D2 Hill Descent warning light on all the time

2003 Discovery 2 Adventurer TD5 manual.

A small problem has come up which I can't find enough information to sort by myself so I thought I'd ask you. The HDC amber warning light is on all the time. All three "amigos" come on with ignition, go off after a few seconds and the the HDC comes back on by itself. My Hawkeye gives the "F4023 Gear information not plausible" diagnostic and the individual switches (Low range, Diff Lock, etc.) all read as Off in the live data section. The HDC switch is not engaged and I've tried rattling the H/L shifter around, all to no avail.

It's an MOT failure, I expect, so any help you can offer would be appreciated. Thank you!

BoldreChris said:

My Hawkeye gives the "F4023 Gear information not plausible" diagnostic and the individual switches (Low range, Diff Lock, etc.)

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Was that in relation with the ABS ECU. If yes it should be a glitch as that's a autobox specific fault code... for the HDC on alone according to the diagnostic protocol it's "brake lamp relay - open/short circuit" fault . If you can understand diagrams and you have RAVE i'll tell you more, if not you can try to replace the interior fusebox cos that relay is soldered into the PCB not simply replaceable but there can be a wiring or SLABS issue too which needs complex troubleshooting

sierrafery said:

Was that in relation with the ABS ECU. If yes it should be a glitch as that's a autobox specific fault code... for the HDC on alone according to the diagnostic protocol it's "brake lamp relay - open/short circuit" fault . If you can understand diagrams and you have RAVE i'll tell you more, if not you can try to replace the interior fusebox cos that relay is soldered into the PCB not simply replaceable but there can be a wiring or SLABS issue too which needs complex troubleshooting

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Thank you, @sierrafery, I don't have RAVE but I'm sure I can get it. Yes, it was from the SLABS ECU. I wonder if it thinks it's got an automatic gearbox and that's what's confusing it - not sure if that's a setting in SLABS but I could check. I guess the HDC function drives the brakes through a relay - perhaps the one you mention on the fuseboard? If that's faulty, I can solder a new one in. Any idea which relay or is it identified by RAVE ?

The relay is not well described nor clearely pointed in any document, it appears as "HDC relay" in some diagrams/descriptions or ''brake lamps relay" in others and there are more relays in the fusebox, actually it's there to activate the brake lamps while HDC is active. I dont konw which one it is cos when a friend of mine had this issue he had a spare fusebox so replaced that which cured the fault. That fusebox used to suffer from water ingress and develop all kind of faults.

Well, I have cleared the warning light though I have to say, it's not clear how! I checked all the switches and gear sensors on the "Live Data" display - all fine - and then looked for a way to tell it that it was a manual gearbox, not auto. No such option appeared and the light was still on. Checked that the brake lights work - they do - still on. So, the only programming possible on the Hawkeye for the SLABS is to set the ride heights by calibration. So I played with that - still on. So I configured the SLABS to think it had coils not air bags and then changed that back. That forced me to calibrate again - just by eye, no blocks available - and after that, lo and behold, it's gone.

Explain that! I can't. Ain't life wonderful? Live it!

It might be a coincidence and the fusebox dried out or something...it's absolutely no relation between your actions and that warning

I am also having a similar problem on my 2004 V8 with an automatic gearbox. I was checking all the connectors on the SLABS module to make sure all were firmly connected while cleaning leaves out of the heater fan. They were but then I noticed a wire with electical tape on the end right by one of the connectors. I looked it up in RAVE electrical library and circuit diagrams. The connector is C0504. I unplugged it and checked it agaist the connector details in the manual. I unwrapped the electrical tape and the wire(which was green with a brown stripe) still had the connector on the end. Per the connector diagram, this wire is supposed to be position 7 on C0504. The schematic shows this wire going to ECU-SELF-LEVELLING/ABS (SLABS) (D163) and then to header 0287 and HDC relay. I re-installed the wire into the connector and plugged it back in. All was fine when I started the truck and went forward. I put it in reverse and still fine until I actually moved the car in reverse. Then the HDC failure light (amber) came and stayed on. I turned off the ignition and restarted the truck. Again no lights on even after putting it in "D" and driving forward. And again putting it in "R" caused no lights but then driving in reverse made the HDC failure light come on. I guess that explains why the wire was removed by a previous owner. I would really like to fix this the proper way. Any thought on how to diagnose and repair this issue?

