: Compressor Function - Cadillac Magneride Controlled Suspension System

04-26-12, 03:43 PM
Can anyone explain how the compressor works in MCS equipped GM cars? I've searched endless hours trying to figure this out. I'm asking principally because the rear shocks on my 2003 STS seem to be maxed out for a sport ride.....very, very stiff. Weight added to the trunk or weight removed makes no difference. The shocks seem to be locked in the stiffest suspension mode. I've read that the compressor has a release valve, and when activated, presumably by a sensor, the stiffness should adjust. This doesn't happen for me. I've read about how magnetized particles within the shocks are energized by a control module to control damping. It doesn't seem that the compressor could have anything to do with this since air is not an issue. So, what is the purpose of the compressor in a GM car with MSC if damping isn't controlled by air?
Can anyone recommend a trouble shooting procedure to resolve this damping problem? It seemed to come on suddenly. One day the ride was perfect; the next day extremely harsh. Because of the sudden change in ride characteristics, I don't suspect the struts or shocks. They only have 41K easy miles on them, and the car has always been garaged.
I would be grateful for any help and advice.

04-26-12, 03:59 PM
The Electronic Level Control - and its compressor - has nothing to do with the F55 MRC suspension firmness. The ELC is strictly to maintain proper rear ride height due to extra weight in the trunk or rear seat. The MRC responds to vehicle attitude, speed, roll, pitch, yaw, and braking demands - among other inputs. The struts and shocks have infinitely variable damping control, caused by a constantly varying magnetic field affecting the viscosity of the "hydraulic" fluid - there are NO variable orifices or valves in these suspension units: the fluid viscosity changes from about 10W oil to jelly in a millisecond and back.

If you suspect a false input (these cars are NOT typically "Cadillac rides") then you need to use the car's built-in code reader/scanner to find any Diagnostic Trouble Codes stored in the system. Use the sticky post up in Seville titled "How to pull codes". Read up on the procedure for your (our) OBD-II system and practice. Write down all codes (suspension codes will be prefixed with a C) and whether the suffix is a C or H. Return to the sticky and open the link with "obd2" in the address - this takes you to a Master Index of all P, B, C, and U codes. Write it all down and post it here.

The compressor, control head, supply/exhaust solenoid head, air lines and shock bladders control height, not stiffness. When you get in the car, stick the key into the ignition and turn it to On or Start, about 8 - 10 secondslater you should hear the compressor run in the RR fenderwell - it will do a 1 - 2 second preload and checkout. After that, unless you place extra weight in the car it should not run again. To test THAT system, do the preceding On sequence. if the compressor preloads, pop the trunk and three of you sit on the open lip. The compressor should level the car. All hop off - you should hear air escaping in the RR fenderwell as the system again levels the car.

If the car has a very exaggerated nose-down rake, it's possible that one or both rear suspension height control rods have come off the sensor arms. (Any tire work just done ???). Click on my username, open my profile. 2 albums, 6 pages of pictures and diagrams for our cars - several of the rear ride height sensors and arms.

Google "lord delphi magnetorheological suspension control" or "cadillac forums seville magnetic ride control".

04-26-12, 04:23 PM
its also possible that the rear shocks were replaced with passive absorbers and resistors were added inline on the connectors to keep the module happy

visual inspection will tell the story there

04-26-12, 08:17 PM
Thank you so much for your detailed reply, and particularly for clarifying the compressor function. I've pulled codes before so I am familiar with the procedure. I've also cleared codes, namely a false "service engine soon" that occurred all too frequently. I currently have no active or history codes, and haven't had any for about two years.
I first noticed the shock stiffness soon after having the oil changed along with a state inspection. When I took the car into the service center for analysis some time later, they reported that a rear sensor had failed and both rear shocks and the bad sensor needed to be replaced. I checked out their itemized quote and found they were planning to use Monroe shocks, #'s 40029 and 40030 at a cost of over $950 each. I checked and found I could acquire the shocks for less than $400 each, and in the process found that they are not suitable for MRC equipped cars. I then took the car to an independent garage, and the stiff shocks problem baffled the owner/mechanic after numerous diagnostic checks. He did verify all fuses were good and that the compressor was working. He said he had some friends who are GM mechanics and that he would check with them for some clues to what might be wrong. I'm still waiting for a response from him, but after one week I've heard nothing. I hope I don't have to take the car into the local Cadillac dealer, but that may be my last hope.
Since the problem occurred suddenly, I think it's most likely a sensor problem. I've read that the steering wheel sensor can fail and trick the system into believing that the car is in a sharp turn, thereby activating stiff suspension. I don't know about that. Like I said there are no codes.
Anyway, thanks again submariner409 for your detailed explanation. I would appreciate any other thoughts. If I can just determine what the problem is and get it fixed without needlessly replacing functioning parts, I'll be very thankful.

