AHC with harsh ride (1 Viewer)

[PADDO]

laptop back and semi-functional… DTC code C1718 present for the Pressure Sensor Circuit. Found this below elsewhere on this board (format is poor):

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CIRCUIT INSPECTION
CIRCUIT DESCRIPTION
DI3GI−06
DTC
C1718 / 18
Fluid Pressure Sensor Circuit
This circuit is sending the data to detect the pressure output from the pump and to judge the abnormality of the fluid pressure by the ECU.

DTC Detecting Condition:
Either of the following 1. or 2. is detected:
1. When detecting the abnormal signal from the fluid pressure
sensor (Fluid pressure sensor terminal voltage of ECU is 0.3 V or less or 4.7 V or more) for every 0.01 sec. and that condition continued for 1 sec.
2. While the motor relay is non−operating, the condition that the fluid pressure exceeds 1 MPa (10.2 kg/cm2, 145 psi) continued for 10 secs.

Trouble Area:
- Fluid pressure sensor
- Fluid pressure sensor circuit
- Suspension control ECU

Fail safe function:
If trouble occurs in the fluid pressure sensor circuit, the height control is prohibited after the ECU has adjusted the vehicle height to the standard.
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Suggestions where to go from here? I don't want to throw money at the problem and just start replacing things if I can further zero in on the problem. This is the first time I've dealt in anywhere near this kind of detail with Toyota codes and manuals (just got Techstream) and have more information than I know how to use.

BTW - my neutral pressures are a little high as well - 7.7 or 7.8 F and R. I'm going to try to fix the front this weekend with the torsion bars, but will have to wait on the rear and take it someplace that can install spacers or refresh the springs.

What is spec on the accumulator pressure?

My cheat sheet lists 1718 as an open or short in the fluid pressure sensor cct so I'd be pulling and cleaning connectors and attempting a reset via techstream as a first pass attempt. The book says check sensor, sensor cct then ECU. Pulling AHC IG 20A fuze for 20 minutes or so (drivers foot well, firewall end of the fuze block for an 03 LHD) may help. I suspect you're not going to be able to progress fault finding and definitively rule in or rule out failed globes until you can get the system out of fail safe. Also try and get front neutral pressure back to 6.9MPa and rear within that 5.6 to 6.7 range. If you have had one or both rear accumulators fail you would have seen a significant drop in the fluid's reservoir level, and it has very little excess capacity between its high and low fluid marks, and the charge(s) (nitrogen I guess) would be entrained in the system too unless it has been properly bled. For the main accumulator I get readings around 10.4 or so.

 

[kgrove]

My cheat sheet lists 1718 as an open or short in the fluid pressure sensor cct so I'd be pulling and cleaning connectors and attempting a reset via techstream as a first pass attempt. The book says check sensor, sensor cct then ECU. Pulling AHC IG 20A fuze for 20 minutes or so (drivers foot well, firewall end of the fuze block for an 03 LHD) may help. I suspect you're not going to be able to progress fault finding and definitively rule in or rule out failed globes until you can get the system out of fail safe. Also try and get front neutral pressure back to 6.9MPa and rear within that 5.6 to 6.7 range. If you have had one or both rear accumulators fail you would have seen a significant drop in the fluid's reservoir level, and it has very little excess capacity between its high and low fluid marks, and the charge(s) (nitrogen I guess) would be entrained in the system too unless it has been properly bled. For the main accumulator I get readings around 10.4 or so.

Good info. Here is my plan:

1) tweek TB to fix front pressures
2) look for any bad electrical connections in system to fix code, though I doubt I will find anything. I already removed the AHC fuse, but only for a couple seconds. The one I removed is in the main fuse box in engine compartment, so will try looking for additional, or maybe '07 is just in a diff location than '03. Will also try a reset in Techstream if I can figure it out (still learning that thing). Hopefully will clear ode, but if doubt it.
3) find mechanic and replace rear springs (6 yrs old) and install airbags at same time. It doesn't make any sense to touch my globes or fix anything major until I have fixed what might have been the instigator of the problem being the AHC can't handle the tongue weight of my new boat. This should also bring the rear AHC pressure back in spec when not towing boat. Maybe fixing pressures and resetting clears the code and I'm good to go.
4) if code still there, take off rear globes per earlier post and see if I can tell any physical problems. I'm not excited on spending a bajillion dollars on globes if Im not sure they are toast. Based on what I find, I'll either take it to a dealer and see if they can locate the electrical problem or I'll order new rear globes and try the repair myself.

