LDV air suspension

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gsx600

Original Poster:

2,740 posts

247 months

Friday 12th August 2005
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Anyone on here have knowledge of LDV air suspension, what's it supposed to do, how's it operate, adjustment etc

As I have a 3.5 LDV Ambulance that I want to check the suspension is working correctly. Always seems too low at the rear. The air pump cuts in when you start the engine and after a while cuts out with the vehicle lifted higher that it was.
There also seems to be a swwitch on the rhs of dash that I think has something to do with suspension, but not sure what !

Hirich

3,337 posts

261 months

Saturday 13th August 2005
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Assuming it's a fairly old vehicle, I can tell you more than you ever wanted to know...

Principles
The LDV system uses a pump to regulate the amount of air in the springs at a particular time, and is controlled by a height sensor fitted to the rear axle. There are two (and a bit) benefits:
- Ride quality. Remember that a van has to carry a huge weight difference on the rear axle between unladen and laden. The traditional leaf springs are single rate - if correctly set for strength and stiffness (for both ride and handling) when laden, they are way too stiff when unladen. Even rising rate coils or leafs would not totally resolve this. The airsprings can though. Therefore, whatever the rear load, the springing is much closer to optimum. This is particularly useful on ambulances - services were refusing to buy traditionally sprung units because spinal injury patients would be in agony (and at risk of more damage) when crossing potholes. The improvement to ride quality with a small payload is very significant
- Ride Height: By adjusting the volume of air via the pump (which also acts as a relief valve), the rear deck would always be at the same height (and with leaf springs it would vary significantly between laden & unladen). So it achieves load-variable spring stiffness without resorting to huge movement or ride height variations.
- and the bit is: because this was early technology, the system was prone to leaks and low temperatures shrinking the springs. The pump could always solve this.

How It Operates:
When you switch on in the morning, the pump is likely to start. It adds air until the sensor on the axle detects that it is at the right height. The pump should not come on again unless you have a leak, or you throw a load in the back.
Thereafter, the springs are effectively sealed pockets of air. The 'piston' pushes up into the rubber sock, compressing the air inside (F=pressure x area), and acting as a spring. Simple.
And just so you know, the pump only reacts if it gets a consistent signal from the sensor for some 30 seconds. So that's why it isn't coming on all the time.

Adjustment
You describe that it sits lower. To some degree it should (compared to the leaf spring, see above). Broadly, whatever the payload, the floor should be almost flat.
Because your van squats overnight, it sounds like you have a leak, but not a major one. You can probably ignore it. Do check the tubing for fraying, just in case. There could be leaks around the sock edge, but there's not a lot you can do about this (apart from stretching the spring and applying some tyre bead seal).
It could also be where the pipes enter a spring, pump or joint. The bare tube pushes into the joint where it is grabbed, locked and sealed. Trying to pull it apart could cause more damage than it solves. A dab of bead seal might help.
If you ever mess with the air system, do release all pressure. At least jack it up on the chassis. There may be a release valve on the pump.

if you want to adjust the normal running ride height, get under the chassis by the rear axle and find the sensor. It comprises a rotational rheostat (inch and a half across) a lever arm and a drop link to the axle. Adjust only by the drop link (which should be threaded like a steering arm). It should be about pro-rata - an inch in the link is approximately an inch in rear step height. Try to find a specification from LDV, as the floor/step height is quite important and you don't want to be jacking it up unnecessarily high. Having said that, it should not need adjusting, as there is no reason for it to drift from factory settings.

What To Watch:
- On the pump, there is supposed to be a filter on the air inlet. Check it's OK. Crap in the system will be a problem. It's supposed to include a dryer as well, but not such a problem.
- Pump failure would be a pretty major problem, though you could at least drive home slowly. Unlikely
- The Panhard rod takes surprisingly high loads. Check for play in the rod ends.
- Take a close look at the cross member, especially around and outboard of the chassis rails. Some huge loads, so look for cracks (and a good dose of Waxoyl to prevent rust).
- The dampers have to act as rebound stops for the springs (which can't 'do' tension). If you regularly get airborne, they can take quite a mighty thump (which gets worse if the damper starts to fail). So replace leaking dampers even more promptly than usual, and check for bushes and any signs of metal damage (though unlikely).
- If the vehicle ever fails to lift, check the pump is working and the pipes aren't leaking. But if they are Ok, there's another possibility. The air enters the spring though the centre of bump stop (inside the spring). The rubber has channels to allow air to enter even if the vehicle is sitting on the stops, but if it has degraded it is possible that the air cannot get into the spring.

