Some tips for a quick(er) repair of the heater control unit

[CharlesStevenson]

On Sunday afternoon I fixed the intermittent heater temperature control on my car & it took about an hour including removal and refitting.

I'd been finding it would either be either too cold or too hot and sometimes by hitting the button, or turning it left-right very quickly, it would adjust but only to max or min levels.

That being so, here are a few tips to make this job easy for you if you have to do it:


Firstly, I found that as opposed to some other posts I'd read, I didn't need to remove the radio. Just pop off the heater fascia, then remove the 4 screws that hold the air vents above and radio panel below. You can then push the unit backwards, turn it 90 degrees so the dials are facing the LEFT and detach the smaller wiring loom. Then wiggle it forwards, keeping it to the LEFT of the hole, and detach the bigger loom. Removing the screws means you can push the panels above and below the unit enough to wiggle the unit out.


Secondly, to refit the repaired unit what I found best was to firstly push out the two buttons / blanks to the left & right of the little oddment slot above the radio, then remove the black buttons from the heater unit to stop them falling off behind the radio during re-fitting (no guesses for why I've added this comment!). You can then push the unit back in by reversing the above steps, ie with the back facing the RIGHT attach the big loom then push it in further and attach the smaller one. Then line it up with the 2 white plastic pins / lugs, one either side. You'll need to put your fingers in through either / both of the two holes where those switches were, and wiggle it so the third "lug" at the top centre of the unit slots into a hole in the underside of the air vents above it. Then line up the 4 screw holes and re-fit and finally pop the fascia back on.


Thirdly, I'd already purchased some de-soldering braid wick wire (£1.68) and a solder sucker tool (£2.00) to help with replacing a headlight relay on my light module and both came in handy for this job too as they make removing the solder and cleaning the holes a much easier task.


Lastly, I'd seen a post which mentioned it was difficult to get the horizontal pots and only vertical ones were available, but eBbay do sell the correct potentiometers, 5K ohm Piher PT15 horizontal potentiometer. I got 2 for £2.50 inc p&p

http://www.ebay.co.uk/itm/Piher-PT15...72.m2749.l2649

For the techy amongst us I've attached the spec sheet for the item.

Might help someone else

[sarumboy]

Thanks for those tips, very useful, especially as I have the exact same fault on my heater,(phase 2 V40 ES). It is either too hot or too cold regardless of where the temp. knob is set.
Just to confirm, does the temperature control knob actuate the Piher pot directly and did it cure the fault ok?
Thanks again, Sarumboy.

[CharlesStevenson]

Hi sarumboy,

I've been running with the replaced pot for the temp control for about a week now and have found that yes, it does seem to have 'cured' the problem, however, it can be slow to change from hot to cold.

Sometimes it'll adjust in seconds, othertimes it takes a few mins, but it does change.

Perhaps my soldering wasn't as good as I thought and I might pull the unit and re-solder it.

So possibly something else's causing me this problem, but compared to the two options of freezing cold or boiling hot of old, I'm happy to wait a few mins for the temperature to change to something in the mid-range.

.. and yes, when you pull the unit you'll be able to remove the plastic spindles from it's front and the Piher pots are the same design for both the fan speed control and the temperature.

Just remember to set the temp to full left or full right before pulling it off the front and align the new pot in exactly the same position before you solder it back in or the knob will be out of alignment with the blue-red colouring.

If yours is playing up as you describe, as a short-term fix, what I did for mine was to remove the front panel so that I could get more 'wiggle' on the temperature knob and then I could rotate it left-right-left etc with more movement.

It used to (sometimes) work for me to let the temperature change until I swapped the pot.

[sarumboy]

Hi Charles -
Thank you for the detailed reply, good news that it is at least working again albeit slow to change temperature. The one thing that I thought of, although you said to position the knob at one end or the other before dismantling, is that the control knob can rotate 180 degrees but the pot is capable of rotating 265 degrees. Is it possible for the sweep to go from 0 ohms to say, 3.5K ohms or at the other end, from 1.5K ohms to 5K ohms. I wonder whether that would make a difference.
When my new pots arrive, I'll take the cover off and take a look-see.
Thanks again,
Sarumboy

[CharlesStevenson]

Yes, it's harder to put into words but is simple when you see the actual items.

There are effectively 3 bits to the front dial / knob and all can be pulled out / off the unit when it's removed on your desk, you don't have to remove the black dials in advance, that's just me being extra cautious when writing out the steps.

The 3 parts are:-
* The black knob
* The spindle that goes through the knob into the centre of the pot to rotate it, and
* A white clip in 'thingy' that holds the knob in place - the 3 are 'keyed' so you can't put one in the wrong space.

All three of those have a full 360 deg range of motion, but the pot is limited to less, so, providing you set the heater control to, say cold before removing the knob & then the board, all you'll need to do is to set the new pot to the same position and it'll line up ok - even if you set it wrong you could, perhaps just refit the black knob in a different position (I don't know if the knob is 'keyed' to stop this, sorry.

Because the new pots are direct replacements they have the same limits on range of motion as what you've removed so all should be ok.

There - I said it's wordy, and yes, once you've got the unit out and the board removed it'll all become clear.

I'm still finding the temperature is slow to respond, but it does & I recollect reading somewhere on here that somebody mentioned the units 'might' be calibrated when first assembled, possibly due to the tolerances that these pots are made to. That being so, when you've done the job you too might find the temp changes slowly, but you should at least be able to control it more than you can at the moment.

One last comment - if you've not got the 2 tools to help with removing the solder I'd get them. It's going to be much easier than trying to heat up the solder and 'wiggle' the 3 legs out one at a time.

[sarumboy]

I've delayed in replying until the new pots arrived and I've now done the job. But thank you so much for the detailed notes which helped understand what to expect having taken the unit out.
Instead of removing the old pot by 'wiggling' all three legs out, I chopped the longer legs off then could deal with one connection at a time. Then with a solder sucker and small PCB drill cleaned out the holes making replacing the pot straightforward.
The net result is that it's now behaving exactly like yours, taking about 30 seconds before any change in temp is noticed but it is changing across the range as it should other than that. My guess is there might have been a way at manufacture to calibrate the unit for the actual resistance of the pot.
But for your original post, I would still have a heater that's either too hot or too cold!
Thanks again.

[CharlesStevenson]

Glad it helped - and you've saved yourself a few quid too - always good when Xmas is getting nearer!

... and yes, in some of the posts I'd read that got me ready to do the job myself there were comments about the board being calibrated to reflect slight differences in the pot.