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Hi folks! Just bought 06 STS 4 V8. I have the dreaded rear MRC shock dilemma. The car rides a bit low. One mechanic said shocks need to be replaced, another said they are fine. Compressor charges on start and then intermittently. Could this be a sensor issue? Many thanks
 

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08 STS-4 N* 1SG, 08 DTS Luxury II, 04 Bonneville GXP N*
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The compressor automatically starts with each ignition cycle for a few seconds (four if I'm not mistaken). The compressor can then operate intermittently depending on how the Electronic Suspension Control (ESC) module determines is necessary. You cannot tell if the rear shocks are leaking without puncturing the air bladder and checking for oil. THIS PERMANENTLY DAMAGES THE SHOCK SO DO NOT DO THIS! It is then a bounce test that the technician is basing his recommendation on? If it's an air leak and the shocks are not rising with the compressor on I'd be checking air lines and connections before jumping on new shocks.

Also look here...
http://www.cadillacforums.com/forums/cadillac-sts-forum-2005-through-2012/595065-rear-suspension-problem.html
 

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2005 STS-4, 2003 CTS 3.2
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From what I understand our rear air isn't always on - right?!

I inspected mine before fairly close and they always seem fully deflated - unless I have about 5-6 people in the car and the back starts to sag
 

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Wow that thread was very thorough. Thank you. Bounce test was it so far. No codes on display but I am hoping it could be just that-a hose leak at compressor.
 

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From what I understand our rear air isn't always on - right?!

I inspected mine before fairly close and they always seem fully deflated - unless I have about 5-6 people in the car and the back starts to sag
The air isn't always "On", depending on your definition. When the car is on the ESC module is constantly monitoring ride height. There are measurements in the service manual as to how high the rear end should be but I don't have that figure off the top of my head. The ESC continually adjusts to meet this specification. There will always be a delay of up to nearly 30 seconds before the compressor will either engage or vent because otherwise every bump in the road would cause the system to take an action. Even filling up with fuel will add enough weight to cause the compressor to adjust the height.

@cronos73 - As I recall, the auto-level portion of the system will not generate a DTC. There are other faults relating to the suspension in general that will however. Therefore, it's not surprising that you don't have any indication of a fault. Also, many DTC's will not set and MIL so this does not guarantee there aren't some codes stored. The only way you'll know for sure is to scan the car with a code reader. Some readers may not pick-up on suspension codes. Cheaper readers are sometimes limited to emissions related faults.

If the ESC detected a potential sensor fault you would have some sort of indication whether it be by a "Speed Limited to ...", "Service Suspension System" warning or other type. The sensors for the Magnetic Ride Control are also used for ride height. My best guess is that your problem is probably not a sensor.

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In addition to the air lines, it would be worth checking the compressor. Just because it can be heard running doesn't mean it's actually producing air pressure. With the cost of these shocks I'd be inclined to check all the other possibilities first. Things like checking the compressor are pretty easy. This can be done by disconnecting a line at one of the shocks and checking for air while the compressor is running. It's worth noting that the compressor puts out high pressure, not high volume. Don't expect to feel a lot of air coming from the line.
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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FWIW, The suspension height sensors - all 4 corners - input to the stability and body control systems - constantly. The ELC (rear shock bladders) does a key: ON preload/test then remains off unless there is a weight change in the trunk or rear seat areas.

If ELC runs intermittently, absent weight changes, you have a small leak somewhere.

Test it - get 2 assistants and some beerz. Pop the trunk. Pop the beerz. You turn the Key: ON - after about 6 seconds, does ELC run for 2 seconds or so ? OK - leave the key on. Now y'all sit on the open trunk lip. The ELC should run and level the car. OK ?? All hop off and you listen for escaping air (RR wheelwell ??) as the system vents off and levels the car. AOK ??? Good to go.
 

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Ahh, makes sense


These don't seem as problematic as some systems I have seen - so far at least
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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These don't seem as problematic as some systems I have seen - so far at least
The MRC part of the system is much longer-lived than the old controlled valve CVRSS types. There are no moving parts to MRC - save for the piston itself - and rod seals.

Wait 'till the car gets to 10 or so years old. Those shock bladders can take only so much road grit, dry air, exhaust heat, and general beating before something pops.

A very common problem is the filter/dryer dessicant can in the compressor assembly - it's a maintenance item but no one will tell you that. Compressing air condenses moisture and overloads the dryer - then nasty, gooey snot carries over to the compressor, air lines and shock bladders - ruining everything.

Get into the Suncore site or maybe Amazon or www.nalleygmc.com and find the part. Takes 2 hours to change out every 5 years.

Yes, I do.

EDIT: Google "lord delphi magnetorheological suspension system". My 2002.5 STS was in the group of test mules, then the system was used by Corvette and Ferrari, then grew to many later Cadillac and other brand models. MRC (F55) began on 01/15/02 for Seville Touring Sedan (STS) only. That carried on until the last real STS K-car was built in 2003. The Sigma body "STS" is 3 initials only - it now means nothing. :rolleyes:
 
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