2012 Yukon XL Denali front suspension seems overly stiff

[dkad260]

think the more important thing to replace is the springs

I agree, made a noticeable difference on my Envoy after 18 years.

The front springs according to my dealer are still avail, part # 20842473. I searched GM parts online and the spring isn't listed for my 2012 Denali but two different dealers gave me the same part # based off my VIN.....

Being only a 9YO vehicle the springs are likely still good, but makes sense to go with new springs with new OEM struts if possible.

 

[91RS]

That is indeed a different number than both my 08 and 12 call for and does show to still be available. I’d order them while you can.

 

[iamdub]

Just FYI... the front struts have absolutely nothing to do with the "air ride" portion of the rear suspension.

The AutoRide system (RPO code Z55) controls the valving of the rear shocks and front struts electronically. It stiffens or softens according to the terrain.

The Auto leveling system (RPO code G69) consists of the rear air shocks and the air compressor behind the drivers rear wheel well.

The AutoRide struts are known to get stiff when they age. I think this is perhaps a failsafe built into the design to keep the truck from leaning so badly in curves with worn out struts.

@iamdub unplugged his one time to check them out. Perhaps he will chime in with his results. I just remember him saying that they were scary soft when unplugged and he didn't recommend driving at high speeds or making fast maneuvers with them unplugged.

Yes, this is my experience. I know for sure that the suspension controller is what made them overly firm. But, I don't know how it decides that they're too soft for normal driving to make it react accordingly. If it was an age/time thing, it could gradually increase the firmness to compensate for estimated wear. It could easily do so by monitoring the cycling of the suspension via the height sensors and estimate the life of the strut. I don't think this is the case cuz new struts would be even firmer unless the controller can detect that the struts aren't the same. I'm probably over-thinking it.

 

[TM355]

Yes, this is my experience. I know for sure that the suspension controller is what made them overly firm. But, I don't know how it decides that they're too soft for normal driving to make it react accordingly. If it was an age/time thing, it could gradually increase the firmness to compensate for estimated wear. It could easily do so by monitoring the cycling of the suspension via the height sensors and estimate the life of the strut. I don't think this is the case cuz new struts would be even firmer unless the controller can detect that the struts aren't the same. I'm probably over-thinking it.

So, from what I have seen in testing, if the front suspension is lower than what the ECU thinks is stock, the shock will get stiffer. Not sure if this is linear as the height changes, but it is a good generalization of what is happening. I think this also increases the damping on the rear shocks too to some degree (this makes sense since the programming would be silly if it only heavily damped the front but not the rear at all).

Suspension height reset from a Tech2 could fix this but I do not have access to one, or else adjustment of the sensor rods could also do the same. Do you have the current voltage values of your four sensors at parked, level ride height? It would be interesting to start gathering that data among a good sized sample of vehicles that have not been modified.

***FYI I realized all of this was happening when I just changed my front to lowering springs in a hurry and then drove the car. It was SO stiff and I thought, wtf the spring rate on these springs is complete shit! I don't know what made me realize the shocks could be the culprit, perhaps it was the rear shocks that also went stiff (but I reconnected a heavy rear sway bar at the same time which is not very analytical!) To check it out I unplugged the shocks and they went full soft and revealed the true spring rate which was right about what it should have been.

If you continue to fool the front into being higher and higher but then exceed what would be stock height, you will start to cause the rear to try to compensate with more air in the shocks, and again you will get a stiffass ride in the rear now. I have not been able to test if the damping gets stiffer, softer, in a linear manner, etc. if the ECU thinks the suspension is sitting higher than what it thinks is stock.

 

[iamdub]

So, from what I have seen in testing, if the front suspension is lower than what the ECU thinks is stock, the shock will get stiffer. Not sure if this is linear as the height changes, but it is a good generalization of what is happening. I think this also increases the damping on the rear shocks too to some degree (this makes sense since the programming would be silly if it only heavily damped the front but not the rear at all).

