No low speed fan when KOEO,Man A/C on, But A/C cools and Fans cycle on-off when engine running

[cherrybomb]

2003 Beetle TDI, A/T and Man A/C. Last summer the A/C worked and cooled good. Car never had overheat light on. Cked the fuse box on battery and noticed the 40 A strip burned so replaced it. Used A/C during last summer and could hear cooling fans running and cycling. Additional research revealed articles by DANG144, Oilhammer and AndyBees that indicated the low speed should be activated by KOEO, A/C Button on and good 30A blade fuse. No Slow Speed when tried. Verified ALL the items in the battery fuse box were clean and working (pwr on both sides). Did not test the individual fans yet due to *trying to understand if the Thermal Temp sensor on Rad. would allow the existing operation if defective. The Fan Control Module (FCM) also would be a possible cause. Both seem to require removal of battery and tray to access from top. The Green Temp sensor under Vac. Pump has been replaced and provides data to ECM since Torque Pro reads a 200 F. temp during driving. *Age and limited mobility limit the ability to get under car or work from underneath. Plan to plug our 9V. memory saver device into the console plug since no radio (Monsoon Cassette only) code came with car. Hope to only remove the Battery and Tray only 1 time. Should tests on both fans show speeds, high and low ok, can we turn to the FCM or TFS (Thermal Fan Switch)? The Ambient Air Sensor (AAS) and A/C High Pressure sensor would have prevented operation last year we think?
* age (1 yr. prior to VW into USA) and limited mobility.

 

[P2B]

Fans should run provided ambient temperature is above freezing and there is some pressure in the system, regardless of thermoswitch. You can back probe the white wire on the pressure sensor as a rough check, anything over 1.5vdc (key on) is enough to allow fan operation.

The fan and AC troubleshooting PDFs linked from DanG144's post are fairly definitive, you can't go wrong if you follow them.

 

[cherrybomb]

Updated: Put the car on ramps to check and see if adequate room to disconnect the TFS of individual fans connectors. My medium size hands ( now scratched) did not fit the available openings. Maybe possible option of removing the L.F. fender liner, but unsure if this is workable ?? Would that step allow access to the TFS & FCM if utilized ? Yesterday the Temps were mid 70's so the car was taken for a test run. Coolant Temps (Torque Pro) ran around 198 to 202F with the A/C activated, cold air blowing. This appears to indicate the (AAS)Ambient Air sensor is working, verify the cooling fans (2) are working and the A/C compressor are working. Upon return to shop, the hose from left coolant flange was very warm and the hose at thermostat was warm. Note: Confirmed the Green 30A. fuse on Bat. Box good and Wiggle tested. We can only identify 3 items that prevent the low speed fans from operating as first tested. #1. FCM, #2. TFS or #3. Both cooling fans low speed defective (Both High Speed work). May try underneath access or fender liner access again before removing the Battery since we need it to operate the Fans and test the TFS. & FCM. The Troubleshooting Guides Lines from above noted members have been invaluable. Now to determine which component is defective. Difficult step is gaining access to test the components. Will update as we progress.

 

[noob_tl]

The resistors in the fans for low speed are failure prone, it's easy enough to test once you get access to the connectors. I thought I was able to get to the connector from above to test, but to change the fans out I put the lock carrier in service position so there was much more room. It's a little more involved on the NB since you remove the fenders and bumper cover as a unit, need two people to lift it off since it's a big flexible plastic piece. It sounds like a lot of work, but it really isn't.

 

[cherrybomb]

Next Step: Raised front of car again and was able to get access to the 3 wire TFS. Unplugged it and jumped 12V terminal to high speed, BOTH (3 wire fan harnesses) fans came on High. Next jumped the 12v terminal to low speed circuit and Neither fan came on.
1. Next we used a fused 12v. feed from battery to the low speed TFS plug terminal and again NOTHING.
Cause- Low Speed Resistor failure in both fans?
The ignition was on (KEKO) with the A/C and Blower fan on also.
2. In the 3 wire fan harness, is the (Brown) a constant ground? When we jumped 12v direct from battery, above note, to the (Low Speed) red/white Small Fan, NOTHING, it should eliminate other components (FCM,etc.)? 3rd wire is (high speed) red/black had to be good since fan ran on high. Hope to use the 1ohm-100w resistor mod others have utilized. Any unforeseen "gotchas" to be aware of?

 

[csstevej]

Yeah both fans have a low side failure.

 

[P2B]

In the 3 wire fan harness, is the (Brown) a constant ground?

Yes. Brown is always ground in a VW.

When we jumped 12v direct from battery, above note, to the (Low Speed) red/white Small Fan, NOTHING, it should eliminate other components (FCM,etc.)?

Correct

3rd wire is (high speed) red/black had to be good since fan ran on high. Hope to use the 1ohm-100w resistor mod others have utilized. Any unforeseen "gotchas" to be aware of?

If you are referring to hacking up the fan wiring harnesses to externalize the low speed resistors, if I was going to that much trouble I would use a PWM circuit (like newer cars) instead - then Murphy's law would probably have the fans fail due to worn brushes anyway

 

[cherrybomb]

Thanks for answering the questions. BTY,, we have no clue how to build a PWM unit, but the Resistor Mod is doable. Care to share a SIMPLE PWM unit build link or provide details? Not versed in electronics. Was 50+ when HDTV in mid 90's

 

[P2B]

As an EE I could design one, but it would need to be potted to withstand under hood conditions and l would use OE connectors so as to enable the harness to be returned to stock - so on reflection the cost savings to be had vs replacing the stock fans doesn't justify the effort. IMO same applies to the external resistor solution.