Sound good? Basically I'm going to fix the Non-AHC portions of the suspension first and then go to the electronics and AHC itself, in part because I know with 100% certainty the non-AHC portion needs fixing where everything else is an educated guess. Fix what I know first and see what problem remains - maybe some problems will disappear.

 

[CappyKD]

Make's perfect sense.

 

[kgrove]

Question: to get the front pressures back in spec, I know it's roughly 0.2mpa per full turn on the adjustment bolts, but there is some ability to easily adjust up or down to hit the target. For the rear, adjustments are not so easy. I'm debating between replacing the old springs and adding spacers. If I go spacers, is there a rule of thumb for how much to achieve a certain decrease in neutral pressure similar to the 0.2/turn rule of thumb on the front? Something like an Xmm per 0.2mpa? Putting in a spacer, figuring out it wasn't enough or was too much, then taking it back off and putting in another is very time consuming (and since I'm paying a mechanic could get pricey) and I'd prefer having a good shot at nailing it on the first try. My rear pressures right now are around 7.7, so I have to lose 1-2 full mpa (assuming those pressures don't change once the front gets adjusted). I'll be putting air bags in at the same time which also could have an impact even when deflated.

 

[Actualeyes]

If I go spacers, is there a rule of thumb for how much to achieve a certain decrease in neutral pressure similar to the 0.2/turn rule of thumb on the front? Something like an Xmm per 0.2mpa? Putting in a spacer, figuring out it wasn't enough or was too much, then taking it back off and putting in another is very time consuming (and since I'm paying a mechanic could get pricey) and I'd prefer having a good shot at nailing it on the first try. My rear pressures right now are around 7.7, so I have to lose 1-2 full mpa (assuming those pressures don't change once the front gets adjusted). I'll be putting air bags in at the same time which also could have an impact even when deflated.

I can't imagine that's possible because it depends on how old and sagged your springs and the weight you have on board. My old sagged 80k spring overloaded were at about 10mPa. Upgrading to a used set of nonAHC rears brought it right on spec... But luck and serendipity, not science

Try the spacer and see... They're not very expensive.

 

[PADDO]

Question: to get the front pressures back in spec, I know it's roughly 0.2mpa per full turn on the adjustment bolts, but there is some ability to easily adjust up or down to hit the target. For the rear, adjustments are not so easy. I'm debating between replacing the old springs and adding spacers. If I go spacers, is there a rule of thumb for how much to achieve a certain decrease in neutral pressure similar to the 0.2/turn rule of thumb on the front? Something like an Xmm per 0.2mpa? Putting in a spacer, figuring out it wasn't enough or was too much, then taking it back off and putting in another is very time consuming (and since I'm paying a mechanic could get pricey) and I'd prefer having a good shot at nailing it on the first try. My rear pressures right now are around 7.7, so I have to lose 1-2 full mpa (assuming those pressures don't change once the front gets adjusted). I'll be putting air bags in at the same time which also could have an impact even when deflated.

My experience FWIW: 10mm spacers lowered the rear pressure 0.5Mpa, but here is the qualifier, I'm stock weight or actually under stock because of no third row seats and as we all know one person's reading doesn't constitute a trend line. In your situation, and as labour costs are a factor, I'd seriously think about installing OEM non AHC coils for $200 (parts) + airbags as mentioned for the towing. Spacers will cost $25 and I suspect you're looking at minimum 20mm, I think max available is 30mm. Labour to install spacers will equal labour for new springs, but I'd guess you would achieve the desired outcome with spacers + bags too. Either option will be better than present. Good luck.

