126 Blower Motor circuit - yet another diagnostic thread!

[DrLou]

This is an '83 (126) 300 SD...

The backstory: Thanks to an over-enthusiastic jump starter ( who connected charge cables in reverse! ), I'm working though some damage likely caused(?) by it...

At this point, the focus is the blower system.

1) Blower motor itself works AOK - tested connected directly to 12v.
2) I had put a fuse in-line at the blower motor plug some years ago - just for good measure. This was burned out; now replaced.
3) Under-hood fuses all intact; no evidence of burn.

Still, no pushbutton setting activates the blower.

Various posts here - most relate to the 124 - refer to other fuses and relays being in play. Where are any/all other fuses which affect this subsystem located?

For example, I cannot find any other in-line 30A blade fuse - or other - in the area of the brake booster. Is there some other fusable link under the R side dashboard somewhere?

Does any of the little canister relays in fusebox 'switch' this system?

Thanks, Wizards!

[SD Blue]

F10 and F11 are the two fuses for the blower. Inspect them closely, especially those with a plastic body, as they are notorious for issues.

Have you tried the High-Low-Automatic settings for blower speed control?

One has to wonder why the in-line fuse was blown. Maybe you have worn brushes in the blower motor. Has it ever been changed?
https://www.startekinfo.com/StarTek/outside/12253/program/ETM/ACC83-85.pdf

[sixto]

The 124 system or even the 86-up 126 systems aren’t a good reference for your ‘83. Those have a controlled voltage signal to a solid state controller. You have a 12V signal to a resistor bank by the brake booster, and IIRC a relay block switch between the PBU and the resistor bank. You need a functional diagram of the PBU (there’s likely one in the FSM) to confirm the PBU is putting out a blower signal.

Sixto
98 E320s sedan and wagon

[DrLou]

Gentlemen!

Thank you both for your feedback. I'm working through the issues...

First, yes: SD Blue, All fuses are intact; F8 and F14 both implicated in this circuit - and test out AOK. It's very likely I'd either rebuilt or replaced this blower motor; but this would have been about 20 yrs ago. As I mentioned, the motor does run just fine with 12v connected directly. I suppose I could actually test current draw...

sixto, I cannot find anything which looks like a resistor bank near the brake booster, nor any relay block switch in that area. Can you point to any photos helping identify locations of these components?

I do have the Electrical (Troubleshooting) Manual for this car; am working through the schematics. Just hoping one of the control units is not at fault here.

[resto108]

Look under the brake booster, down in the bottom between the firewall and false firewall. You'll see a 2-3" metal box with 1/2"? holes in it. This is the blower resistor. Service manual has the procedure for getting it out.

[funola]

What was the jump starting device? A battery charger or a battery or another car with engine running? How long was it connected reverse polarity? Was the key turned to on position and did you try cranking the starter with reverse polarity?

If you did not turn the key to on position, the only item that can be damaged is the diode pack in the alternator. You need not worry about the blower motor, it will just spin backwards if power was applied and suffer no damage.

To check if the alternator was damaged, check for AC voltage with engine running. Anything over 10 millivolts AC may indicate a damaged alternator.

[DrLou]

Originally Posted by resto108

Look under the brake booster...

Ok, will start digging deeper... I worry that I may have a brake booster removal in my near future? ( Oh well, silver lining? This would be my chance to replace those 35yr-old sway bar bushings I've been wanting to replace... ! )

Originally Posted by funola

What was the jump starting device?

Worse than even a battery charger; it was a high-amperage starter. Yes, alternator was definitely burned out in the process. Long since replaced. Voltage output and battery charging are definitely AOK at this point. Car runs just fine* - I probably didn't mention. In fact, have just refurb'ed lower ball joints, shocks, tie rod ends, etc. Getting 'er ready for her next 500k miles!

Only issue at the moment is I have no blower motor activation. It's worth noting that vacuum switch/valves are activated by various PBU settings, so I don't think the PBU is totally roasted.

( though I seem to have a slow-amperate-leak short somewhere still, probably due to a wet door switch or similar. )

[funola]

Originally Posted by DrLou

Ok, will start digging deeper... I worry that I may have a brake booster removal in my near future? ( Oh well, silver lining? This would be my chance to replace those 35yr-old sway bar bushings I've been wanting to replace... ! )

Worse than even a battery charger; it was a high-amperage starter. Yes, alternator was definitely burned out in the process. Long since replaced. Voltage output and battery charging are definitely AOK at this point. Car runs just fine* - I probably didn't mention. In fact, have just refurb'ed lower ball joints, shocks, tie rod ends, etc. Getting 'er ready for her next 500k miles!

Only issue at the moment is I have no blower motor activation. It's worth noting that vacuum switch/valves are activated by various PBU settings, so I don't think the PBU is totally roasted.

( though I seem to have a slow-amperate-leak short somewhere still, probably due to a wet door switch or similar. )

Per this HELP! W126 Climate Control Schematic Please. the W126 use a solid state motor speed controller. If you had a high amp battery charger connected reverse polarity and had the blower on, good chance the solid state controller is damaged.

[Diseasel300]

Originally Posted by funola

Per this HELP! W126 Climate Control Schematic Please. the W126 use a solid state motor speed controller. If you had a high amp battery charger connected reverse polarity and had the blower on, good chance the solid state controller is damaged.

Only the Gen II (1986+) models use the solid state along with the ACCIII control unit that lasted into the mid 90s. The pre-86 W126 has the same system that the W123's did.

[funola]

Originally Posted by Diseasel300

Only the Gen II (1986+) models use the solid state along with the ACCIII control unit that lasted into the mid 90s. The pre-86 W126 has the same system that the W123's did.

Thanks for the correction. His blower works, so must be the pbu or wiring. Should be easy with a wiring diag. Do you have a link?

[SD Blue]

Check for +12vdc at pin 12 of the Blower Speed Controller. (It's located to the left of the blower motor.)
Also, to rule out any Temp. Controller input and Cold Engine Lockout Switch, place the Pushbutton Controller to the Defrost Mode.

[Diseasel300]

Originally Posted by funola

Thanks for the correction. His blower works, so must be the pbu or wiring. Should be easy with a wiring diag. Do you have a link?

PDF attached, much more legible than the image in the above link. Note that the '83-85 have the 2-piece ACC setup with the PBU in the dash, and another relay module elsewhere to control the resistor bridge to the blower. The '86+ models have a single piece ACC electronic setup with a transistorized blower control. I haven't worked on the earlier 126, so I can't say where the blower control or the resistor is located.

[funola]

Originally Posted by Diseasel300

PDF attached, much more legible than the image in the above link. Note that the '83-85 have the 2-piece ACC setup with the PBU in the dash, and another relay module elsewhere to control the resistor bridge to the blower. The '86+ models have a single piece ACC electronic setup with a transistorized blower control. I haven't worked on the earlier 126, so I can't say where the blower control or the resistor is located.

Your pdf is the same diagram as one of the diagrams in the link I posted above.

Anyway, I said that it has a solid state controller because the diagram calls it "electronic unit for speed control", not that I have ever looked inside one and saw transistors. Still worth checking that controller out. I don't think Mercedes would call it "electronic" if it's just mechanical switches.

[sixto]

The size of the box and external resistors suggests it’s not very electronic. Then there’s the 722.3/4 called 4G-Tronic.

Sixto
98 E320s sedan and wagon

[funola]

I have no experience with W126's only W123's. I did take apart the pbu in my 83 W123 5 years or so ago,and, as I recall, it has mechanical switches, relays and diodes. Call it electronic, non electronic or whatever you want. If it has diodes, it can be damaged by reverse polarity.