I have a couple of issues with my HEVAC that I've yet to work out.

The one I'm stumped on currently is the LCD backlight is dead - it's not the bulb, I've checked and replaced it. I've checked all the connectors are properly seated and reconnected them just in case. Also checked the plug to the instrument cluster, and that's fine too.

Someone has replaced the original 97-98 unit with a 2001 part at some point. I thought the A/C compressor and software were the only real changes between earlier and later units? Is there anything that might explain my lack of light?

My car is a bit odd, but every other date stamp I've come across is either 97 or 98 so far.

C244 (up to 99MY)
C0792 (from 99MY) 8 pin white
1 Ignition switch position I power
2 Batter power
3 Ignition switch position II power
4 Ground
5 Left heated windshield relay control
6 Control panel illumination
7 Compressor clutch control
8 Condensor fan motor control

Put a Volt tester on pin 6 and check with lights on and off. If pin 6 is getting signal the problem is in the HEVAC unit itself.

From memory the HEVAC LCD lamp uses an inverse PWM signal from the instrument cluster to control the brightness when the side lights are turned on... but that being said it should be on all the time in some degree so that you can see what's going on the display (or at least see how many dead pixels there are!!)

If it's not working at all then it could be the driver transistor (as I'm sure it probably has one!) has gone faulty or there is a bad connection somewhere.

I'm planning on playing with a couple of HEVACs over the weekend as I have a LCD Pixel repair kit that I've bought and want to try out. If it works, then I'm going to see about getting a load of them in bulk and hopefully add that to the list of bits that I repair/rebuild...

But whilst I'm in there, I will see if I can come across anything that might cause the backlight not to illuminate at all.

Marty

Thanks guys,

I have the unit from the old car I could try - that one certainly works. Failing that I'll break out the multimeter again.

I can just about make out that most if not all of the segments work in the LCD if I shine a light on it.

On the up side, since resetting the faults, half of the heated windscreen sort of works! Quite a few lines not working but hey, better than the last one. Hopefully the other side has a wiring issue and isn't completely dead.

I haven't forgotten about looking into this... been caught up today with a few other things and been out this afternoon giving Snog a bath (though I missed a few bits by the look of it, but it's so hard to see where you've brushed on black paint when it's wet!

Also now got 2 of my 3 keys... only taken about 10 weeks... but they reckon the 3rd one is due in a couple of days which is nice... A bit of a mental relief to have a spare key for my one now!

I'll be popping back down to the garage a bit later to try out this HEVAC screen fix so will have a poke about on the board then and see if I can find out how the backlight is driven.

Marty

Well, Have had a bit of time today to have a look at my HEVAC screen fix, and the good news is that it works a treat.

The bad news is that the supplier I got this from on eBay sells them from France @ 20 Euro a pop.. which to me is a little bit steep for what is probably a few quid's worth of connector. So I've going to see if I can go direct to China or somewhere like that and get a bunch of them made, so I can then hopefully either offer them cheaper (for the DIY'ers out there) or offer a HEVAC screen repair service to tack onto the other bits I do...

You can see all the pixels now.. it probably has logged nearly every error under the sun as I was powering it up on the bench with none of the motors/sensors connected...

So then I started looking into the backlight and have found the positive for the backlight goes off to a small transistor on the board - it's T005 in the picture below

I am still working on tracing it from there back, but I would say it's the first place to go looking if the backlight isn't powering at all.

I'm going to look up the part number for it and also availability on replacement units. I'll let you know when I've found where it traces back to and if there are any other possible failure points.

Marty

Edit - just to add that tracing the transistor wiring... The feed comes in on pin 11 on the big black connector on the main PCB as an inverse PWM signal from the instrument cluster. This then ends up on one side of D020, going through that to R047, which is 6.8K ohms. The other side of R047 goes to one side of R046 (10K ohms) and to the bottom/left pin of the transistor T005. Bottom/right pin appears to be the power and top tab + bottom/centre pin are joined and are the output drive to the lamp itself.

T005 appears to work out as an Infineon BCX69-25 small power transistor.

That looks good with the possible LCD fix!

Thanks for looking into the backlight so in depth!

I need to try my spare unit to see whether the issue is within the instrument cluster (or cabling between it I guess too) and the HEVAC unit itself. At least there doesn't seem to be much involved inside the unit itself if so. I'd like to keep the 2001 unit as I believe the later software in it sorts out an issue with hot restarts where the HEVAC would blow hot even if it was a hot day.. or something like that?

No problem - I had it all in bits anyway, so didn't take me long to trace it down.

Wiring... I think you would be pretty unlucky if it's a wiring break, as from memory there aren't any additional connectors between the cluster and the HEVAC on the dimmer line for the LCD as it's all in the fascia loom. But it doesn't mean a pin hasn't backed out of either of the connectors or something simple like that.

I can't fully remember the 'hot start fix' but something tickles in the back of my mind that this has something to do with the position of the ambient temperature sensor being moved from the inlet of the LH blower housing to down by the front of the radiator. And it was just something along the lines of a short 'time out' period where it ignored the external temp sensor reading to allow for latent heat from the engine affecting the reading.

