Having just read Krash989's interesting post, something has been worrying me since I read an interesting article on this suspension system.

I just can't remember if it was on this forum or not, but it was months ago. The guy had gone to the lengths of putting a loudspeaker on the control wires for the shocks and listened to the note of the square wave vary as he changed his driving from hard to easy cornering etc..

I think it was the same post, but whatever...he put a resistor on the appropriate wires, which promptly burnt out. He finally concluded that a wirewound resistor was required–as per Krash's pics. This is the worrying bit.

This is a lot of current, and I just cannot see why this is needed for such a simple detector. Is the voltage across that resistor DC, or is it part of the same signal frequency that controls the shock? I seem to remember 2400 Htz. If it were AC, and the device was a coil of some sort, then there would be much less current / load on the control box output, and simply putting a fat resistor in it's place may stack up problems for later...apart from the fact that it often doesn't work!

Does anyone have any more detailed data about that sender and it's fix?

RB 94SLS

Ive currently been trying to recycle my old sensors off of my old shocks, since i never had a code with them in (even with them being shot)... after inspecting them i found that they have either a variable resistor (changes the amperage depending on the pressure) or they are simple on/off switches (meaning that if you were to connect the two wires together no codes should display).... I did not have time to test which one it is yet because i was unexpectedly called in for work today, nor will i have time tomarrow, but wednesday i may be able to test it... I did a stupid thing when changing my shocks i took pliers to the plastic where the wires go into the sensors so it is going to be very difficult for me to reuse them, but no sense in giving up now.

Ohh yeah, the button has very little movement... I would say about 1/16 of an inch at most, but that still doesnt rule out the variable resistor possibility... it could have only two different amperages that it can transmit. either that or off and on.

You might have been talking about my posts, they were quite a while ago.

I hooked up a speaker to the actuator inputs to listen to how the CVRSS (96 and up) system works. I had concluded that under low levels of shock travel the system sends a test pulse every 16 seconds. Heavier levels of bounce alter the PW of a fixed frequency signal (I cannot remember the exact freq). I later corroborated this information with an online tech manual.

The speaker never burnt out, however on gmforums.com (rip) Dan O. had stated that he used a low power carbon resistor in place of the actuator - which burnt up after a bumpy drive. The signal becomes a DC current under very heavy usage. I used (still use) high power wirewound 50 watt resistors (max dissipation of the circuit is 23 watts). I have no reliability worries about the setup.

The earlier systems have a different design and the resistors may not work. I cannot comment anymore on them since I do not own any car with these systems.

It may well have been your post. I know that interesting experimentation always sticks in my mind.

I went back to Krashed's post as he had raised some interest with some of the regulars. This was the gist of my post, but I didn't want to hijack his thread. My post was a dim memory possibly of your post. Following someone using injectors as the load. I wrote

"The injectors are indeed an inductive load, and this is what I was getting at on ‘....RESISTOR'. I just get the feeling that the system is a lot more complex than it would at first seem, but I'm guessing.

The thing about a coil is that if there is a signal instead of plain DC, the coil will offer a greater resistance as the frequency of the signal rises. ( 2 pi f L I seem to remember.) Even if it was quite a small coil, the AC resistance (reactance) could be around 100 times the 12 ohms that I see on the pics. This means proportionally less current / load on the controlling black box. To put a wirewound resistor with heat sink on a circuit that only has to sense something just seems crazy. 12 ohms / 12v = 1 amp. 1 amp X 12v = 12 watts. a typical radio resistor is a quater watt.

I wish I could remember where I saw that original post about the loudspeakers, cos I feel sure that the guy said he listened to a square-wave signal continuously varying the damping. The frequency remained constant, but the mark-space ratio varied. There was more on than off time when the dampers were at the soft ride. (got that a bit wrong ! ...and I had the figure of 2400 Htz fixed in my mind since reading your post all that time ago )

If this were true, there has to be a variable feed-back loop of some kind rather than just a plain switch, and I notice that my senders seem fairly large, not just a pressure switch. This is where we find that there is a major difference between two similar cars!

I have a 94 SLS and if time permits I'll try to find out the hard way. Having said all this, I'll probably find 4 lengths of string from the springs to the computer."

I hope I didn't misquote you too much. Rob

For CVRSS systems the sensing is actually quite simple. I figure that the computer looks for a window of voltages created by a voltage divider. One resistor is inside the computer and the other consists of the solenoid. To get the right voltage one needs to force the output leg of the circuit to the correct voltage. A resistor is the easiest way to do this. This is a realitively high power circuit to drive the coils.

I guess you could open up the rss computers and change the internal resistance to something much larger and get by with less current - but why.

The system for the 94's is different, my guess is that it operates as some sort of buck converter circuit. This takes a high voltage and converts it to a lower voltage and the coil has nearly constant current through it (actually a triangle wave).

