a/c blower motor works only speed 2

[lancalots]

i have a 1976 300d and i cant get my a/c blower motor in any other speed but 2 ,any ideas ? i have been told it has other fuses but where ? thanks for any help !

[300SDog]

How many speeds has it got? If memory serves me well, the 114/115 dash AC/Fan knob is gradual/graded after the 2nd click.

Irregardless that's where I'd start - with the dash knob, taking it apart and cleaning it up with small brass brush and some kind of evaporative solvent/cleaner like lighter fluid or alcohol.

Just gotta be careful pulling the switch knob center from the rim so it doesnt get scratched.

[Mark DiSilvestro]

No separate fuses for the different fan speeds. The W114-115 cars use the same fan switch for heat and AC, with a vacuum operated change-over switch, mounted at the side of the heater box, above the gas pedal. Depending on whether the AC knob is on or off, the change-over switch switches the circuit between the heater blower and AC blower. If all the HEATER blower speeds still work, it's not the fan switch. Most likely it's the AC blower resistor or change-over switch.

Happy Motoring, Mark

[lancalots]

Originally Posted by Mark DiSilvestro

No separate fuses for the different fan speeds. The W114-115 cars use the same fan switch for heat and AC, with a vacuum operated change-over switch, mounted at the side of the heater box, above the gas pedal. Depending on whether the AC knob is on or off, the change-over switch switches the circuit between the heater blower and AC blower. If all the HEATER blower speeds still work, it's not the fan switch. Most likely it's the AC blower resistor or change-over switch.

Happy Motoring, Mark

Mark , thanks for the insight i think you are on to something here , it very well maybe the blower resistor because all heater blower speeds work and i wouldnt think it would be the change over switch because it does change over to a/c but just to speed two only . now this brings up another ? does anyone off hand know where this blower motor resistor is to the a/c motor ? and do you think i could bypass it to make sure thats what i need . it would sure be nice to have a high speed on the ac !!! thanks again !!!!

[Mark DiSilvestro]

I dug out my 114/115 shop manual and found the Heat/AC wiring diagram and I'm still not completely satisfied. Here's what I've been able to figure out -

There are two separate fuses for the AC and heater blowers (but not for the different fan speeds). My book shows them in a small fusebox, under the hood on the passeger-side firewall. I know you said the AC fan worked on low, but it could somehow be feeding back through the other circuits if the AC fuse is blown or corroded, so check these fuses anyway.

There's only one blower resistor pictured, attached to the heater blower wire harness and fastened at the drivers-side firewall. There's no separate AC blower resistor shown in the diagram. It looks like both blowers share the same resistor.

Depending on position of the change-over switch, 12 volts is supplied directly to either blower. The resistor and fan switch varies the ground circuit.
High blower for both heater and AC blower is by a direct ground through terminal 3 of the fan switch, bypassing the resistor. According to my wiring diagram, separate heater and AC fan circuits ARE connected through the same fan switch, so it COULD be the fan switch.

Hope ths helps as I currently have no way of posting the wiring diagram.

(To get high AC blower right away, you could disconnect the wires from the AC blower motor and wire it, temporarily, to a sparate switch, fuse, and 12 volts, bypassing it completely from the original AC circuit.)

Happy Motoring, Mark

[lancalots]

Mark , want to say thanks again i will most definatly look into the fuse and about bypassing that switch to see if this is what i need ! going on vacation tomarrow and won't be back for a week but when i do get back i will post what it takes to fix . can't say enough about this board it has helped me fix several things on the ole girl !

[diametricalbenz]

Originally Posted by Mark DiSilvestro

I dug out my 114/115 shop manual and found the Heat/AC wiring diagram and I'm still not completely satisfied. Here's what I've been able to figure out -

There are two separate fuses for the AC and heater blowers (but not for the different fan speeds). My book shows them in a small fusebox, under the hood on the passeger-side firewall. I know you said the AC fan worked on low, but it could somehow be feeding back through the other circuits if the AC fuse is blown or corroded, so check these fuses anyway.

There's only one blower resistor pictured, attached to the heater blower wire harness and fastened at the drivers-side firewall. There's no separate AC blower resistor shown in the diagram. It looks like both blowers share the same resistor.

Depending on position of the change-over switch, 12 volts is supplied directly to either blower. The resistor and fan switch varies the ground circuit.
High blower for both heater and AC blower is by a direct ground through terminal 3 of the fan switch, bypassing the resistor. According to my wiring diagram, separate heater and AC fan circuits ARE connected through the same fan switch, so it COULD be the fan switch.

Hope ths helps as I currently have no way of posting the wiring diagram.

(To get high AC blower right away, you could disconnect the wires from the AC blower motor and wire it, temporarily, to a sparate switch, fuse, and 12 volts, bypassing it completely from the original AC circuit.)