04-27-12, 11:11 AM
the sensors are all monitored circuits so they would produce a code if they were not working properly

are you sure everything is connected properly? if a sensor link gets disconnected you can have no codes but improper operation

04-27-12, 11:08 PM
To answer one question, the last time the car was in for service it was at a nationally fanchised service center, and in addition to an oil change, a state inspection was done....that required at least one wheel to be removed. Unusual stiffness in the rear suspension hadn't been noticed before the service, but became obvious sometime afterwards. I'm not clear on how long afterwards. I suspect the servicing mechanic could have dislodged something, but the reports from two subsequent and different sets of diagnostic tests fail to mention that anything is dislodged or disconnected.
There are no current codes, yet it seems the rear shocks stay energized continuously. Visual of the height at the rear appears normal but the rear end can barely be nudged up or down; an inch at most, probably less, when I push down with all my weight. It is extremely firm. I rechecked the codes again today and noticed a TIM (Tilt and Telescopic Steering Wheel Module) U1016 history code which I understand is a loss of communication with the Powertrain Control Module. Could this indicate that the steering wheel position sensor has failed?
Last night I took the car out for a drive and put it under some duress with sharp turns and quick acceleration and at one point this did cause a "Stability Control Engaged" message to be displayed for an instant. Does that tell me anything about the condition of the sensors?
By the way submariner409, the Borla exhaust system you put on your 2002.5 STS is awesome.


Chris is there any reason to believe that more than one sensor link could have been disrupted during a single shop visit for service? Since the rear end stiffness seems uniform on both sides doesn't it seem likely that the steering wheel position sensor would be most suspect?

04-28-12, 09:57 AM
i don't think any sensor is disconnected since that would cause a code... it's got to be something mechanical that the electrical systems can't monitor
Uxxxx codes are nothing to worry about unless they are current and stay current ... a U code in history is nothing to really think twice about.

also, the tilt and telescoping module is unrelated to the steering wheel position sensor ... the steering wheel position sensor is at the base of the column and detects what angle the steering wheel is turned to that T&T module controls the tilt and telescoping features of the column and is inside the column up close to the steering wheel

another idea i just had is that it's normal for the shocks to feel this way ... i think that the shocks might stiffen up when the car is not moving
sub can test this for you since he has a 2002.5 STS - he also may know off the top of his head

04-28-12, 04:34 PM
the Borla exhaust system you put on your 2002.5 STS is awesome.

CORSA - www.corsaperformance.com (http://www.corsaperformance.com)

www.autoanything.com (http://www.autoanything.com) or www.AutoPartsWarehouse.com (http://www.AutoPartsWarehouse.com) Not cheap, but you would be a happy camper...............

Just checked - my car has been in the garage for 36 hours or so - with the key off Karen and I can sit on the open trunk lip and move the rear up and down maybe 1 1/2 - 2 ". With the key on and ELC proved out we can move it maybe 1" or so. It's stiff with power applied to the constantly active suspension. Either on or off the car sits with its characteristic slight nose-down rake.

04-28-12, 07:39 PM
I really appreciate the effort. I'm almost certain I don't get that much movement; probably less than an inch. I'll check again though with some assistance. The rake is consistent with what you describe. I'm thinking of starting a new thread in hopes of catching the attention of a member who has had to deal with a similar problem. Thanks for all your help.....and the exhaust link.

04-29-12, 08:28 PM
My 2003 STS ride went from smooth as silk, typical Cadillac, to being very harsh and jarring. When on the road, the rear shocks now seem to stay fully energized regardless of terrain or road conditions. That's my take on it. There are no current codes, and all history codes are of the UXXXX variety. The car only has 41,000 easy miles and is always garaged. After inspection, a local national chain service center concluded that one rear sensor and both rear shocks need replacement. Their quote of almost $2700 listed two Monroe shocks, 40029 and 40030, which I subsequently determined are not compatible with a MRC equipped car. Therefore, I have no faith in their diagnosis. An independent garage mechanic could find no problem and has no explanation for the continuous stiff ride.
I would appreciate comments from anyone who has had a similar problem, or has some thoughts about what might be wrong.

04-30-12, 12:19 PM
starting a new thread will not help, each time someone replies to this thread it pops to the top again so you're not going to gain any exposure by starting a new thread, only divide the discussion (plus it's against the site rules and regs... for those reasons)

05-01-12, 05:11 PM
I was talking about a new thread that has nothing to do with the function of the compressor in MRC equipped Cadillacs. I don't believe that would be against the forum rules and regulations. Let me know if I'm in violation. I don't want to do anything that is out of order.

05-01-12, 05:26 PM
no, a new thread for a new issue is just the right thing to do, i must have misunderstood you

maybe put a link to this thread in the new thread but that's just a suggestion to help people follow you and where you've been with this car


05-02-12, 05:49 PM
I just came across an interesting, and I believe, relevant 11-month old post by SwampeastMike in thread “STS suspension questions” under Cadillac Seville/Cadillac Eldorado Forum Discussion of Past Cadillac Vehicles. He suggests that if your ride feels like it’s “driving on rims“, a failing steering wheel position sensor may be suspect. He’s speaking from personal experience. A response from submariner409 is supportive of SwampeastMike’s claim. This could be the information I’ve been hoping to find, and with the suggested testing and follow-up could help to resolve my bone jarring ride issue.

05-02-12, 10:08 PM
Sounds promising.

02-02-17, 12:53 PM
i have 2003 STS with MRC suspension. same problem, stiff rear. no codes. compressor and dump valve ok. disconnected rear shock wires and still stiff. race the height 3/4 in in rear and shock not stiff. need to take for road test and check. i will see if i can get back on this later and let you know what happens. i am a dinosoures with computers & bad spelling. str.repair att.net.