Tests for High Resistance in Blower Regulator Supply Line
The way to read the circuit diagram below is to start with what you know, ie the Blower Motor A32m1 and work backwards until you get to the battery.
Here is my analysis;
Blower Motor A32m1 has two wires going directly to the Blower Regulator A32n1. These wires are not designated and are obviously internally connected.
The connections to the Blower Regulator A32n1 are:
Next the Blower Fuse Connector X64/1 gets its power from a RDYE (Red Yellow) wire going to MR1 Pin 2 in the Right Front Fuse and Relay Module K40/7.
MR1 Pin 2 is internally connected to Fuse f33 in the Right Front Fuse and Relay Module K40/7.
Fuse f33 which is a 40A (Amber) fuse which is designated as "Automatic temperature control; Blower motor AC" in the Fuse Chart. For the Fuse Chart see;
WIS 54.15 Fuse and Relay Box - W220 Encyclopedia
Fuse f33 40A gets its power from Circuit 30 which is designated as "Battery +ve continuous" in the STAR Circuit Wiring Diagram Abbreviations Table. For the Circuit Wiring Diagram Abbreviations Table see;
Circuit Wiring - Electrical System - W220 Encyclopedia
IN SUMMARY
The Blower Regulator is connected to a
constant supply of +12V via Circuit 30 in the Right Front Fuse and Relay Module K40/7.
In your case from your posted measurements, the voltage measured at A32n1 Pin 3 (RDYE (Red Yellow) on the Blower Regulator) is NOT CONSTANT but is subject to how much current is being drawn ie how much load is applied.
You have shown that with a very light load such as when the blower motor is off you measure a good 12V at A32n1 Pin 3. The conclusion from this test is that the regulator supply wire is intact and able to deliver 12V but the wire may be damaged or contain a fault.
HOWEVER under HEAVY LOAD such as when the blower motor is commanded to go flat out you measure 0V at A32n1 Pin 3. The conclusion from this test is that the regulator supply wire has a high resistance somewhere between the A32n1 Pin 3 and the battery.
The possible high resistance is most likely to occur at the weak points which are at the various junctions in this supply line. The analysis of the circuit diagram above reveals the weak points to be:
- Pin 3 RDYE (Red Yellow) in the Blower Regulator A32n1.
- Both in and out terminals of Blower Fuse Connector X64/1.
- MR1 Pin 2 in the Right Front Fuse and Relay Module K40/7.
- Both sides of Fuse f33 in the Right Front Fuse and Relay Module K40/7.
- And wherever circuit 30 goes from the Right Front Fuse and Relay Module K40/7, but we will leave determining that until later.
TEST 1
The aim is to ascertain if there is a high resistance in the original supply line to the Blower Regulator A32n1.
The method is to bypass the Blower Regulator supply line completely and see if the blower then works when commanded from the ACC Controller.
The best way to do this is to take a reasonably heavy wire from the +12V in the foot well and connect it to Pin 3 RDYE (Red Yellow) in the Blower Regulator A32n1. It would be a good idea to use a 40A fuse in this hot wire connection just as a precaution. Now that I have analysed the circuit diagram I conclude that it is not necessary to remove the original Red Yellow wire from Pin 3 for this test. Just remove Fuse f33 in the Right Front Fuse and Relay Module K40/7.
Once the hot wire is in place and the Fuse f33 removed, switch on the ACC and command the blower to run at maximum speed.
If the Blower still does not work we have to start the diagnosis again.
If the blower now works properly you will have positively confirmed that there is a high resistance in the original supply line and the challenge is to then find exactly where it is. My guess BTW is that it is a burnt terminal under f33.
Remove the hot wire bypass and reinstall the Fuse f33.
TEST 2
The aim is to find where the high resistance is occurring in the original supply line to the Blower Regulator.
The method is to make a series of voltage measurements on the supply line starting at Pin 3 on the Blower Regulator A32n1 and working backwards towards the battery.
Test each point with the Blower Motor off and then on and record the results.
Make up a Results Table as per;
- Test Point: Voltage with Blower OFF: Voltage with Blower ON
- Pin 3 A32n1: ?V: ?V
- Blower Regulator side of X64/1: ?V: ?V
- Fuse side of X64/1: ?V: ?V
- X64/1 side of Fuse f33: ?V: ?V
- Circuit 30 side of Fuse f33: ?V: ?V
It may be a bit difficult to determine which side of X64/1 is which, so just measure both sides separately and record the results.
The same applies to Fuse f33.
Post the results of Test 1 before starting Test 2.
Best of luck.