Weak blower even after regulator replacement - Mercedes-Benz Forum
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post #1 of 16 (permalink) Old 03-30-2019, 10:22 AM Thread Starter
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I recently replaced the blower motor regulator with the help of Gadha's excellent DIY guide. The original problem was that after an hour or so of driving, the air flow from the blower got very weak. After replacing the regulator all seemed well, but a couple of months went by when I only drove very short distances.

Today I drove a bit longer. The weather here is starting to resemble something like spring (finally), and I noticed at some point it's getting a bit hot in the car. I soon realized the blower wasn't doing that good of a job even at max speed. It was blowing cold, but the air flow was far from what it should be. Later, after the car had cooled down, I tried again and it was much better.

Did I buy a bad regulator or could it be something else, like the blower motor itself? I've understood problems with the motor usually cause either intermittent operation (on/off) or noisy operation etc, while the symptoms I'm seeing are more typical of a bad regulator that acts up when it heats up.

The regulator I bought was from eBay from a seller in the UK. It was new, probably a Chinese product, just the regulator without a heat sink. Are there sellers or brands that are known to be good? Or bad, for that matter.

Last edited by juuso; 03-31-2019 at 08:16 AM. Reason: Typo
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post #2 of 16 (permalink) Old 03-30-2019, 10:47 AM Thread Starter
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One more thing to add: there was a tube of thermal paste included with the regulator and I applied an appropriate amount of it. I'm a software engineer now but in my teens I built more than my fair share of PCs so that stuff is something I'm quite familiar with...
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post #3 of 16 (permalink) Old 04-02-2019, 09:13 AM
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I'm not a big proponent of the blower setup after 4 refurbishings.

Whenever I need to service the blower/regulator when it fails, I always check the operation of the blower fan by itself with a 12V source and a visual inspection of the motor brushes. The Behr fans do have a long service life, but the brushes can wear out faster that the motors or bearings themselves. I also make sure the bearings are lubed before putting a used (used by me) fan back into service. At least I know the fan isn't the problem.

After 2 not long lived replacement aftermarket regulators, I've switched to older original regulators by getting the early (and easier to replace) non integrated regulator. The integrated (heatsink/regulator) parts don't seem to be as robust.
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post #4 of 16 (permalink) Old 04-02-2019, 10:50 PM Thread Starter
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I suspect my new regulator was bad from the get go. I ended up ordering a new one, I don't know which brand it is but it should be OE quality and it was more expensive, around 150 euros (from a reputable MB parts dealer). I'll hopefully get it next week. I also ordered a new motor as I don't want to go back there anytime soon.

When I changed the regulator previously, I removed the old regulator by force, ground down the alignment tabs on the heat sink and tapped new holes for the new regulator. I trust I can keep using the old heat sink with the new regulator too. One thing I'll do differently this time is to use some Arctic Silver thermal paste and ditch whatever comes with the regulator.
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post #5 of 16 (permalink) Old 04-03-2019, 05:23 AM
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Being formerly in IT, I too use Arctic Silver.
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post #6 of 16 (permalink) Old 04-29-2019, 08:23 AM Thread Starter
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New regulator, new motor, and still it happens. After an hour or more of driving yesterday, I noticed there was barely any air coming from any vent. The center vents felt cold though so some definitely was getting through.

I stopped the car and it sounded like the blower was running fine and it responded quickly to controls. After letting the car cool down a half an hour, everything worked again.

Now I'm thinking whether I might be dealing with a vacuum leak that causes some valve(s) to close when the car warms up. Or could the control unit itself fail like this?

I'm aware of the vacuum distributor block behind the firewall that tends to develop a crack. I replaced it just last summer.
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post #7 of 16 (permalink) Old 04-29-2019, 10:55 AM
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Originally Posted by juuso View Post
Now I'm thinking whether I might be dealing with a vacuum leak that causes some valve(s) to close when the car warms up. Or could the control unit itself fail like this?
I don't believe any leak or valve failure can plausibly throttle the volume of air flowing into the cabin.

The control unit could be faulty. It's the very first thing you should have checked before touching the regulator or any other component.

By the brake booster I believe there is a terminal block with three wires going to the blower: (1) power, (2) ground, (3) control. With the blower on its highest speed setting measure the DC voltage between the middle wire (control) and the brown ground wire. You should measure at least 5 volts -- maybe considerably more.
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post #8 of 16 (permalink) Old 04-29-2019, 11:04 AM Thread Starter
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I don't believe any leak or valve failure can plausibly throttle the volume of air flowing into the cabin.

The control unit could be faulty. It's the very first thing you should have checked before touching the regulator or any other component.

By the brake booster I believe there is a terminal block with three wires going to the blower: (1) power, (2) ground, (3) control. With the blower on its highest speed setting measure the DC voltage between the middle wire (control) and the brown ground wire. You should measure at least 5 volts -- maybe considerably more.
The thing is, it's a bit difficult to replicate this issue as I don't regularly drive the car for 1h+ and the problem doesn't occur before that. But yeah, I should have measured the voltage. I even have a multimeter with me all the time but didn't think about that. Anyway, it did sound like it was going at full speed, but it seemed something was preventing the flow of air.
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post #9 of 16 (permalink) Old 04-29-2019, 12:53 PM
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I can relate a similar experience. After driving for around an hour, especially in mild temperatures wit a/c controls fully automatic, the amount of air would deminish to nearly none, even when I ran the fan to max manually. I discovered my evaporator was freezing up, thereby blocking airflow. After turning compressor off and giving it a bit of time, the ice would melt and the airflow would return to normal. I then put the system in diagnostic mode and discovered my evap temp sensor and heater plenum temp sensor both were faulty. Replaced each and cured the problem.
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Where is it again that we are going... And why are we in a handbasket?

Last edited by Ears; 04-29-2019 at 12:56 PM.
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post #10 of 16 (permalink) Old 05-08-2019, 03:06 AM Thread Starter
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Well that's an interesting finding! Probably not the same as mine though, because my problem also happened in the winter when the compressor wasn't running. I haven't done so in a while, but the last time I checked the sensors were giving plausible readings too.

Last week I ordered a refurbished AC control unit from eBay. This morning I took the car out for a longer drive with the new unit installed and the issue occurred again. At the end of the drive I removed the interior air filter and based on what I saw and heard, the blower was blasting at full speed like it was supposed to, so it's confirmed that something is just restricting the air flow.

Next I plan to do some vacuum diagnostics. Let's see if I find anything.

I'm getting a bit desperate here. In 2 weeks we're boarding a ship to Germany, and this is probably the only chance I will ever have to introduce this car to the Autobahn. If this issue doesn't get sorted out by then, well, it's goodbye to that dream.
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