Hi guys, I am having an issue on my 1991 350SDL, for a couple of weeks, every so often when I start it up and take off, the blower for the AC won't do anything for a mile or two. After it does start up, it runs just fine until I shut the car off. Well, it seems to have progressed further, as now I can drive 15 to 20 miles and it still won't come on. I checked the 30 amp fuse, removed and cleaned, and checked that I have voltage on both sides of the fuse. I also checked fuse #5, it is new, but checked anyway, as well as checking voltage on both sides of it, so think the fuses are not the problem.

It is really weird in that once it takes off and starts running, it will go all day long, as long as I don't shut the car off, and it doesn't do it all the time. Any ideas as to where I should start looking?

Thanks!

Does the fan come on high speed if you push the defrost button? That should bypass any logic in the pushbutton unit and force the fan on high speed. If it does, the pushbutton unit may be malfunctioning. If it doesn't, you may have a bad fan motor or bad transistor pack.

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Current stable:
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I haven't checked the defrost, I will do that in a while. I have checked that on just about everything else.I do know that the compressor runs in AC mode, cause while going down the road I can move it to vent after trying AC for a few minutes, and the air forced through the evaporator from driving is ice cold for a bit. But I will check defrost. Thanks!

Well, must be regulator or blower motor, since defrost does the same thing with fan on high. Will let you know what I find out.

You might want to take a look at your ignition switch. Slightly wiggle your key around while the car is running. I've seen this problem a few hundred times.

Thanks for all the support, I appreciate it!

It turned out to be the blower motor, the brushes were pretty much gone, and they had worn the armature pretty good, so I have a new Bosch motor on order.

Have you checked the blower strip fuse for the classic crack that causes intermittent or no blower function?

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Garage:

2017 Chevy Colorado Diesel (nanny state emissions)
2005 Volvo S40 T5 AWD, 77k
1987 Mercedes-Benz 300D turbodiesel, 4 sp auto, 156k - 28.7 mpg
1996 Tracker 4x4, 2 door, 16v, 3 sp auto. 113k - 28.6 mpg

WARNING: this post may contain dangerous free thinking.

Sounds about right, worn motors can behave that way (won't start when initially powered, but run fine once spinning). Coincidentally URO just released Blower Motor Assembly 126 820 0542 for these cars, which retails for about a third of the OEM unit. Pelican doesn't have the new blower in its system yet, but they can add it immediately if someone calls and wants to order it.

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URO Parts online catalog: https://apaindustries.com/catalog

Pelican now has this blower available: https://www.pelicanparts.com/More_Info/1268200542M253.htm?pn=126-820-05-42-M253&bt=Y&fs=0&SVSVSI=0814

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URO Parts online catalog: https://apaindustries.com/catalog

Bumping this thread as I'm dealing with a similar issue in my '85 300D. I have played with the HVAC buttons on cold car, warm car, off car, running car..

I removed the blower motor this afternoon and looked around it. I'm not sure how to disassemble to the point that the brushes are accessible.

Anyway, I put the motor in my bench vice and hooked it up directly to 12V source. It not only spun, but spun with some gusto! The motor is definitely not the problem.

I also tried measuring power at the plug for the motor (connects to the motor under the dash). FWIW, I'm an electrical idiot, but this seemed to make some sense. I stuck my multi-meter leads into the female ports of the plug and tried various defrost/fan speed settings with the key in the 'on' (not running) position. For all of these tests, I showed 0.00 on the meter. Not sure if I did this correctly or not. I did try swapping the leads from side to side to make sure they would have been in the correct position.

I have tried jiggling the key in the ignition with the key in 'on' (not running) position, and cycled through the HVAC setting. No dice.

Fuses are good.

When I press one of the HVAC buttons on the CCU, I hear a click somewhere in the engine bay (near the firewall? Hard to tell). So, to my small mind, that tells me there's some signal going from CCU and getting lost somewhere in the middle? Dunno, could be wrong.

I see that testing the resistor on the PS fender under the hood is an option. Same for the sensor (or switch?) on the thermostat. I'm not sure how to test these. I pulled the plug from the resistor and stuck the positive end in each pin-hole, grounded the negative, and turned the defrost 'on'. I got 0.0 for each pinhole. Again, no clue if this is an adequate measure of testing the resistor.

I removed and disassembled the CCU today. I did not notice any solder points that looked corroded, cracked, burned, broken, etc. Truthfully, I was surprised to see how nice it actually was inside. I have emailed JamesDean to ask about testing/repair services (he's about to get my cruise amp and actuator as well).

I do not have an Aux pump. I have deleted the monovalve system completely and that went with it. Worth noting, the HVAC has worked beautifully with the monovalve delete for over a year.

Any specific, detailed, advice is highly appreciated. I can do a ton of my own work, but with electrical specifically, just pretend I'm an idiot.

Thanks.

I don't know if you will be taking your motor apart or not since it is out but my 84 300D has a Siemens motor in it instead of the Bosch Motors that others have.
On the Siemens Motor the brushes are up front and you need to remove the blower cage/fan to get at them. Unfortunately I cannot remember the details on taking it apart as I did it a long time ago.

If it has a Bosch Motor there is a few threads on taking them apart. The Bosch Motors have the Brushes in the rear.

This does not cover the whole tear down. I do remember that in order to change the ball bearing I had to drill out some rivets and there was also dirt inside that was binding it a bit.

http://www.peachparts.com/shopforum/diesel-discussion/308273-need-advice-removing-84-123-blower-motor-fan.html

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84 300D, 82 Volvo 244Gl Diesel

Another thing to check is the vacuum system. If the air flow diaphragm does not fully engage the micro-switch the blower will not be powered.

The diaphragm, micro-switch and relay are accessed most easily by removing the glove box.

If the blower engages when the defrost button is pushed then you most likely have a vacuum leak. The usual culprit is the center vent diaphragm.

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Ok cool, this is new info I hadn't found yet. Thank you.

I assume you're referring to the little vacuum pods in the dash, right? If one of those had gone bad, would my other vacuum functions still work? Ex, my locks all still work just fine with the key.

Also, are you saying the defroster would work regardless of the vacuum status? Because defrost would be the default in error, correct? If that's the case, I have no defrost at all, so wonder if it's actually an electrical issue.

Lastly, how can I test the three components inside the glove box space?

Yes, the other vacuum functions would still work as they are supplied before the climate control. They would be a bit slower but since they are supplemented by the vacuum reservoir you probably wouldn't notice the delay.

The fact the fan doesn't engage using the defrost function casts doubt on the pods being the problem. Might be worth checking them out while you are in the process.

You say you removed and checked the CCU. Are you referring to the push button unit mounted in the console?

The actual control/amplifier is located in the right hand side of the dash behind the glove box. Is that the part you checked? There is a relay located on the cabin side of the firewall also located behind the glove box. That might be what you are hearing "click".

To eliminate the pods causing the trouble the easiest way is to remove the glove box and physically observe the movement of the diaphragms. Obviously the vehicle need to be running. The micro-switch is located on the ducting to the right of the main duct work where it joins the center air box.

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

Blower motor resistor replacement. (Thank you Kent!)

https://www.youtube.com/watch?v=6Cxmfa2FMQQ

From Peach Parts:

https://www.pelicanparts.com/techarticles/Mercedes-W123/12-ELEC-Blower_Motor_Resistor_Replacement/12-ELEC-Blower_Motor_Resistor_Replacement.htm

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now