Suspension height reset from a Tech2 could fix this but I do not have access to one, or else adjustment of the sensor rods could also do the same. Do you have the current voltage values of your four sensors at parked, level ride height? It would be interesting to start gathering that data among a good sized sample of vehicles that have not been modified.

***FYI I realized all of this was happening when I just changed my front to lowering springs in a hurry and then drove the car. It was SO stiff and I thought, wtf the spring rate on these springs is complete shit! I don't know what made me realize the shocks could be the culprit, perhaps it was the rear shocks that also went stiff (but I reconnected a heavy rear sway bar at the same time which is not very analytical!) To check it out I unplugged the shocks and they went full soft and revealed the true spring rate which was right about what it should have been.

If you continue to fool the front into being higher and higher but then exceed what would be stock height, you will start to cause the rear to try to compensate with more air in the shocks, and again you will get a stiffass ride in the rear now. I have not been able to test if the damping gets stiffer, softer, in a linear manner, etc. if the ECU thinks the suspension is sitting higher than what it thinks is stock.

I do not have any voltage readings. With mine at stock height, I very precisely marked the sensors. After lowering, I made my own links that put the sensors back where they were so the controller would think the system was at its expected height.

 

[TM355]

I do not have any voltage readings. With mine at stock height, I very precisely marked the sensors. After lowering, I made my own links that put the sensors back where they were so the controller would think the system was at its expected height.

Yeah I wish I was familiar enough a few weeks ago that I took the voltage readings before I messed with anything. They are typical 5v sensors and it seems like the **** stock numbers may be in the 1.9 to 2.8 range. Tenths and hundredths of a volt are critical here, so it's very sensitive and might not be able to be measured externally/mechanically with required accuracy?? Not sure. The fronts might be higher voltage than the rears @ static. Higher voltage = lower actual suspension height or longer sensor rods. I'm currently guessing fronts to be around 2.5-2.7 and rear to be 1.9-2.2 for stock. Again it would be great to start sampling this data at some point. Many of the bypass circuits for conventional shock kits set this at 2.5 volts all around.

On another note, I suspect that in some cases, spring sag over time causes the shocks to be stiffer, contributing to some people's harsher ride complaints. This makes some sense as an old suspension will usually be softer or bouncy or floaty rather than stiffer, unless it is hitting on the bump stops or something like that.

At the end of the day, the only real way to truly satisfy the ECU is to do a reinitialization of the system with a Tech2.

 

[iamdub]

Yeah I wish I was familiar enough a few weeks ago that I took the voltage readings before I messed with anything. They are typical 5v sensors and it seems like the **** stock numbers may be in the 1.9 to 2.8 range. Tenths and hundredths of a volt are critical here, so it's very sensitive and might not be able to be measured externally/mechanically with required accuracy?? Not sure. The fronts might be higher voltage than the rears @ static. Higher voltage = lower actual suspension height or longer sensor rods. I'm currently guessing fronts to be around 2.5-2.7 and rear to be 1.9-2.2 for stock. Again it would be great to start sampling this data at some point. Many of the bypass circuits for conventional shock kits set this at 2.5 volts all around.

On another note, I suspect that in some cases, spring sag over time causes the shocks to be stiffer, contributing to some people's harsher ride complaints. This makes some sense as an old suspension will usually be softer or bouncy or floaty rather than stiffer, unless it is hitting on the bump stops or something like that.

At the end of the day, the only real way to truly satisfy the ECU is to do a reinitialization of the system with a Tech2.

This is true that the scale seems to be a narrow range. But, it must not matter a whole lot since Belltech and probably others make sensor ride height links that are a specific length. They're a standard size for a particular drop and used across many lowered vehicles. Pretty much every vehicle sits differently. So, even though the sensor scaling has a hundred-thousandths resolution, the suspension controller probably only monitors incremental blocks of that scaling.

 

[dkad260]

Do you have the current voltage values of your four sensors at parked, level ride height? It would be interesting to start gathering that data among a good sized sample of vehicles that have not been modified.

While not directed at me, I'm all in with acquiring some data as well. I can probe the sensor leads and see what I come up with.