Ok, I've had enough. It's been bloody cold here (3 below F predicted for the am) and I'm getting damn tired of cold feet and tush. Since neither I nor anyone else suffering from a non-functioning blower under 20 degrees have been able to diagnose the problem, I'm about to resort to a redneck solution. I'm considering hot wiring the blower directly from the battery, probably with a toggle switch in the glove box. My plan is to simply tie the hot wire into the existing blower wire without disconnecting anything. Can anyone give a considered opinion on whether such a tactic will damage anything in the climate control system, especially the amplifier and the evil servo? If it doesn't it should work as a temporary solution. In fact, since the blower works above a certain temperature, if I use the switch to fire up the blower, once it raises the temperature inside the car, there's a good possibility it will start functioning normally. It's dangerous because if that possibility is realized, it could easily turn into a permanent solution despite the fact that I'm generally opposed to porch appliances.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Why not go to a truckstop and get a 300-500w electric heater to use until you fix the blower. As a bonus, you'll have heat right away.

Wiring the blower directly from the battery should be OK although I would disconnect the wire from the servo (or whatever normally controls the blower) and I would fuse the connection to the battery at about 20 Amps or so. If you want to get fancy, use an SPDT switch so that you can toggle the blower between "direct – battery" and "normal." As you say, maybe a few minutes of direct connection will convince the blower to work normally and you can switch back until "next time."

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

If you do it, for sure use a proper fuse! Redneck mods often cause redneck car fires!

__________________
-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

at least carry some marshmallows along so you have something to do while you wait for the red trucks.

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1984 123.193 372,xxx miles, room for Seven.

1999 Dodge Durango Cummins 4BTAA 47RE 5k lb 4x4 getting 25+mpgs, room for Seven.

Yes, I was going to fuse it. The three way switch is the ideal solution.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

I have mine fused and switched at 20A. It blew a 15A instantly, so it probably close to 20.

__________________
$60 OM617 Blank Exhaust Flanges
$110 OM606 Blank Exhaust Flanges
No merc at the moment

Why do you have yours fused and switched?

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

i recently fixed someones car who the previous owner did this exactly and ruined their climate control unit so I would think about it and try to fix whats wrong the climate control. this was from an 83 and i tried resoldering the board but didnt fix the problem. Found burn marks on the circuit board as a result. Acc II is different in the respect that depends a lot on the vacuum system working 100% or other wise nothing will work.

if this is for the 77 then it could be a vacuum issue, to see if this is the problem, switch the ignition switch on but dont start the car, then hit defrost button. blower motor should start. then start the car and hit the auto hi button. the blower motor will turn on when the coolant is @ 40 c(104 F).

I like forceds idea

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1986 300SDL, 211K,Dealership serviced its whole life
1991 190E 2.6(120k)
1983 300D(300k)
1977 300D(211k)

Thank you. Any idea what happened to ruin the climate control? What did you have to do to repair it? It's not a vacuum issue. Blower doesn't run in any position when cold. In warmer weather it works flawlessly. I jumped the main vacuum control switch and it made no difference. I replaced the amp to no avail, initially thinking it was burned out. So, either the second used amp is flawed or the original was ok and the problem is elsewhere, or both amps were/are flawed. I'm guessing it's a bad connection or cracked solder joint somewhere which in cold weather contracts enough so it doesn't conduct electricity.
Problem is on my 79SD which has the servo unit.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Have you ever learned how the system works? Think it will answer some, if not most of the questions.

As for the reason the climate control unit failed on the 83, im guessing the hotwiring of the blower motor overloaded the diode that regulated voltage on the circuitboard of the climate control unit. The po hotwired the blowermotor with like8 guage wire and protected it with a 20 or 30amp fuse. All the wires for the blowermotor were cut
.

__________________
1986 300SDL, 211K,Dealership serviced its whole life
1991 190E 2.6(120k)
1983 300D(300k)
1977 300D(211k)

I'm pretty familiar with most of the details of the system. Is there a reason for thinking that the amplifier in the one you worked on had not failed prior to the hot wiring? It seems to me that is the likely scenario, accounting for why the person hot wired the blower. The amplifier fails when the auxilliary water pump seizes so it's a pretty common problem. In my case, I have the water pump disconnected until I get the fuse harness to reconnect it and protect the amp.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

The amplifer evens out the resistances of the temperature sensors agianst a fixed resistor value when they are not in balance which activates the servo.
Now its quite possible that amplifer failed or one the temperature sensors or potentiometers failed when the blowmotor had nothing to do with the problem since the blowmotor only comes on when command to by the servo.

Another possiblity is that the cold engine lock out wasnt working properly and was misdiagnosed as a blower motor problem

__________________
1986 300SDL, 211K,Dealership serviced its whole life
1991 190E 2.6(120k)
1983 300D(300k)
1977 300D(211k)

Dead CCU, I gutted the whole system in favor of an on/off switch and space for gauges. I'll hook it up to a rotary switch one of these days.

__________________
$60 OM617 Blank Exhaust Flanges
$110 OM606 Blank Exhaust Flanges
No merc at the moment

16 amps is max. Over that you need to repair or replace the blower motor to keep it from frying.

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1983 123.133 California
- GreaseCar Veg System