And Finally, What's That Switch For?
Assuming this is the magic switch, it reveals a really neat function. It is supposed to short the axle sensor. This convinces the pump that there is far too much air in the system. The pump dumps the air, the suspension collapses onto the bump stops, and the rear end magically kneels by several inches, improving access significantly (especially if you're carrying some bloke on a stretcher).
Return the switch, the pump realises that it has been suckered and repumps up the springs ready to leave.
You need to find a PCB behind the dashboard, which connects to the sensor and pump. I think it might be on the passenger side. There should be a connector on it (either to your switch, or unused). Even if it isn't the right switch, the connector should still be there and you can jury rig it (all PCBs have the feature). Work out the connections, run a flylead to your switch. Result.

So to summarise the key answers:
Seems Low: It's supposed to be, but you may have a slow leak, especially if it droops overnight. May not be worth worrying about.
Pump comes on then switches off: It's supposed to. The pump isn't supposed to be on much of the time.
Odd switch: Based on the option price, the most exensive switch and flylead ever.
And how do I know so much: Because I bloody designed it! Well, actually a later version that was never used. So I also know what's really wrong with it, and how much better it could have been.

gsx600

Original Poster:

2,740 posts

247 months

Saturday 13th August 2005
quotequote all
Wow thanks for all that info, I knew I could trust pistonheads for the 'knowledge !'

Extremly comprehensive, and now gives me something to work on. You were right it is a failry old LDV (1994 ish)
The pump seems to be ok as it does raise the bed ok everytime you drive, although I still think a bit too low but does seem to be within spec according to the sticker on the van.
Now I know about the dump valve (switch)I will be leaving that alone as I shouldn't have the need to lower quickly
When I measure underneath for sping height I think one side is longer than the other and hence the van sits at a slight lean what would be a cause for that ? Airlock ?
Again many thanks for the info and at least I know the system is functioning as it should all be it with a slight leak over a period of time (worse if we sleep in the back overnight !), a slight lean, and too low for my liking (far too much room between front wheels and arches)

Hirich

3,337 posts

261 months

Saturday 13th August 2005
quotequote all
I think that lean is just one of those things you have to accept. From memory, although there are two pipes from the pump to each spring, there is only the one height control mechanism, near the axle centre line. Of course, this cannot detect lean.
There was consideration of using two height sensors, out by each wheel. But cost considerations prevented that (I can't remember whether the dual sensor design required a different pump). It would be worth looking at the pump to see whether there are a pair of pressure adjusters by the outlet, but I don't remember such a feature.

There were a few assumed reasons for lean:
- Manufacturing tolerance. Always assumed to be in the springs.
- Given that the springs are much softer when the vehicle is unladen, the system is much more sensitive to variations in load and specification left to right. Such loads might include the camber in the road, even a side-mounted spare wheel or fuel tank.
- If it happens overnight (sinks noticeably on one side) then recovers when the pump comes on, then it would be due to a leak. But as previously stated you can probably live with this (unless the pump is coming on frequently)
It also looks worse with the low floor height and lots of long horizontal lines to reference against the ground.
So I would take a nose around for some adjuster on the pump. It might be worth contacting a dealer or LDV Service direct to see whether there is such a thing or whether they have some formal fix.
Alternatively you might consider:
- Loosening and (after a hammer tap and some rocking of the vehicle) retightening the trailing arm front bolts (where it mounts to the chassis). It is possible that you have different pre-load in the bushes, and this might relieve it.
- Shimming one side with a plate above or below the shorter airspring. I wouldn't recommend this as the loads are pretty impressive. Use a high strength steel and if you fit it to the crossmember chamfer the edges to avoid creating a stress concentration in the member. And this would only really work anyway if the lean is consistent through all load conditions.

