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Discussion Starter #1
Hi, I removed the HEVAC display unit to replace two bulbs, the ones for the temperature control switches. The display was working before I took out the unit, but the display has stopped working when I put it back in and connected the plugs.

I then removed the panel again, and took it apart completely, to see if there was anything burned out but everything looks fine. I put it back together, reconnected it but still nothing.

All of the switches and controls work, and, although I can't see the display, the air con is working OK as I tried yesterday when it was very warm.

Would anyone have an idea what could have happened ? I read a lot of the threads about missing pixels and replacing the ribbon cable but on my P38 there seems to newer type of ribbon cable connector. Mine is June 2001.

I am not sure that replacing the ribbon cable will help because the display was working properly until it was unplugged, and now everything has disappeared.

Any ideas would be appreciated, even the idea that I should buy a second, working, unit.

Pierre3.
 

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It sounds like your ribbon cable glue has come undone. MartyNZ makes an upgraded zebra strip to repair it. I think his site is P38shoponline.com
Install seem quite easy as there is a you tube video to show you how.



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it could also be as simple as the back light bulb for the display.
Does the screen light up, but no pixels, or is it dark? If dark, change bulb.
Bumping an old filament around can break it.....
 

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Discussion Starter #5
Thanks to you guys, for the information.

Bolt - there is only a blank, black, screen at the moment. I have changed the bulb twice, but again I don't guarantee the reliability of those bulbs either, even though they came from a motor factor. I think that I will see if I can buy more bulbs locally [Dublin]. If not I will order from Amazon or Ebay.

ColoradoCrow [and Richard_G] - I am hoping that your suggestion about the ribbon cable is the issue. I will look at MartyNZ's website and probably order the parts.

Could it be that when I was pulling off the plugs, from the back of the HEVAC unit, that the ribbon cable came loose ? I would like to think that that is the answer, but I suppose that I just find hard to think that pulling out the plugs could dislodge the ribbon cable. However, there are a lot of threads referring to exactly that.

Thanks again, I will look for Marty's website.

One small, completely irrelevant, aside - I bought one of those key fob repair kits, two batteries, a rubber front piece and the battery back.

It didn't work. When I looked at my key I could see that there was nothing to press down by the rubber front piece, I currently use a pencil to press down on the very small switches. I think that there should be another small switch cap between the rubber front and the pcb. So that bit of new kit was no good.

Then I changed the batteries [after a huge fight to get the back off - now it has a big hole in the middle ], and nothing happened. It was only after I had replaced the original batteries and got the key fob working again that I happened to check the battery packaging. The battery "use-by-date" was 2017 !!!!

Pierre3.
 

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shine a torch through the display. Ideally behind via the bulb hole, but can work from the front. If you can read all pixels, then the ribbon is ok.

Bulb check is easy . .. . connect it to 12V battery with wires
 

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Discussion Starter #7
Hi, thanks for the good suggestion, I will try that first thing in the morning.

Pierre3.
 

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+1 on the torch through the back trick! Simple, and effective!
A blank black screen is not being back lit......
If the unit is installed, a bright torch shone at an angle at the front of the screen should show up the display well enough to confirm things are working.
 

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Discussion Starter #9
Hi, I tried the light through the back of the display and I could see the figures, poorly, in the screen. They were not very bright even though I used a small LED torch with a very bright light.

I put in a new bulb and the display has come back on but it is very dull. It is hard to see in normal daylight, and even at night, with the lights on, it si difficult to see. It is as if the bulb needs to be a bigger wattage. I checked the back of the bulb and it says it is 2 watts, which as far as I am aware, is the correct wattage for the display.

Can anyone suggest what may be happening ? Should the bulb be 3 or 4 watts ? Could it just be the display screen ? I don't think it is the screen itself because before I took out the whole unit to change bulbs that had gone out the display had worked properly.

So, perhaps someone might know what is happening. I can see that the usual icon of an open book is showing, now that I can see the display so that will be the next issue to fix !!

Pierre3.
 

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As you have the thing apart, double check that all of the pins in the various plugs are clean, as in no corrosion......Also, as you have shifted wires around, be sure that you have not dislodged a spliced earth wire as I seem to recall, the HVAC is fed from a spliced earth lead, trace it back to be sure. Also, clean and re tighten the earth point under the HVAC as it is now easy to reach.
Are you positive the shop has not sold you a 24 volt bulb? I experienced this once in Oz.......Can you test it outside of the controller?
 

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Discussion Starter #11
Hi, I used an electrical cleaning spray on all the plugs and sockets. To be honest, they did look OK, but I thought that I would give them a clean anyway.

I will have a look at the earth connection as suggested.

I will also order a couple of new bulbs from a different parts shop, just in case, as you suggest, the bulbs that I have are 24v. It is a possibility. I know that of 3 bulbs that I have one of them is certainly 12v 2w. However, I can read the other two.

Thanks for the suggestions, I will have another go at this tomorrow.

Pierre3.
 

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Discussion Starter #13
Richard - thanks for the link. I have ordered the set.

I was talking to someone today, and he suggested checking the main display dimming wire going into the right-hand side plug, on the back of the HEVAC unit. I will stick a meter on the connection, if I can get the probe into the socket, and see if, and what, voltage is present.

I am assuming that if I get any sort of voltage then it is working, but I am not sure how the dimming works. Is it the same as a rheostat, reducing the voltage from 12 volts to , say, 5 volts ?

Pierre3.
 

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If you got a reply from an eBay seller, in relation to replacement HEVAC units, then it was me, as I replied to someone about a unit with the same kind of issue as you have.

The dimming wire from the instrument cluster to the HEVAC is (from memory) a PWM wire, and it is pulled DOWN to ground. it uses the pulses to drive a transistor on the HEVAC board, which then effectively dims the backlight lamp. So in theory, measuring between 12V and that wire should produce a voltage - measuring between that wire and ground will show very little, if anything.

Also worth checking the dimmer switch on the indicator stalk, in case it's been knocked and turned the dimmer down, and that's causing the problem - but the HEVAC display should never go fully out, so you have something weird going on alright!

I'll try and have a look at the specs for the dimmer wire tomorrow

Marty
 

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I replaced my ribbon cable on the weekend with one of the rubberised zebra connectors and had a black screen - took it apart and put it back together 4 times before I realised in my case it was a dodgy bulb connection. Found it was this by swapping the white bulb holder behind the display for one of the black ones behind the program button light that I knew was working. My simple 30 mins job turned into a 3 and a quarter hours PITA one.
 

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The last time I had my HVAC unit out I just soldered white 12volt LEDs directly to the board and used 2 for the backlit LCD screen no more issues and no more bulb changing. Just a thought. It was quite easy to do and didn’t take long once the unit was out.


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Discussion Starter #17
Hi Marty, thanks very much for your post. It was indeed myself who contacted you through Ebay, and you did send a reply via Ebay.

I have not had a chance to try the suggestions that you made, but I hope to do so before the weekend.

Pierre3.
 

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Discussion Starter #18
Hi Tats, where did you get the rubberised zebra connector from ? It may be worth putting a spare one in the spares draw, in the garage.

Pierre3.
 

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Discussion Starter #20
OK, thanks for that. I will look it up. I have bought from LandyFrance before, so maybe they have them. Otherwise I will go with the one Marty sells.

Pierre3.
 
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