First of all what was the problem which made you check the connections at the SLABS? and are the reverse and brake lights working ?...if not check fuse F25... also do you see all of the 3 amigos on ignition self test ?

JohnHZ3 said:

The schematic shows this wire going to ECU-SELF-LEVELLING/ABS (SLABS) (D163) and then to header 0287 and HDC relay.

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check again and you'll see that the GN wire goes from C0504-7 to the reverse light switch and from 0287 to others which need reverse gear input nothing to do with HDC relay

sierrafery said:

It might be a coincidence and the fusebox dried out or something...it's absolutely no relation between your actions and that warning

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No sign of any staining or dampness in there and we've had no rain here for months now. I couldn't see a connection either. Still off after 10 miles today.

We can put this one too on the list with LR oddities then ... though if there was no fault code stored it's possible that it was just a bad contact on the HDC warning lamp's circuit

sierrafery said:

First of all what was the problem which made you check the connections at the SLABS? and are the reverse and brake lights working ?...if not check fuse F25... also do you see all of the 3 amigos on ignition self test ?

check again and you'll see that the GN wire goes from C0504-7 to the reverse light switch and from 0287 to others which need reverse gear input nothing to do with HDC relay

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I decided to check the connections on the SLABS after reading about someone having a loose connector. I wasn't having any issues, just decided to check since I already had the kick panel off. All of the three amigos are on during the ignition self test but go off after starting the truck as they should. The reverse and brake lights are working properly(just checked). Any other ideas?

So confirm that the amber HDC comes on alone only when you select reverse ?...any fault code stored in the SLABS ECU ?

sierrafery said:

So confirm that the amber HDC comes on alone only when you select reverse ?...any fault code stored in the SLABS ECU ?

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The only fault was 4023 Gear Information not plausible (other than a few engine misfire codes ). The truck does have 4.37 gears. I checked again and the light comes on after putting it in R. Maybe the ECU is bad?

Try to measure voltage on that wire which was unpinned when you select reverse, see if you get 12v on it then or not

sierrafery said:

Try to measure voltage on that wire which was unpinned when you select reverse, see if you get 12v on it then or not

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I will check it tomorrow. I appreciate your continued help!

It's a strange thing on your's cos normally the HDC warning alone has to do with the HDC/brake lamps relay circuit but as you describe the behaviour i'm thinking that the SLABS ECU doesnt get the reverse input on that wire but it gets the selected gear input on the data link from the engine ECU so it is a missmatch between inputs hence it triggers a warning and LR didnt describe that particular case inth diagnostic protocol...we'll see

Ah, yes, that would make the diagnostic text tie up with the symptom. Maybe what I did, swapping from bags to coils and back again actually forces a hard reset of the controller and re-establishes the link to the ECU. If anyone has the source code for the SLABS controller, I'd be happy to go through it and find out. Surely someone has managed to sneak a copy out

BoldreChris said:

Ah, yes, that would make the diagnostic text tie up with the symptom. Maybe what I did, swapping from bags to coils and back again...

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IMO it doesnt make any sense cos according to the diagnostic protocol decribed by LR the HDC warning alone is exclusively for the brake management side of the SLABS ECU when it gets open or short circuit input on the HDC relay coil's circuit it has nothing to do with the SLS side of the ECU nor with the reverse input but as we know LR works in mysterious ways.

sierrafery said:

Try to measure voltage on that wire which was unpinned when you select reverse, see if you get 12v on it then or not

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Got home from work in time to check this out. The wire does get 12 V when reverse is selected. The wire reads 0V on all transmission selector positions. I'm assuming there should not be 12V on this wire unless there is some kind of probelem? Hopefully this points to a fixable issue.

It is supposed to get 12V on reverse cos that wire is connected to the reverse light circuit so that's ruled out... did you drive it only forward since and you can confirm that the HDC warning doesnt come on that way too ? at this point i can't find any logical explanation why this happens only on reverse and if the wire is unpinned it doesnt happen unless it's an ECU misbehaviour