You are only partially correct about your reactance statement. A square wave consists of an infinite number of frequency components. One with a dc offset (0 to 12 volts) has a dc frequency component. The other voltages are not continuous, but vary over time. So unless there were high pass filters and peak detectors within the sense circuit - the dc is the easiest and cheapest portion to measure. Also, the frequency components will vary with pulse width.

My best guess is that there the voltage across the load (should be nearly dc) is measured or that the system looks for a negative voltage swing across the inductior during the off phase of the PWM signal in the 94 suspension system. I do not think that the sensing system is overly complex utilizing feedback from the actuator, which in this case would be positive feedback making the system inherently unstable.

Now I'm REALLY intrigued, I love to work out how designers set about a problem. :hmm: Sadly, most of my toys are in boxes back in the UK, but when I have time I'll try to borrow a scope and see what's going on, though it'll be a week or two. RB

Well i dont know but i think i got lucky today... my dad has a friend who has a cadillac of the same year as mine (dont know what model) and he just replaced all of his fuel injector coils because a couple were acting up... so i might be able to do the injector coil thing with my car... if that doesnt work i think i might be able to find some (im guessing) 12watt 2ohm inductive resistor coils on this site...

www.allelectronics.com

do you guys think that ceramic resistors of the same kind would do... im totally in the shallow end of the knowledge pool with these kinds of electronics

Having a moment to reflect on Maydog's reply prior to finding the time to have a look myself, I'll just make a couple of comments though they won't be a lot of help. Just for fun tonight.

Of course, if the circuit is designed to run at these currents, there would be no point in worrying about reducing it. Finding an R that was in the right range and woomphy enough is all that it would take. What worried me was the possibility that there was a signal on the detector / sender that would have severely reduced the current IF there had been a coil in the device. So that's IF there is a wave on the detector line and IF the detector offers an inductive load. A sawtooth wave would certainly behave as mentioned a square wave agreed, is a little different.

Krashed, I wouldn't worry too much I'm being picky just for interest's sake. Maydog's comment on square waves becomes a mathematician's game when, what they call the rise time is very short. An infinitely low rise-time is equivalent to having an infinite band of frequencies on it's edge, and has little effect on the job in hand because it can't be achieved anyway. However the electronics designers have enough problems protection the hardware from spikey ‘noise' in the real world. It's akin to the old coils. If you were silly enough to turn the ignition on in an old car, and then walk away for ten mins...and the contacts were closed, the coil would get hot...even damaged. But as soon as the engine was started, that same 12v would now be cutting on and off at high speed and the coil would take less current for two reasons. One, it's not of for so long, and two, the coil acts quite differently to a fluctuating current. It is because those old contacts make a crude square-wave that there had to be a condenser across the points.

My wife alway says ‘why don't you look at the instructions / manuals.....where's the fun in that?
I always look afterwards to see if I was right. :bighead:

I have no idea if the RSS system for '94 is the same as my '98, but if they are similar, this might help understand the design intent. Understand that I am reading between the lines from a '98 Service Manual with more than a basic understanding of electrical / electronic systems, including RF.

The damping force of the strut / shock is adjustable by controlling the average current flowing through a solenoid device that GM calls a "damper actuator". The solenoid is probably fine tuning a hydraulic orifice inside the strut / shock.

Low average current levels to the solenoid yield minimum damping.

The DC voltage to the solenoid is PWM modulated on a 2 KHz signal.

The CVRSS module sends a test signal every 16 seconds (more or less) and the feedback voltage is monitored. If the feedback voltage is higher than programmed for 2 test cycles, a DTC is set and that strut / shock is set to minimum damping (low current).

At first blush, it would appear that the ideal "resistance" needed to fool the feedback system would be the "impedance" that the coil presents during average road conditions at 2 KHz.

Whatever that turns out to be.

Interesting thread and looking forward to any reverse engineering experience that is shared.

Thanks for the data JimD and also to maydog for his ingenious method of monitoring the system in the first place. I’m not sure of the changes over the years, but when I’m finished rewiring my kid’s house I’ll get back on the puzzle. (this is a strange land where red electricity comes out of black wires and the wires are a thick as girders…it’s going to take a week!!)

I was intrigued by maydog’s comment that the loudspeaker did not burn out. Also, the frequency is very near to the 2,400 that I had mentioned. I recalled that it was an audible note. If I recall from the dim distant past, that speakers are classified in their reactance at 400 Hz, so 2k may have saved it??

I can see that the shocks are modified by what must be a fairly hefty PWM current. What I’m not clear on is the nature of the ‘signal’ on the detector line. I realize that I may have made a totally wrong assumption but cannot see the need for such hi currents on the CVRSS module / feedback circuitry.

It all seem a fantastic system, but when I asked if anyone serviced them, I was disappointed to see a…vigorous comment on the quality of the actual shock itself.

RB