AHC Still Soft in the Rear

[2dogs68]

Hi,
Hoping for a little help with my AHC, last winter a hydraulic hose gave up on my 2001, 180k miles, 100 series, here in the UK we have MoT annual inspection and on previous tests I'd been advised the rear springs were corroded (nothing serious just something to keep an eye on). The hose burst about the same time as the MoT was due so got the mechanic to replace the springs also, ever since then it was really soft in the back and really hard over potholes and speed bumps. Since then I have found it difficult to find mechanics that are prepared to tackle actual repairs and most just want to do services, which I sort of get but doesn't help me. Since I decided a while ago I was going to keep this truck for a very long time I decided to replace the globes and the accumulators for each globe. After that the ride was still bad. Then discovered the ABC of AHC video and went through that process, sure enough the pressures were too high. But the LH torsion bar was adjusted to the limit and still low on that side, so I reindexed the torsion bar. I repeated the ABC of AHC process again and the height sensors were out and badly corroded so I replaced those also. Now I have pretty much replaced 80% of AHC parts.

So after all that work and a steep learning curve (which has been great, as I've got to know my truck a lot better) I've still got a softer than expected ride in the rear. I used iCarsoft to measure all the heights and pressures and they're in specification. I'd say it's inconsistent too, some days it's firmer than others. I use the truck daily driver, during the week no more than 50 miles to and from work, no additional weight, original specification VX model, I checked everything after a week following the work and everything is still in spec.

I have an idea that the new springs are possibly incorrect but with no markings on them, it's a guess. I'm at the head scratching stage now and grateful for any suggestions anybody may have.
Thanks

 

[SteveH]

When the PO replaced my rear springs, he also installed spacers to get the correct height/pressures in the AHC system. The springs were stock OEM replacements. I don't think rear springs are super expensive (at least not in the US), so you might consider replacing (again) with known OEM springs, instead of 'what's in there now', of whose pedigree you are unsure.

 

[2dogs68]

When the PO replaced my rear springs, he also installed spacers to get the correct height/pressures in the AHC system. The springs were stock OEM replacements. I don't think rear springs are super expensive (at least not in the US), so you might consider replacing (again) with known OEM springs, instead of 'what's in there now', of whose pedigree you are unsure.

Thanks Steve, I was leaning towards this conclusion purely based on what else could it be? Most "mechanics" seem to be service outlets these days. I had a BWM 7 series years ago were a front spring actually snapped, the "mechanic" fitted the wrong ones, they were obviously too high, the car looked ridiculous, when I questioned it he said, "that's what the parts guy delivered".

 

[neo2uzfe]

Hi,
Hoping for a little help with my AHC, last winter a hydraulic hose gave up on my 2001, 180k miles, 100 series, here in the UK we have MoT annual inspection and on previous tests I'd been advised the rear springs were corroded (nothing serious just something to keep an eye on). The hose burst about the same time as the MoT was due so got the mechanic to replace the springs also, ever since then it was really soft in the back and really hard over potholes and speed bumps. Since then I have found it difficult to find mechanics that are prepared to tackle actual repairs and most just want to do services, which I sort of get but doesn't help me. Since I decided a while ago I was going to keep this truck for a very long time I decided to replace the globes and the accumulators for each globe. After that the ride was still bad. Then discovered the ABC of AHC video and went through that process, sure enough the pressures were too high. But the LH torsion bar was adjusted to the limit and still low on that side, so I reindexed the torsion bar. I repeated the ABC of AHC process again and the height sensors were out and badly corroded so I replaced those also. Now I have pretty much replaced 80% of AHC parts.

So after all that work and a steep learning curve (which has been great, as I've got to know my truck a lot better) I've still got a softer than expected ride in the rear. I used iCarsoft to measure all the heights and pressures and they're in specification. I'd say it's inconsistent too, some days it's firmer than others. I use the truck daily driver, during the week no more than 50 miles to and from work, no additional weight, original specification VX model, I checked everything after a week following the work and everything is still in spec.

I have an idea that the new springs are possibly incorrect but with no markings on them, it's a guess. I'm at the head scratching stage now and grateful for any suggestions anybody may have.
Thanks

Is your car diesel? There can be difference in diesel rear coil spring. When you correct rear pressures, the front might come in spec.

 

[2dogs68]

Is your car diesel? There can be difference in diesel rear coil spring. When you correct rear pressures, the front might come in spec.

Yes the car is diesel, I have ordered some genuine Toyota springs, I'm pretty sure the current ones are incorrect.

 

[IndroCruise]

Yes the car is diesel, I have ordered some genuine Toyota springs, I'm pretty sure the current ones are incorrect.

My Right Hand Drive 2006 LC100 ("Sahara" in Australia, equivalent to "VX" in some countries) with AHC and 1HD-FTE turbodiesel came from the factory in Japan with Part Number 48231-6A770 Rear coil springs, designated with a PURPLE patch, same free length of 485 millimetres on both sides of the vehicle (measured when the springs were removed).

These springs seemed to soften with age and were replaced with the higher spring rate yellow KING KTRS -79 Rear coil springs (also same length both sides) to cope better with extra load. When the vehicle is not carrying the usual touring load the Rear AHC pressure is about 5.3 MPa, which is slightly below the FSM-specified range. The ride is slightly firmer when the vehicle is unloaded but still very acceptable, and excellent when loaded, with the extra load correcting the Rear AHC pressure. I also added airbags inside the Rear coils for occasional very heavy loads (such as the weight on the Rear towbar for a caravan etc) but these are very rarely inflated above their shape-holding pressure of 5 psi.

There are also some informative posts by @uHu concerning springs (search "springs" and @uHu). He has identified other springs for AHC-equipped vehicles, including Part Number 48231-6A780 W(ACTIVE HEIGHT CONTROL); H=500, BROWN PATCH. He notes that even with the "Brown" springs fitted, it is easy to oveload the AHC system.

Left Hand Drive vehicles commonly have different Rear springs (longer on LHS) to give a pre-load which better balances the weight distribition of driver and fuel in these vehicles.

The image below is taken from the FSM and gives some guidance on AHC load limits. This is easily overcome with Front torsion bar adjustmnent (or re-indexing if necessary) and with higher spring rate coil springs in the Rear. Spacers give some, but not much, additional load-carrying capacity. It is worth noting that the 1HD-FTE turbodiesel engine is much heavier (+100kg?) than the the 2UZ-FE gasoline/petrol engine. So the "payload" is less in vehicles fitted with the 1HD-FTE engine, if staying within the legal load limits. In Australia, maximum legal Gross Vehicle Mass (GVM) for any 100 series vehicle (LC or LX) is 3,260 kg or 7,172 pounds (slightly less for 105 series).

 

[2dogs68]

Thanks for all your comments, I fitted the genuine Toyota springs (48231-6A770) and the ride is now as it should be.