 

[uHu]

Question: to get the front pressures back in spec, I know it's roughly 0.2mpa per full turn on the adjustment bolts, but there is some ability to easily adjust up or down to hit the target. For the rear, adjustments are not so easy. I'm debating between replacing the old springs and adding spacers. If I go spacers, is there a rule of thumb for how much to achieve a certain decrease in neutral pressure similar to the 0.2/turn rule of thumb on the front? Something like an Xmm per 0.2mpa? Putting in a spacer, figuring out it wasn't enough or was too much, then taking it back off and putting in another is very time consuming (and since I'm paying a mechanic could get pricey) and I'd prefer having a good shot at nailing it on the first try. My rear pressures right now are around 7.7, so I have to lose 1-2 full mpa (assuming those pressures don't change once the front gets adjusted). I'll be putting air bags in at the same time which also could have an impact even when deflated.

If you're paying someone to do the job, the only sensible thing is to change the springs - probably cheaper than changing spacers twice - and a better result suspension-wise. If you are also putting in airbags, there is no use for spacers or stronger coils either (unless you have added a lot of extra weight).

So, the easiest, fastest and cheapest (considering labour and the wish for air bags) would be to change coils to original new ones and add airbags of a known brand.

 

[kgrove]

First part complete. Tightened the torsion bars today. Four turns took the front pressure from 7.8 to 7.1... Made the fifth turn expecting to nail the 6.9 goal but it went all the way down to 6.6. I'm going to leave it alone for a few wekas and see if it stays there... If it does I'm still thinking 6.6 is close enough and I'd rather be a shade under than over. Opinions?

The other good news is some combination of fixing the front pressure and running the Techstream utility cleared the 1718 DTC code I had for the pressure sensors. Interestingly between fixing the front pressure and removing the third row seats, my rear pressure also fell very close to spec (6.9? Didn't write it down). It might ride slightly better now, but it's still more harsh than I recall.

Airbags are on order, so as soon as they come in I'll replace the springs and install the bags at the same time. My suspicion is it will still be riding harsh and I'll have to replace those rear globes.

 

[kgrove]

I finally got part 1 complete - replaced the springs with new and while there had air bags installed. I checked the AHC pressure and with nearly deflated air bags had pressures of 6.8 or 6.9 in both the front and rear. The ride is still a little rougher than I like - not as smooth over sharp bumps as I recall - but I will test it out a little further over the coming weeks. Hopefully all it took was getting the pressures back in spec, but I'm still suspicious I ruined the rear globes/accumulators by towing with such a tongue heavy boat. I want a week to drive it plus at least one time towing the boat to test the airbags, then test it again afterwards, before I mess with anything else.

Side note - I think the mechanics screwed up the air bag install... will tackle that in an air bag thread.

 

[dfrelich]

A wild shot. How's your battery?

Reason I say this... my truck has been bit more bumpy when in the cold. I contributed that to aging truck and had planned to tackle ahc issues in the later summer.

Then a deep cold a couple of weeks ago forced me to change out my battery. (the battery was little under 12 volts when I tested it - almost 5 years old) As soon I replaced my battery, I drove out in the cold and was amazed how smooth the truck felt.

I'm still skeptical but last few drives was so nice. Could be that my alternator is going out and is not handling the electrical load the truck demands when in drive? Now with fresh battery, it is helping the truck with the demand?

 

[kgrove]

A wild shot. How's your battery?

Reason I say this... my truck has been bit more bumpy when in the cold. I contributed that to aging truck and had planned to tackle ahc issues in the later summer.

Then a deep cold a couple of weeks ago forced me to change out my battery. (the battery was little under 12 volts when I tested it - almost 5 years old) As soon I replaced my battery, I drove out in the cold and was amazed how smooth the truck felt.

I'm still skeptical but last few drives was so nice. Could be that my alternator is going out and is not handling the electrical load the truck demands when in drive? Now with fresh battery, it is helping the truck with the demand?

That's weird. As a matter of fact my battery is barely limping by... Tried replacing it today in fact but both places I stopped by were out of the correct size. I'll report back if the problems improve after I install a fresh battery but I can't for the life of me think how a battery could impact how the AHC system is functioning.

 

[dfrelich]

That's weird. As a matter of fact my battery is barely limping by... Tried replacing it today in fact but both places I stopped by were out of the correct size. I'll report back if the problems improve after I install a fresh battery but I can't for the life of me think how a battery could impact how the AHC system is functioning.

Yes I struggle. But the logic sounds plausible. For example, before the battery swap, the dome lights would be dim unless I rev up the engine. With new battery, the dome shines bright.

