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Joined: Apr 23 2019
Posts: 387

I have the dash off my RR.
Q1. The blend and distribution motors were 90% ok before I started (periodic service book symbol), but anyway, now that I have everything out the HEVAC is showing me “Feedback circuit on RH blend motor faulty” (or some such – reporting via Nanocom).
I'm not an electronics expert, but possibly know enough to be dangerous. I can move the motors back and forth with a 9V battery. I took the 10k ohm pot off the board of the RH blend motor (ie desoldered it), and it goes all the way from 0 to 10k ohms (measured on multi-meter), but when I soldered it back to the board it goes from 0 to 3k ohms mid-range, and back to zero again (measured at the circuit board) - looking at the ETM there is a dotted line going from the pot to the motor, so maybe something is messing with the way the DMM measures resistance. Thing is all 3 blend motors seem to be like that (I didn't remove the pots from all 3 though!!). I’m just measuring this on a Uni_T multi-meter 20k ohm range – the fact that all 3 are the same may just mean my meter’s getting it wrong. Or maybe my test is wrong (just measured between the white/ wiper and one of the reference legs of the pot)?
Anyway – if the feedback error is not due to a dodgy pot then what else ? wiring? I think I may scrap the RH blend motor anyway, as the case has been cooked by a hot heater matrix manifold at some point (although the innards look fine ) but the principle is so simple that it then annoys me I can’t crack it !! As future-proofing I'm considering just installing 3 brand new ones at this point - just because accessing the blend motors is such a PITA.
Q2. Has anyone had any luck freeing the Distribution mechanism - I've greased all the pivots i can see, but it's still quite stiff - the gearing in the blend motor shifts it without complaint though. There seem to be two black toothed wheels driven by the main grey one, how do the black wheels come off ? Pull hard? I found a link to someone who had dismantled the whole heater box to do this, which i'm not very keen on.

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Joined: Dec 30 2015
Posts: 4133

The dotted line shown in the ETM diagram just signifies that the motor moves the wiper. I've repaired the feedback circuit errors simply by a squirt of switch cleaner inside the pot and working it back and forth before putting the gears back. Any slight dead or dirty spot on the track will cause an error.

Just checked a spare I have here. Had to lift the board out of the housing slightly to get a probe onto the wiper pin but measuring between the wiper (the terminal lurking under the rounded side of the pot) and the terminal on the end of the track nearest the motor, I get between 26 Ohms with the pot fully one way and 10k at the far end of the travel. Don't forget that when in use it doesn't use all of the travel from one end to the other. Make sure you get the arrows on the gears lined up when you put it back together too.

Distribution flap usually feels a bit stiffer than you expect but the blend motors can generate quite a bit of torque. The motor itself drives a worm gear and is then further geared down so can move a lot more than you'd think from such a tiny little motor.

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Joined: Apr 23 2019
Posts: 387

Thanks for looking Richard, I'll try with a different meter.

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Joined: Feb 25 2020
Posts: 42

A note about "new" blend motors - which I am sure most of you know already.
The original Valeo (French ones) have the brand like "etched" in the plastic (wrong word, don't know the right one), whereas the new "original" Valeo ones are Chinese and the brand is just a label glued on top.
The quality differs, and they are not as durable as the others were ... oh well, that's normal for most parts nowadays.
Also, open them up anyway even if new to check them out, I have a couple of cases where the gears were not aligned ...

Member
Joined: Apr 23 2019
Posts: 387

Replaced battery on my DMM and suddenly the range tests ok, moving the motor with 9V - anything from about 2k to 9k ohms resistance (motors fitted, so limited by the movement of the flaps). I also redid a pin on the LH blend motor and replaced the connector block. Now all is fine, no service book symbol.

I've got the new ones coming anyway, so I may just put those on to test them and see what resistance values they give. Thanks for the tip Leo, if I'm fitting them I'll open the new ones up for a quick check first. This UK house-arrest thing has put a damper on my P38 work as I'm now home-schooling two kids.