I'll see if I can find the actual documentation on it... I would imagine the older one would have an issue though with the AC compressor clutch drive - as the earlier HEVAC units would drive the clutch directly, so would expect to have a load on the AC Clutch pin (which is why when you put in the upgrade harness to power the clutch off a battery feed via a relay driven from the HEVAC you need to put a power resistor in as a 'dummy' load so it doesn't throw a fault) - whereas the later models had the extra relay and fuse in the underbonnet fuse box - so I presume the later version of software just had this 'error' condition deleted so they could just drive a relay and not have to put in a big resistor into the factory wiring loom...

You might be able to get away with swapping just the whole front panel and PCB between the 2 units... all the driver components for the backlight are on the front panel PCB and then have a direct link through the ribbon cable to the HEVAC connector to the fascia loom. The chip with the software is on the bottom board with all the driver chips and all that... So I don't see a reason why you can't use the older front panel with the newer main board and have the best of both worlds... And then I'll buy the old faulty one off you to try and repair it and do the LCD fix and all that if/when I get some of these connector strips

Let us know how you get on...
Marty

Well that hopefully means I don't need to go looking for a random connection in the loom between the cluster and the HEVAC unit hiding in the dash somewhere :)

That sounds like what I've read re. the sensor and delay. I'm not sure where the sensor is on mine, but I really must replace the pollen filter on that side - I did the driver's side while investigating what had flooded the carpet and found it was completely full of crap. I guess the other side will be the same... so I will see if there is a sensor in there sometime. I do have the little relay in the fuse box though for the A/C clutch. Doesn't seem to run so I guess it has a leak somewhere. Future problem.

That sounds like an idea swapping the front half over - that sounds like it should make for an easy fix if its the transistor etc.

Currently with the weather ranging from 'cold' to 'very bloody cold' I'm making do with turning the blower up and down. I've no idea what the temperature is set to on either side but it comes out hot so that'll do! :)

The temp sensor location move was pre-'Thor' days, so your ambient temp sensor should be down in front of the radiator, just inside the LH chassis rail (the bracket is bolted using the bolts for the crash can).

A copy/paste from RAVE about the ambient temp sensor: (as it's easier than typing it out!)

Ambient Sensor <br>
This sensor measures the temperature of the
external air flow into the vehicle. <br>
For vehicles with VIN’s up to 381430: <br>
The ambient temperature sensor is located behind the
drivers side of the fascia. <br><br>
For vehicles with VIN’s from 381431: <br>
The ambient temperature sensor is located behind the
LH side of the vehicle in front of the condenser. The
revised location improves the response time of the
sensor. The new HEVAC software will not update the
ambient temperature sensor readings unless the
vehicle is travelling over approximately 15 mph (25
km/h). The HEVAC software will therefore not auto-matically
correct temperature readings when a faulty
ambient temperature sensor is renewed.

That was taken from the '97 edition of the ETM - so I think is somewhere in the '97/'98 MY - as it has an amendment at listed at the start of the ETM dated 02/98 with one of the changes relating to the ambient temp sensor location. So your vehicle should definitely have it moved... I was close on the delay for using the temperature reading - but instead of a timeout, it goes on vehicle speed so it knows it's getting a fresh air supply over it before it uses the value for any calculations.

Re A/C - again Thor models had the updated wiring installed in the vehicle loom with RL10 and F27 used to drive the AC compressor, and the HEVAC wired directly to the relay, via the trinary switch, so if it isn't working then there is either: No gas, dodgy trinary switch, dodgy AC Compressor, or a leak somewhere.

You can unplug and jumper the trinary switch to force the compressor to run - to either diagnose/rule it out and then it comes back to the switch, gas, or leak :)

Marty

A/C is on the list for the warmer weather :)

Something else has my attention now with regards to things heating up and cooling down... time for a different thread for that!

Finally got around to doing something about this. I tried the hevac unit from my old car and it lit up fine, ruling out the instrument cluster and loom between the two. I traced pin 11 back to the diode near the transistor and that was fine, but beyond that I'd run out of soldering skill to change components that tiny. I guess the transistor has failed, but I went about sorting it another way.

Seeing as there is an ignition position 2 switched supply, I took a feed from there, and de-soldered the tabs to the light in the plastic housing from the PCB, and attaching my new feed to them. Used a 2 pin disconnect so the two halves can be separated for future repairs (when the lcd inevitably fails etc) without inducing rage etc.

Innovative fix!

If you want me to take a look at it at some point, then I have SMD soldering equipment that I can use to replace the smaller components...

I haven't updated my side of things on the HEVAC either... I ended up getting a bunch of the LCD connectors made in China (minimum order was 500!!) and have used them in 4 or 5 HEVAC units now, with good results on all of them for fixing faulty pixels.

I have also found a way to light the LCD with white LED's - and can now also do an option of either standard green backlight, or white (for the LCD):

I have also got for testing/fitment checking replacement ball bearings and springs for the rotary fan knob 'clicking' as I've had a couple where either the ball bearing or the spring has gone 'ping' when I've taken the unit apart... so figure it might be handy to be able to supply spares of these at some point!

Marty

It's a little bit of a bodge I guess, but it works. I did try the working unit in the car and dimmed the dash lighting, but it didn't seem to make any noticeable difference to the LCD backlight. Even if it did, I never dim it anyway.

Good to hear the zebra strips worked out! That LED backlighting looks very good too!

Have you ever done anything with regards to backlighting the controls? I have LEDs in all of my other switchgear - along the top row etc and window switch pack, but the HEVAC is comparatively stuck in a very 90s level of dim. So is my instrument cluster - but only because I still haven't moved the lighting part over from the old car.