Hirich

3,337 posts

261 months

Saturday 13th August 2005
quotequote all
And I should also thank you. Having reminded me of one of my bete noirs, I was awake in bed all night remembering all the things that were wrong, and the different designs I considered...

andylebb

3 posts

173 months

Monday 19th October 2009
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thanks for the info, i also have a problem with my ex ambulance suspension. its got a mind of its own, lifts when it feels like it but now it aint lifting. its got that low , its knocking loudly on the chassis. with your help now, i think maybe the sensor on the axle maybe need cleaning/replacing?

HiRich

3,337 posts

261 months

Monday 19th October 2009
quotequote all
Yes, first thought would be the sender unit - check input voltage/earth, and then sender function. Then check pump operation (power in). Finally, there might be a small pcb behind the dash.

andylebb

3 posts

173 months

Saturday 12th December 2009
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hi i still had no joy with the suspension does any one no of a specialist in the wirral area?

andylebb

3 posts

173 months

Saturday 9th January 2010
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ha after all this, ave hooked up a small electric compressor that plugs into the ciggy lighter up directly into the system an it seems 2 work

sharlene

1 posts

170 months

Sunday 31st January 2010
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hi
just bought a ldv 400 ex-aa speclift recovery truck.
there are two buttons at the rear on both sides.
absolutely no idea what there for any ideas would be really grateful
many thanks

shafi khan

1 posts

167 months

Wednesday 14th April 2010
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hi i also have a ldv 400 series (1994) speclift
and again the rear will not lift nor is the button by rhs of the dash working
so with the vehicle on or off the the rear wont lift
an ldv dealer quoted me a whopping £200 to only diagnois the problem so anyone
have any tips for me please

paul gardner

1 posts

161 months

Sunday 10th October 2010
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hi i have a ldv convoy 400 minibus the suspension pump is running but not lifting the suspension ca n u help me before i spend a lot of money on a new pump thanks

drivealot

1 posts

161 months

Wednesday 20th October 2010
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It maybe that the pump is operating correctly but you have a major air leak. You need to get underneath and get dirty I'm afraid and check the system for hissing noises with the pump running. However this maybe well worthwhile should a leak on a pipe be found and can be repaired quite cheaply. Good luck!

Terry34

1 posts

85 months

Friday 17th February 2017
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Can anyone tell me were I can find a pair of half leaf springs for a air suspension ldv 400 minibus 2000 reg. having major problems finding anything anywhere Terry.

Ldv400camper

4 posts

13 months

Tuesday 28th February 2023
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Hi there I have an ldv 400 converted to a camper, I am having trouble with my air suspension one of the bags has some sort of whole of break of the seal, after a lot of searching I’m finding near on impossible to find the part even with the part number from ldv. So I was wondering if I could change the air suspension to more conventional spring suspension. Any thoughts? Thank you

Scrump

21,889 posts

157 months

Tuesday 28th February 2023
quotequote all
Ldv400camper said:
Hi there I have an ldv 400 converted to a camper, I am having trouble with my air suspension one of the bags has some sort of whole of break of the seal, after a lot of searching I’m finding near on impossible to find the part even with the part number from ldv. So I was wondering if I could change the air suspension to more conventional spring suspension. Any thoughts? Thank you
Are you sure the airbags are a standard LDV part? I ask because many vans have aftermarket air bags fitted when they are converted to campers. There are many suppliers of aftermarket airbags, the difficult bit is usually finding one with the right brackets to fit each van. If you already have the airbag brackets and fittings then it may be easiest to fit new aftermarket airbags.

For instance I have air ride kit fitted to my van (not an LDV) and if you see here they say on their website they can supply air bags for any van:
https://airride.co.uk/mh-finder/

Ldv400camper

4 posts

13 months

Tuesday 28th February 2023
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Thank you that is very helpful much appreciated

Ldv400camper

4 posts

13 months

Tuesday 28th February 2023
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I think it may be an original part as I got the part number from ldv but they don’t stock the part

Ldv400camper

4 posts

13 months

Tuesday 28th February 2023
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So my air suspension works fine but is leaking air are you saying if I can find the right size bag I could just change that. Do you know of a place I can find the right size that holds the right weight

Scrump

21,889 posts

157 months

Thursday 2nd March 2023
quotequote all
Yes, you could just change the airbags, if you can find an aftermarket set which fit.
Airride are a supplier of a wide range of bags. I would ask them if they have one which would fit.