the information you’ve provided (as to the fact it may be the regulator, or the input into the regulator), I think this is where I will start.
This next part might be super confusing, so bear with me:
Are you familiar with any possible issues around jumping the fan? If I have the fan not start up, would running temporary jump wires from the positive input before the regulator to the positive output past the regulator, and doing the same with the ground help pinpoint if it’s the regulator? That would, I think, tell me if the issue in the regulator, or if the signal is bad before entering the regulator, yes?[/QUOTE
I don't think you will gain anything by jumping the positive terminals on the regulator. It still won't tell you whether its the regulator output not working or the lack of input from the HVAC controller. I believe that it's the negative side that's switched instead of the positive. You might be better off jumpering the blower motor negative lead to a good solid ground when it doesn't work, but it doesn't isolate the problem. Regardless of which side you are jumpering, you have effectively just tied the motor directly to power. You have bypassed the regulator, but it doesn't help you determine whether the input is missing to the regulator or the switch circuit in the regulator is bad. Without a scope and knowledge of what the input waveform should look like, the best bet is substitution of the regulator assembly. If I had to guess, I would bet on the regulator being bad because it's a part that sees a lot of thermal cycling. Still, a bad component on the CanBus or the HVAC controller could cause intermittent problems like you are seeing. As these complex vehicles age, they can generate lots of issues.
One thing you might try before you spring for new regulator is getting a spray can of air like they use for blowing dust off of electronics parts and using it to cool the regulator housing when it doesn't work. If that works, then you know its the switching circuit in the regulator.
Well, that's a peculiar looking module. It certainly may have more than one function.
Originally Posted by LadyS
Perhaps it does double duty? It’s mounted under the glove compartment, just before the air filter. There’s even a foam gasket, I assume to cut down on air leaks. I think where it’s positioned is before the air goes past the heat exchanger, so it’s cabin temperature air.
photos attached of my old one for added clarification.
Well, that's a peculiar looking module. It certainly may have more than one function.
The back of the module is designed as a heat sink that is cooled by air blowing from the blower. Lots of bumpy surface area to aid in cooling. That design is probably left over from when the Regulator/Resistor was just a series of high wattage resistors that turned the excess current into heat. Each resistor would sit in one of the indentations. If you look a MB blower resistor modules from the 80s and 90s, they all had a similar design except that each indentation was several inches deep to house the resistance which would be cooled by air from the blower. Nowdays, they use an SCR to control the current duty cycle and don't need the long resistor housings.
I guess I don't understand where it is in the air stream. If hot air passes over it when the heat setting is on in winter it would be self-defeating to a large extent, I would think.
Originally Posted by ZZGlk
The back of the module is designed as a heat sink that is cooled by air blowing from the blower. Lots of bumpy surface area to aid in cooling. That design is probably left over from when the Regulator/Resistor was just a series of high wattage resistors that turned the excess current into heat. Each resistor would sit in one of the indentations. If you look a MB blower resistor modules from the 80s and 90s, they all had a similar design except that each indentation was several inches deep to house the resistance which would be cooled by air from the blower. Nowdays, they use an SCR to control the current duty cycle and don't need the long resistor housings.
I guess I don't understand where it is in the air stream. If hot air passes over it when the heat setting is on in winter it would be self-defeating to a large extent, I would think.
MBK - where it sits in the air stream is before the heat exchanger. So it has either air from outside, or air being drawn in from the cabin. Even with the heat on, it’s going to be in the relatively cool air stream of cabin air (on recirculate), or the excessively cold outside air.
The motor failed to start again today, so I checked the output from the regulator.
It also read 0.00, with an input of 12.45.
I decided to leave the meter attached to the regulator output, and restart to vehicle to see what it gives me.
The regulator initially puts out a burst of voltage, and when the fan fails to engage, drops back to 0.00. So I likely was getting 0.00 from the original regulator, because I wasn’t connecting it to the system until after that initial burst of voltage, which dropped when the fan didn’t respond.
I’ve attached a video of what it does. Fan still spins when spun by hand.
Sorry to hear that. I had pretty much the same problem years ago on my Saab fairly early-on from new. Replaced the solid-state fan speed regulator, provided better mounting and heat sink compound and moved the regulator to where it had more air circulation (it was always a stand-alone component not in the air stream). Worked fine and never had another failure.
Originally Posted by LadyS
Or not. I guess I can’t attach a photo from iPhone.
Basically, the voltage climbs from 0.00 to 13.42, and then quickly drops back down to 0.00 if the fan doesn’t engage.
If the fan does engage, it will continue to supply between 12.46, and 13.42.
Well hopefully I’m not about to go on a wild goose chase! I contemplated replacing the motor at the same time, to avoid additional shipping costs, and this is the Universe punishing me for not doing that.
If the replacement blower motor doesn’t correct the issue, it’ll have to go to the shop for proper diagnosis, and repair.
Well hopefully I’m not about to go on a wild goose chase! I contemplated replacing the motor at the same time, to avoid additional shipping costs, and this is the Universe punishing me for not doing that.
If the replacement blower motor doesn’t correct the issue, it’ll have to go to the shop for proper diagnosis, and repair.
Have you tried hooking up the blower direct to a 12 source? That will tell you if the motor is good. It should just run at high speed. The only reason you need the regulator is to for all the variable speed stuff.
Have you tried hooking up the blower direct to a 12 source? That will tell you if the motor is good. It should just run at high speed. The only reason you need the regulator is to for all the variable speed stuff.
I had mentioned that idea earlier, about bypassing the regulator to see if it was the fan motor, or the regulator, but I haven’t attempted it as the motor functioned when I was driving today. But since it’s intermittent, it could possibly work if I jolted it around to remove it.
It will typically run for 5-10 drive cycles before cooking out again, so I’ll have to wait until it does. Either way, a blower motor will be here Tuesday.
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