Ok, so I have a weird one for you...

Working on replacing some potentiometers in some blend motors, so I can send them off to someone who wants a set. Replaced the pots, and set them up, checked resistance across the range of travel, and all good - nice and smooth end to end.

Hooked them up to a HEVAC controller to make sure that it was driving them properly and they would calibrate, and that's where the fun started. Distribution motor would calibrate, but the temp blend ones fail and then chuck up an error, saying the stall position is wrong. Look in the live values, and their stuck at a max of 75/76.

Figure OK, something must be up with these pots, even though they are Piher NV15's 10K ohm, as mentioned many places. Decided I'd swap them for an older set of (new) pots, which have a different centre - so needed to grind them out for the thumbwheel gears to fit in.. No trouble, I've done it on a set before, and they've worked fine.

Put the other set of pots in and the same thing happens... tried with 2 different heater boxes that I've reconditioned - so no sticky flaps etc. and a couple of different HEVAC controllers - same deal...

Here's the rub... I've got an older set of blend motors, with original (working) pots... put them on the same HEVAC controller, and heater box, and they will calibrate fine... sit them next to the new ones, and measure resistance range vs resistance on pot, and pretty much the same.. even if I tweak the new ones, so they are the same, then no difference when trying to calibrate.

I've tried swapping one of the motors from old-new in case there was something up with that which was causing the issue to do with stalling etc, and again no change.

I'm really at a loss now - they are pretty much, for all intents and purposes 2 sets of working (when checked on a multimeter etc) blend motors... any ideas on why set, on the same heater box/HEVAC controller won't calibrate the end points on the blend motors? I know they only use about 50% of the actual motor range, but seems happy enough on the other set of motors, with original pots...

I've now spend 2 days pissing about with these things, and not actually any closer to a solution...

They're not my old ones are they?

Actually, you know what... they are...

The drivers side one has a bit of a fault in the motor I think, as I swapped that for one of the other ones in the tested set I have and the motor runs properly... bit swapping the pots out, and just doesn't want to know.

Going to try measuring the resistance or the actual wiring, and may even the current the motors are drawing to see how it compares with the other set.

Something tells me I'll still be stumped tomorrow...

Only thing I can think of, and I'm sure you've checked, but are the two outputs from the pot the right way round? Is it possible the pots you picked up are 'backwards' in this regard somehow?

For test purposes, as the motor in the distribution position calibrates OK, have you swapped around the motors and tried that as a blend motor and tried the blend motors in the distribution position?
I did post the test currents that I managed to achieve when I did my overhaul somewhere (in my heater O rings thread?) which might be useful to you for reference purposes. Don't think I recorded stall current though.
EDIT- blend motor currents here

:( now I feel guilty...

Don't be - it's obviously something strange going on, or the tolerance on the pots that I've got is too big, and thus they are causing the HEVAC to not see enough of a swing on the input to let it calibrate.

Today I tried a few more things, including trying to swap one of the temp blend motors to the distribution motor position, and that motor still wouldn't calibrate properly.

I took some voltage/resistance reading at each end stop compared to the one that did work properly, and there is about 1K Ohm difference between the old potentiometer, and the new one at one end of the travel - which at that kind of range is a fair amount, since the actual output rotation is just over probably 90-120 degrees.

So, I don't quite know what the next thing to try is...

Not sure what type of pots we use there Marty (and you may have to dismantle one to find out for sure) but if they are the carbon track type in particular they can develop 'flat' spots (ie. in/on the area/track they are used the most)...

When you are checking the resistance across the range of the pot (ie. to ensure it is a smooth/steady change in resistance) it may be quite hard to spot a flat with a digital multimeter: Try an analogue meter instead; Not saying it will tell you for sure if the pots are iffy but it can't hurt to try.

(I have encountered this issue myself checking the likes of flappy air flow meters, fuel tank senders etc; ie. Seems 'OK' with a digital meter, only see the flat/worn tracks for sure with an analogue one. It can happen with wirewound pots too but depends on their construction).

Finally are these pots. linear ones ? - The other issue I have noted is folks sometimes test/replace LINEAR ones with LOG ones of the same value...

Yes, they are linear pots, they seem to be the only type that people sell. If you want a log pot in this style, then they seem to be special order from the manufacture.

The pots I have that are playing up are brand new Piher PT15 NV15 10K ohm. They are carbon track, yes, but both versions I have are brand new, not pre used ones in trying to test.

The old ones in the other set of blend motors (that will calibrate) are the originals and test fine on analogue/digital multimeter.

I'll check the new ones on my analogue meter too, but I think it would be strange that at least 4 brand new pots from 2 different batches are faulty...

Interesting - but excuse me Marty while I ask some more stupid questions (but which I have been known to do before...)

Have you used these same Piher pots successfully before ? Are you sure they have the same mechanical/electrical characteristics as the originals ? (eg. a 240 degrees turn is max. to min. value etc) Definitely LINEAR and not LOG or ANALOG ?

http://www.piher-nacesa.com/pdf/14-PT15v03.pdf

'Trimmer Pots' are just that (in that they allow you - manually- to make small(ish) changes to a circuit) rather than being mechanically linked (eg. to a flap etc) for 'circuit feedback/servo' purposes and so whatever the replacements are they will need to be identical to work properly.

Yes, I have used one set of the Piher pots before (which also aren't working properly in this set of blend motors now - I had bought new ones which had the hole for the thumbwheel/gear, whereas the older version I had needed some modification for the thumwheel to fit, but it's both sets that aren't behaving now.

Piher trimpots are the type used in the original blend motors aswell - so these are replacements that are supposed to fit. The difference on the ones with the thumbwheel slot already there is that the wiper is offset from the RR ones - so instead of lining the marks on the gears up, you centre the trimpot and then fit the big gear in it's centre position to match.

I have that datasheet on the trimpots, and they even show a circuit in there about using them as a feedback circuit. My next thing I'm going to look at is the board in the HEVAC controller, but i'm fairly sure it must be something to do with the blend motors, as it's doing it on 3 different controllers, from various years/part numbers.

Ah...ok. if these same pots. have worked OK before and they are identical to the originals then it must be something else then !

-You may be surprised though how if suppliers think you are just using them for 'something fairly basic' they may well often supply you with what is convenient for them - and 'presumed' to be an equivalent (eg. CW pots rather than CCW pots which does not matter that much for most circuits but will obviously not work in a positional servo situation etc)