I haven't been able to discern any pattern, so I'll just relate what happens.

In general: When first starting he car, all is well. Make a short trip, and park for a few minutes. When I come back there is no cabin air blower. I've tried manually changing fan speeds, turning climate control off and back on, and shutting the car off and restarting it. I even tried switching to heat and defrost. Nothing seems to work. I think it may be just the fan because you can feel a little "coldness" if you hold your hand over the vent while driving. If you drive long enough and fuss with it enough, eventually it comes back on. I have yet to have it come back on if just left alone (leaving it alone is tough to do in 90 degree weather...). Once it hadn't come back by the time I got home. Every time it has sat for at least an hour or so it works normally.

All panel indications are "normal". When you manually set the speed it shows the expected speed even though the fan is not running.

I have yet to have it quit while it was working, although I think there was one case where fussing with it got it working, and further fussing caused it to quit again, in a matter of seconds.

There were two times when it didn't quit after the first stop, but did after the second. Some first trips are "long": more than 10 minutes, others are short: under a mile.

Not sure where to start; any suggestions appreciated.

 

...

 

That's pretty much what my wife suggested...

I'll take a look...

 

Good chance that'll work. The brushes get worn down to the point they're very intermittent, and often that whack with a semi-solid object will jar them back into position enough to get the motor working again (for a very short time, of course).

 

Do you think so? We're not talking about a super high mileage car here; it only has 84,000 miles on it.

 

It fails when hot, it could be a bad blower, something shorts when hot, or more likely the fan controller , you could open it all up and see if there is power at the motor when it fails.

 

84,000 miles can be a lot of different things. That's a few years of my road trip mileage (probably 1,500 operating hours, much of which not needing the fan running, or it running very slowly) or if it's a NYC vehicle, that fan motor could have an eternity of hours, much of it blasting on high to keep the occupants from freezing or boiling to death. ;-)

That applies to the rest of the vehicle as well - I'd suggest that most 84,000 mile GLKs in NYC (or similar) are looking and acting "well-used" barring extraordinary maintenance, while those like mine (garage queens from the desert southwest) will look and drive like nearly new cars.

 

Just a quick update.

Since the heat wave has ended, it hasn't occurred once, so, I'm thinking it has to do with something getting heat soaked. I am a total klutz with WIS, and can't figure out how to get a schematic of the blower circuit. (I found something, and it seems to show I have two blowers, which I don't think is correct.) Regardless, it shows a solid state controller. Wonder where that is?

The cabin temperature was lower after the brief stop than it was on the initial start, so I'm wondering if it isn't a component in the engine compartment that is failing, or a component that generates heat while the system is running. Any guesses?

 

The blower motor controller sits right to the left of the motor when looking forward. The red and yellow wires (on a GLK350, at least) from the motor plug into it.

Just be aware that the problem could have originated in the fan speed controls above the console. I had to replace mine at some point. There are plenty of good used ones available on eBay. It displayed what looked like the right settings, but it just didn’t have correct outputs. The fan ran on high and driver side temp was high heat regardless of settings.

 

That could all still point at a typical fan motor problem. When the brushes get really worn down, a small fraction of a mm of clearance can be enough to keep the motor from running - and that can happen with thermal expansion /contraction. I'd still recommend a good whack to the blower motor - if it starts up, you have the culprit dead to rights.

 

It's not acting up any longer, and I don't have easy access to the motor with a hammer. I don't want to be driving around with the dash opened up for the next 3 years...

I think maybe I'll run a pair of wires off the motor to an led somewhere visible as a monitor.

 

I took my fan assembly out and flushed it in rubbing alcohol last year (some will remember I wrote about this). I let it dry in the sun for an hour (don't want any fires, do we!) and it worked great after that. Brushes had loads of wear left after 225000 km. It has just started acting up again. Seems to start up after turning off for a few minutes and jostling it over a few bumps.

I think it is the motor bushings that are worn and not providing contact for current flow through the motor (had this exact issue in a GM starter years ago). You can determine this by removing the under dash cover and give it a little bump (haven't had the chance to do that yet). Checking for voltage is a good start, however. If voltage is present and a bump starts it, a cleaning might help for a while. It's amazing how much carbon brush crap gets stuck on the motor assembly in there! Not sure if the bushings are replaceable.

 

Just a quick update:

I have done nothing since it started working again. The last 4-5 days we've been back into the 90's again, and it has continued to work as expected. I'm not complaining, but it doesn't give me that warm (no pun) and fuzzy feeling...

I'm planning to hook some test wires up to the fan motor when I replace the cabin air filter, but it's too hot right now...

 

Be happy and fortunate ... it's 105° F here right now.