I have a 1991 420 SEL which has recently been doing some strange A/C things...when I first get into the car in the morning and it is cool outside everything works just fine...turn on the AC and it works and the thermostat controls the cabin temp and the speed of the fan. When I come out after work and the car has been sitting in the sun the AC blower will not come on no matter which control buttons I push...it has me hot under the collar...the blower moter is good and so are the controls first thing in the morning in a cool vehicle...but not after sitting in the sun...any help or gouge would be appreciated...

I would suspect that you may have some corrosion built up on the strip fuse. Or the fuse has a small hairline crack. In the engine compartment, locate the fuse box. Just forward of the fuse box you should see a small rectangular plastic cover with a wire exiting from each end. Open the plastic cover and you will will see the aluminum fuse strip. The fuse is very fragile and breaks easily. Check for any hair line cracks on the fuse and corrosion at the connection points, if cracked, replace. If corrosion is found at the connection points, remove the fuse carefully, remove the corrosion and replace. If you need to replace the fuse be sure to replace w/same amp rating. These are cheap at the stealership, might as well get two so you have an extra. Hope this advice helps you.

Brgds,

BENZSTER

Well i cleaned all of the blower electrical contacts in the fuse box and replaced the fuse...the A/C and blower will still come on first thing in the morning when it is cool outside and runs just fine including the Temperture control...but once the car sits in the sun and warms up, when I get into it in the evening the blower will not come on...is there any other suggestions out there I could try...thanks in advance

Well I tried another suggestion...that when I put in the key to start the car...move it around to make sure the contacts were not dirty...that did not help...drove it yesterday and the blower ran like a champ in the morning from home to work and not at all in the evening after sitting in the parking lot...I hope there are additional things i can try before i take it into the shop...thankfully it has been a cool summer in VA...

Does your compressor engage when it has been sitting in the sun ? In otherwords, is it the blower not blowing or is it the compressor not working or both ?

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Saumil S. Patel

This was in the little black fuse box, separate from the regular fuse box, right?

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2012 E350
2006 Callaway SC560

The compressor does engage when I go from economy to A/C...I just added Freon and in the morning it blows Ice Cold Air...the compressor has no problemn with the heat...it turns on and off as required...

The fuse I replace and the contacts I cleaned were in a large fuse box on the drivers side by the Brake Fluid holder...if there is another fuse box I need to look at can you telll where to look...thanks

All I have been tring to debug and A/C blower motor problem on my 1991 420 SEL...a number of responses allude to multiple fuze boxes on this model...I have found the large one by the brake fluid fill point...but I have no gouge an where any others may be located and might be the point of failure causing my blower motor not to come on in the afternoon after the car has been sitting in the sun...can you point me in the right direction on where the additional fuze boxes are on my car that I need to look at...thanks

The blower motor's fuse is seperate from the main fuse box. It is a small black plastic holder along the driver side fender, roughly above the ignition control unit. The top part of the holder pops out and you will see a metal fuse clip. Benzster has also specified the location using a different scheme.

If it is not the fuse then:
If your car is like my 87, 260E, then you can determine if the problem is in the signal that comes from the push button unit OR the problem is downstream (either the blower regulator or the blower itself, to find this you will have to get to the blower).

There is a 3 terminal connector on the firewall just behind your brake booster. Disconnect the wire terminal. The first terminal (closest to the engine) on the firewall connector is ground. The middle terminal should have battery voltage when the car is in running. The third terminal (closest to the fender) is for controlling the speed and is sent by the push button unit. When you have selected low blower speed, this voltage should be about 1V. When you select high blower speed, the voltage should be between 6-8 volts, could be higher (dont remember the exact numbers right now).

Now when your blower stops:
if you dont have the close to 12V on the middle terminal, the problem is the ignition switch.
if you dont get any voltage on the third terminal, the problem is that your push button unit is not sending the correct control voltage
if both the voltages are o.k., then the problem is either your blower motor regulator or the motor itself.

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Saumil S. Patel

Thanks for the detailed explaination...I will look for the fuze and if required do additional trouble shooting...thanks again and I will report back with findings...

BTW, I had my blower become erratic once. Turned out that the 3-pin connector on the frewall simply had some corrosion.

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2012 E350
2006 Callaway SC560

Once I found the Blower "Strip" Fuse...and thanks for the help on that one, the fuse was brittle, in fact it broke in my hands. Went to the Mercedes dealer and bought two "Strip" fuses and installed...and now I have a back up...and yes it worked...the blower blows and the A/C works...thanks to all who responded...very much appreciated...a cool 420 SEL owner

OK the motor has been at best intermittent...still works when it is cool and not when it warms up sitting in the sun...I figure it is time to dig in and look at the blower motor...any gouge on the procedure to get to the blower motor on a W126 1990 420 SEL and how hard it is to get at...thanks in advance

I think before digging the blower motor, you could test the connector behind the firewall (if your car has one) and determine whether it is downstream from that connector or upstream. I am not sure why the motor itself would care about ouside temp, but the climate control unit in the car will and what if it does not send the voltage needed to start the blower.

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Saumil S. Patel