Thread: Blend Motor Potentiometer repair

First you need to identify and remove the faulty Blend Motor. All info is at RangeRovers.net
Having just removed my RH Temp Blend Motor I would emphasise the need for a ratcheting cross head screw removal tool around 2cm deep as Ron shows at RR.net or get ready to cut through the plastic duct to get at the screws with anything longer if its the right hand temp Potentiometer. I cut the cable to the blend motor and rejoined with crimps on reassembly as Stokholmviews did, Telstra phone cable ones are perfect if you can get hold of 5.
I read about the possibility of changing the Potentiometer at Range Rover P38 Maintenance repair improvements and tips learned by experience by ownership. so full kudos to them for providing the info on the part and pics.
For those like me who have no knowledge of electrical components, I thought a Potentiometer was something mystical however with a bit of reading its quite a simple thing.
The potentiometer in our Blend motors is a 15mm 10k Ohm linear unit
There are 3 terminals on the Potentiometer, 2 on one side are permanently linked through the 10k ohm resistive layer. The 3rd terminal on the opposite side is the wiper terminal. As the wiper is turned it splits the resistance in two in proportion to the position of the wiper, so half way through the turn or wipe there is 5k ohms resistance on each side, at ¾ turn there is 2.5k one side and 7.5 k the other etc. With this change of resistance the hevac unit knows where the blend flap is in relation to its open and closed position and can adjust as necessary.
I presume then that over time as the potentiometer resistive layer wears the total resistance value becomes more and more until the hevac cant recognise the value . My faulty potentiometer had a value of 14k ohms, not much more than spec but the blend motor did not work

The new potentiometer I sourced from an Ebay seller in Australia. He was selling them at 4 for A$3.90 so with postage they cost A$1.65 each The listing was for …..............10K Piher trimpot 15mm top adjustable resistors
Fitting was simply opening the blend motor case, ease the motor and small circuit board out and unsolder the old and resolder the new Potentiometer in place. The New Potentiometer terminals were spaced further apart from the original ones alignment but were very easy to bend once soldered in.

The Potentiometer needs to be positioned near perfectly central so the small gear fits and engages but its easy to adjust to centre when soldered into place and once resembled it is held securely by an axle once the cover is replaced

Reassembly of the unit is straight forward however it is absolutely necessary to get the gears and Potentiometer position correct. Therefore I would suggest place an OHMs meter across the White and grey wires connected to the Potentiometer and turn the pot until it reads the full 10k ohms. Assemble the blend motor without the black Potentiometer gear. Run the motor, Black and brown wires and watch the outside white flap locating dog turn fully clockwise ( see pic ). It is important to reverse the dog slightly by reversing the wires to take out any tension on the gears. Then open the blend motor and without disturbing the gears insert the Black potentiometer gear and reassemble.


Now test your work. Run the motor so the dog turns fully the opposite way to the picture. Ohms across the white grey should be 0 ohms or very low. Run motor the other way fully till it stops and your reading should be 10k ohms

That is it. The only hard part is getting it out of the car and back again.
It took me 3 hours from start to finish and thankfully I now have fully functioning temp control blend motor again all for A$1.65
If you are going to attempt this I would highly recommend reading the info at Range Rover P38 Maintenance repair improvements and tips learned by experience by ownership. first

The Ebay seller is BennyInternational Home
e-mail enquiry@bennyinternational.com
This seller knows that this 15mm 10k ohm Piher make potentiometer fits the P38 blend motor so if you cant find a listing on e-bay e-mail him.
For a better explanation of how a potentiometer works see this Video,
[ame=http://www.youtube.com/watch?v=rUkrpqEmXb8&feature=related]Potentiometers - How They Work, Disassembly and Exploration - YouTube[/ame]

A big thanks to AULRO member Glen Hendry for donating the blend motors I used to help gather info to help me and hopefully others.

I would highly recommend cutting through the Duct covering the blend motor using a rope saw. This makes removal an refit a much easier task. Using the rope saw to cut through the duct takes 30 seconds and the duct can be rejoined using duct tape easily .

Mate it sounds like you go high spec ones anyway

These are the kind of thing I mean

[ame="http://www.amazon.com/Potentiometer-Audio-Solid-Alessandro-High-End/dp/B00E3X5QXQ/ref=sr_1_1/178-6794549-4572469's=industrial&ie=UTF8&qid=1412457906&sr=1-1&keywords=10K+Potentiometer+Specs"]Potentiometer - 10K Audio, 2 Watt MIL. Spec, Solid Shaft, Alessandro High-End: Amazon.com: Industrial & Scientific@@AMEPARAM@@http://ecx.images-amazon.com/images/I/51km%2BzYeI5L.@@AMEPARAM@@51km%2BzYeI5L[/ame]

Interesting that a search on "Mil spec pots" turns up heaps of results from guitar forums, I guess that is a harsh environment

Steve