Auxiliary Electric Radiator Cooling Fan does not come on - Mercedes-Benz Forum
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#1 (permalink) Old 03-23-2013, 12:31 PM
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Auxiliary Electric Radiator Cooling Fan does not come on

Hello Everyone!

This is my first post on this forum, but I have been watching it and using advise found here for some issues with my wife's 2001 ML320 truck already.

I already researched this topic here, but I could not find answers that would satisfy and resolve the problem that I am having.

To make the long story short, the electric auxiliary radiator cooling fan does not turn on ether when the AC is activated or engine temperature is high.

My AC is operating properly otherwise. The compressor comes on and cold air comes through while the condenser is cool. It works just fine when the truck is on the move, but it stops blowing hot air at idol, when the condenser warms up. As I already mentioned before, the fan does not come on when the engine gets hot either. I know that because the clutch for the main cooling fan is dead and does not engage. On a hot day, the engine temperature goes well over 100 degrees, but electric fans stay off. I already ordered a replacement clutch for the main fan and I will put it in in due time. But, that's a side note.

I already checked the fan fuse (it has the 40 amp one) and the relay. There is current at pin 30 and fans come on just fine if I jump the relay. The relay itself is operating properly when I test it outside the circuit.

I considered an idea that I have low refrigerant pressure, but fans should also come on when the engine gets hot (especially with dead main cooling fan clutch), but they don't. Also, the AC is blowing pretty cool air when the truck is moving at speed.

Currently, I fear that the AAM is faulty, but I really want for something simpler to be wrong. There fore I need an outside perspective to point out a troubleshooting step that I might have missed. Please share your wisdom.

Thank you in advance.
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#2 (permalink) Old 03-23-2013, 12:38 PM
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It does need to get hot before AC fans will come on. They will not work if refrigerant is very low. How high over 100 has it got to.
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#3 (permalink) Old 03-23-2013, 01:47 PM
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It does need to get hot before AC fans will come on. They will not work if refrigerant is very low. How high over 100 has it got to.
It got to 105 - 110 degrees may be. If I understand correctly, those fans activate independently if either the AC is on or the engine temperature gets to high even if the AC is not on. Please correct me if I am wrong.
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#4 (permalink) Old 03-23-2013, 04:44 PM
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Yes the aux. cooling fans should come on when you are overheating.

You have already determined that pin/slot #30 on the fuse box has voltage. You must now determine if pin/slot also has voltage. If it doesn't then you have a problem with the fuse box.

If it does then you should remove relay #21 (blower motor) and replace it with relay #26. If the blower motor works then the relay is good and your problem is that there is no signal coming from the AAM to ground the relay.

The reason that you do not have AC in traffic is because there is no cooling of the condenser and the radiator. The reason you have AC at hwy. speeds is that the in rushing air through the grille is cooling the condenser and the radiator.

Also fuse #44 should normally have a 30 amp fuse unless your truck was fitted with a modified harness inside the fuse box. So you will have to check to see if you have it.
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File Type: pdf AC Fuse #44 Harness #1.pdf (112.4 KB, 221 views)
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#5 (permalink) Old 03-24-2013, 03:36 PM
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Thank you 43sqd, per your suggestion I tested pin 86 on the relay and it does have power as well. Pin 85 has no continuity with the ground, which is expected I guess. I also, as you suggested, swapped out relay k21 with k26, and in-cabin blower worked, so this confirms that relay k26 also works as expected. This confirms that related fuses, relays and wiring inside the fuse/relay box are trouble free.

I checked for the auxiliary fan harness and it only has one electrical connector at the front, which, I guess, means that the harness was not upgraded. The fuse #44 is 40amps confirmed. We bought the truck used, so the previous owner must have installed it. May be he already had issues with it. Also, I can't remember if I ever saw those fans working, but they must have because we had the truck for years and I only noticed the problem last summer. It was not an issue during winter, so I only returned to it right now.

So, what is next? Right now it seems evident that AAM does not activate relay k26, but why? The issue can be, again, with low refrigerant pressure and/or problem with engine coolant temperature sensor, which prevents the AAM from getting an accurate reading. But, how much are the odds that both systems are having a problem at the same time. May be with twelve year old truck not that bed and worth checking.

So, how can I test the coolant temperature sensor and is it the same sensor that drives the coolant temperature gauge on the dash?

Thank you.
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#6 (permalink) Old 03-24-2013, 11:29 PM
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But, how much are the odds that both systems are having a problem at the same time.
Excellent. At least someone is thinking on this forum.

Pin #8 from the AAM is responsible for grounding relay #26. And the system works as follows:

1. During normal use of the veh. without AC, the Coolant Temp. Sensor and the ECM monitor coolant temp. When the coolant reaches a predetermined degree. it will send a CAN message to the AAM to turn on the aux. fans, via pin #8. When the coolant reaches another predetermined degree, the fans will be turned off. And so on , and so on.

2. With the veh. in AC mode, the AAM is the only thing responsible for turning on the fans, via pin #8, when the Refrig. Pressure Sensor reaches a predetermined pressure at the sensor. And so on, and so on.

Although the same grounding pin is used for either grounding, they are actuated by different means. And therefore both the Coolant Temp. Sensor and the Refrig. Pressure Sensor would have to be faulty at the same time.

But in any event the Coolant Sensor is working correctly because it is showing high temps. on the cluster.

Yes there are tests for the Coolant Temp. Sensor but it involves testing both at the sensor and the ECM. Back probing of the ECM and DVOM will be necessary. Let me know what you want to do with these tests as I will have to set something up in another post.

In the mean time, pull up the fuse/relay module out of the box and look for the 4 numbers affixed to the AAM. The number will be 163 54 xx xx. Start hunting on line for the same exact number.
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File Type: pdf AC Fan Control up to 31-8-01.PDF (17.6 KB, 155 views)
File Type: pdf Coolant Temp. Sensor Testing.pdf (16.0 KB, 114 views)
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#7 (permalink) Old 03-25-2013, 01:37 PM
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Ok, So, I got the numbers off of AAM and I will look for the replacement.

Inside that fuse/relay box I discovered two more controllers back to back. One of them is probably an ECM. What is the other one? And which one is which?

Also I would still like to get some understanding if the coolant temperature is read correctly by the ECM. I have an OBD II USB interface and a Scan XL Pro software on my computer. I should be able to read coolant temperature information as ECM sees it and compare that value to one that instrument cluster displays to see if they match. This will give me some reassurance that temperature is read correctly and instrument cluster gauge is working properly.

I cannot do this at the moment because I have the fuel pump out of the tank trying to repair the fuel level sending unit. Hopefully I can put it back together this week and do the scan.

Do you think that this idea will work?

Thank you.
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#8 (permalink) Old 03-25-2013, 01:45 PM
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#9 (permalink) Old 03-25-2013, 02:05 PM
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I found one on e-bay for cheep, I think anyway, but it sells with a disclamer that it will have to be reprogrammed by the dealer. Is this true or the AAM should swap right out?

Thank you.
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#10 (permalink) Old 03-25-2013, 02:08 PM
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Originally Posted by 43sqd View Post
Location.
Ok, thank you. So, what is the unit directly to the left of the ECM then?
Thank you.
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