Happy Motoring, Mark

I know this is an old thread but do you happen to have the picture of exactly where the resistor is and or what it looks like? Is it enclosed or open by chance?

Thanks!

[250 Coupe]

This is for a W114/W115. I don't know if it's the same for your 240D.

It's mounted to the firewall so that the resistors (which look like little coils of wire) are air cooled. From inside the engine compartment, look for a 4 (6?) pin connector plugged flush to the firewall. It should be above the booster and toward the center a bit. Two screws on opposite corners hold it in but I don't remember if it goes into the plenum or pulls out into the engine compartment.

Michael

[Mark DiSilvestro]

The resistor mounting screws are on the engine compartment side of the firewall, but it removes from the air duct side, as it's permanently wired to the heater blower. My wiring diagram shows a 4-pin connection. The resistor has a plated steel cover with holes for air to pass through and cool it. I don't know if it's availible separately. The few new heater blowers, I've handled for these cars came with a new resistor attached. It is possible to pry the steel cover from the resistor and unsolder it from the blower wires if you have a replacement or want to try and repair the old one. However, as I mentioned before, if the heater blower still works on all it's speeds, it's probably not that resistor.

Happy Motoring, Mark

[diametricalbenz]

250 Coupe - This questions pertains to my W115 not the W123

Mark - The AC nor the heater fan doesn't work at any speed 1-3. I get nothing from the motor. I checked all the fuses and they're all okay. I don't remember seeing anything caged in metal but I will look everything over again. I am wondering if the change over switch is stuck hence no actuation of the fan motors. Can it get stuck and allow no fan operation whatsoever?

In the event that the resistor is not there on the firewall, are the heater motors with the attached resistor able to be tested by accessing them by removing the AC/Radio/12V console from inside the cabin? Would the whole motor need to be replaced or can the resistor be removed and soldered in with an axial resistor?

Thanks for the help guys.

[250 Coupe]

You will need to pull the aluminum grill at the base of the windshield to see the resistor assembly.
It will look like this.
Click the image to enlarge.

Check the vacuum operator above the throttle pedal. It needs to be at one end or the other of it's travel to select either the heater or A/C.

Michael

[Mark DiSilvestro]

If you have to pull the vent grill from the base of the windshield, it's tucked under the aluminum windshield trim and secured by the wiper shaft bushing nuts and by 5 or 6 plastic rivets below the rear of the hood.
To remove the plastic rivets, push the plastic pins from the rivet centers, then the rivet buttons should easily pry right out of the holes. Once the grill is off, you'll have to fish around the bottom of the air plenum for the pins to re-secure the rivets. Hope you find them all and, of course, that the 34 year-old plastic rivets are all still intact and don't disintegrate when you remove them.

Happy Motoring, Mark

[diametricalbenz]

Well, I got underneath the steering column with a flashlight and looked around in lieu of stuffing myself with corned beef.

I found out that the heater fan fuse blew out at some point. I changed the fuse and still nothing. I manually actuated the vent flap as far up and as far down as it would go and still nothing. I can get nothing from either the AC cage fan nor the heater fan. I started prying at one of the fasteners holding the vent grill in front of the windshield to get to the heater fan but the screwdriver was scratching the aluminum.

Is there a special tool that you guys know about that would minimize the damage to get those pins off?

Are there other relays or fuses to check?

So it seems as if the problems may be narrowed to:
Fan switch (1-3)
Fan resistor
Changeover switch (where is this?...is it inside the box within the console? it seems to be inacessible)

[Mark DiSilvestro]

You need a skinny punch or a nail or stiff wire to push through the plastic pins from the center of those plastic rivets. Then you should be able to easily pry them out with your fingernail.
As for the changeover switch, it should be located at the left lower corner of the heater box, just above the accelerator pedal.

Happy Motoring, Mark

[Tristar1959]

114/115' s:-
Summary of salient points for logic troubleshooting:-

BOTH the heater muffin fan and the A-C squirrel cages use the SAME resistor bridge up in the air plenum - they are cleverly forward-fed for the Heater's feed and back-fed for the A-C's; both via the Off-Hi-Med-Lo fan knob;

So, depending upon the position of the springloaded "switchover" switch, as mentioned above, which is activated by a tab on the rod which controls the main airflow diverter - THAT rod is activated by a vacuum unit, and IT is gated vacuum according to the position of the A-C rotary switch - full off will allow the heater's radiator cores air flow as well as the (plenum) muffin fan's operation; any A-C position in its temp range should both divert airflow up through the A-C evaporator and enable operation of the (center console) squirrel cage blowers. As mentioned above, if the vacuum element is not operating properly, the rod can be moved manually to effect both electrical and airflow switchover, but be sure the switch is mounted/adjusted such that it actually switches when the rod "bottoms out" and the tab hits the switch's button. [test by having rotary fan switch on, say, 2nd click (med) - move rod back and forth - you should hear fans switch back and forth]