Cabin Fan Resistor Circuits

Status
Not open for further replies.

Aqua Puttana

Poly - Thread Finder
T1N Cabin Fan Circuits

I have noticed a couple posts about no cabin fan operation at all as opposed to high speed only fan operation. It turns out that there appears to be two different fan motor speed resistor configurations. The following schematics include some notes which may help people to understand the operation.

Added.
A quick check if the Cabin Fan will not operate on all speeds.

The REST and A/C system both utilize fan speed #1.

Enable REST or your A/C system. If the Cabin Fan operates on low speed, but doesn't respond to the other speeds it is likely an ATC module internal fan relay problem. It is NOT the resistor pack because the REST speed #1 needs the resistor pack to be complete for that speed to operate. (It uses all the resistors in series for the circuit.)

Edit:
Always use pin numbers AND wire colors to verify

I find the diagrams less than intuitive, but maybe I'm just not familiar with the conventions. Labels aren't consistent though. I have added some Blue letter designations for this discussion. S209 = Splice #209. C = Connector.

Note that in 8W-42-5 Pin #2 (Blue "A") is labelled "High Side Driver" within the ATC dotted box. In 8W-42-6 (also Blue "A") there is no label at all. (That happens alot.) They are exactly the same point. For me it took referring back and forth using wire colors to 100% confirm that. I find that by MB using one diagram to cover the different flavors the overlapping combined information takes some effort to verify 100%.

As a specific example refer to 8W-42-5 "Blower Motor Resistor Block" dotted box. For the typical NAS aka NAFTA Sprinter fan/resistor circuit, only the pin #4 "Blower Motor +" DG wire applies. The pin #2 C1 has absolutely nothing to do with the circuit. It is for the other variation.

8W42-05wNotesBlue.jpg

Troubleshooting Tip

With this 8W-42-6 configuration: If the fan works with the key off and REST button on (the relay is fed from Fuse #8), but not otherwise then it MAY be fuse #21 (ATC 12V feed), the actual fan speed selector switch, or the ATC fan relay shown. There could be many other things wrong, but this REST test is a quick and easy way to verify that the blower works at all. If it works on REST, then the problem is not the blower itself, or the resistor block either for that matter.

One step further. If the fan operates on high, but the REST doesn't work then you probably need a new resistor block.

8w42-06wNotesFanResistorBlueA.jpg

8W42-07wNotesA.jpg

8W42-08wNotes1.jpg

Note the reference to Splice #S362 which is shown in the post.

8W42-09 wNotesRev2a.jpg
 
Last edited:

Aqua Puttana

Poly - Thread Finder
Cont.

Some other drawings while I'm at it.

8W11-34Rev1a.jpg

8W42-10 wNotesA.jpg

8W42-11 wNotes.jpg

8w-42-17 FanSplice201.jpg

Information I found indicates that this is what the pump diode looks like (next to Circle 1). The picture is the relay rail under the driver seat.

SprinterAuxPumpDiode.JPG

Note: The diode is in the circuit for both the Dash Inset Control aka Program Timer and the Booster Heater aka Except Program Timer OEM Espar heaters. That said, my Booster Heater diode isn't mounted as shown above. I have yet to locate the diode, but my testing shows that it dies exist.

The Booster Heater has no Stage 1 Fan Relay. The Stage 1 (Low) fan speed is provided internally within the ATC module circuit/logic. The Booster Heater also has no Circulation Pump relay. The Circulation Pump is powered by the ignition key position or the ATC in REST mode.
 
Last edited:

Aqua Puttana

Poly - Thread Finder
From a recent thread.
https://sprinter-source.com/forums/showthread.php?p=375077

Hi Vic,

On the bottom row the first pin on the left appears to be half black and half blue with a green stripe however like you indicated it is not really all that dark green but is the closest and only apparent match. Is this the wire and pin position you would like for me to test?
That sounds like it. 16 AWG wire Pin #21 on Connector C1.

8w-80-7 ATC Connector.jpg

...
AC is operating which is fan speed 1. Both of which should be producing current on the BK/BL/DG wire and connection pin you refer to.
Ok. That helps to give direction. There will not necessarily be power on Pin #21 when the fan is running on speed #1 lowest speed.

The internal fan relay is needed for all 4 speeds. There is a pin which feeds only fan speed #1 (slowest) for REST [and apparently with A/C enabled]. That connection doesn't need the ATC module internal fan relay to run the fan. That is probably how your fan is running.

Added. The control is probably designed to operate the fan on speed #1 whenever the A/C is enabled. That would assure that there is always some air flow over the evaporator coil. That would help prevent icing of the evaporator coil.

See BK/VT (Black/Violet) wire. "B" triangle in this diagram.

8w42-06wNotesFanResistorBlueA.jpg


I will measure the voltage in the various settings and also check for current when the AC compressor is off and blower is on heat / vent for the 4 fan positions and measure current on BK/BL/DG wire for each fan setting during the various operations mentioned.

Am I on the right track? Can you think of any other testing that needs to be done at this juncture other than what I have listed?
With the fan only running on speed #1 it should be enough to just check for 12 v on the BK/BL/DG wire.

From what I gather we are checking the fan current into the ATC fan relay and if it is there when fan is working and is not there when fan wont blow then the problem is some where else.
...
Correct. If you find 12 volts on the BK/BL/DG Pin #21 wire and the fan will not operate on all speeds then that indicates your internal fan relay coil is getting proper control signal power, but is not powering the fan switch for whatever reason. (It could be a bad coil, loose connection, bad contacts... who knows?) At any rate the solution at that point is to install the replacement ATC module.

A DIY repair could be to replace the internal relay with an external relay, but that involves circuit board level modification. You have the ATC module part so you should just use it.

vic

Added:
For future reference a quick check if the Cabin Fan will not operate on all speeds.

The REST and A/C system both utilize fan speed #1.

Enable REST or your A/C system. If the Cabin Fan operates on low speed, but doesn't respond to the other speeds it is likely an ATC module internal fan relay problem. It is NOT the resistor pack because the REST speed #1 needs the resistor pack to be complete for that speed to operate. (It uses all the resistors in series for the circuit.)
 
Status
Not open for further replies.

Top Bottom