 

[Texasoil]

I too have a harsh ride on my 2003 LX 470 with 60k. I did replace the AHC fluid few weeks back but not much has change. I would be interested to know what might be wrong with your system. I even thought of replacing the front and back shock absorbers.

I am in Bellaire and would be willing to test you suspension out (takes 5 min), no charge of course. Just give me a call You DO NOT need to replace the 'shocks'
Keith B&B Suspension (832)651-7576

 

[TXLX100]

I have a similar harsh ride over small bumps. 1998 with 135k miles, TBs never been touched and have new rear springs. The read out for pressure right now are 10.0 in the front and 8.6 in the rear. I expected the rear to be a little lower, I suppose I can add a shim to the rear springs to help.

I'll crank on the front a bit, seems like a long way to go to get to 7 mpa-g. Has anyone noticed a difference in steering or had to get an alignment after adjusting that much?

 

[PADDO]

I have a similar harsh ride over small bumps. 1998 with 135k miles, TBs never been touched and have new rear springs. The read out for pressure right now are 10.0 in the front and 8.6 in the rear. I expected the rear to be a little lower, I suppose I can add a shim to the rear springs to help.

I'll crank on the front a bit, seems like a long way to go to get to 7 mpa-g. Has anyone noticed a difference in steering or had to get an alignment after adjusting that much?

Why would you need an alignment after getting the front pressures back to spec? The front end geometry doesn't change, your height sensors set the overall height, not the torsion bars.

 

[SWCruiser]

I'm also not sure why I'm getting high rear pressure after installing new springs. I'm going to crank the front down again this weekend along with adjusting the height sesnors.

 

[TXLX100]

Why would you need an alignment after getting the front pressures back to spec? The front end geometry doesn't change, your height sensors set the overall height, not the torsion bars.

I don't know, that's why I asked the question. More tension is being offloaded to the torsion bars from the AHC shock, it's plausible the alignment specs could change if the bars are tightened enough.

I didn't check the pressures before I added the new coils. I have another LX with same miles I can check that hasn't had the coils changed, I'll report back when I check it.

 

[00LX]

Is there an AHC maintenance write-up with pictures?

 

[uHu]

I have a similar harsh ride over small bumps. 1998 with 135k miles, TBs never been touched and have new rear springs. The read out for pressure right now are 10.0 in the front and 8.6 in the rear. I expected the rear to be a little lower, I suppose I can add a shim to the rear springs to help.

I'll crank on the front a bit, seems like a long way to go to get to 7 mpa-g. Has anyone noticed a difference in steering or had to get an alignment after adjusting that much?

As we can extract from Paddo's reply, alignment will not be affected, as the hight stays the same.
If the rear springs are new, and the pressure is too high when the car is empty, the height is most likely adjusted too high. Always measure the height before concluding anything regarding pressure readings. It's the load taken up by the steel springs (TBs & coils) that decide how much there is left for the ahc to lift (up to the set height). So, if either the steel springs are too weak, or the height adjustment is too high, the ahc pressure will be too high.

 

[TXLX100]

As we can extract from Paddo's reply, alignment will not be affected, as the hight stays the same.
If the rear springs are new, and the pressure is too high when the car is empty, the height is most likely adjusted too high. Always measure the height before concluding anything regarding pressure readings. It's the load taken up by the steel springs (TBs & coils) that decide how much there is left for the ahc to lift (up to the set height). So, if either the steel springs are too weak, or the height adjustment is too high, the ahc pressure will be too high.

Good suggestion. I will see what the rear reading is in Low. I'm probably not going to mess with the ride height , it's still stock from the factory so I don't know why it would need to change too much. There isn't much weight in the rear, one seat and a fridge & slide probably weighing 100lbs. I think I'm about 19.5" in the front and 21" in the rear, I'll have to double check.

Update: I couldn't get the reading to change from neutral to low, the system only reads when the pump is running and it doesn't have to run to go from neutral to low.

I had more normal readings today, I cycled from low to high several times and got consistent pressures.

In neutral:
Front - 8.0
Rear - 7.6
Fender to hub
Front 19" (both sides)
Rear 20.75" (both sides)

In High
Front - 10.7
Rear - 4.2

Not sure why the rear pressure went down in High.