VCC Problem
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Cadillac DeVille 1985 to 2005 including:
1985-1992 Fleetwood, 1993 Sixty Special, All FWD Forum Discussion, VCC Problem in Past Cadillac Vehicle Discussion; I know there is a lot out there, but I haven't seen my particular issue. 1991 Deville 4.9L. When I ...
  1. #1
    N0DIH's Avatar
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    VCC Problem

    I know there is a lot out there, but I haven't seen my particular issue. 1991 Deville 4.9L.

    When I am going from crusing on 2 lane roads averaging 50 mph to jumping on the freeway to head to 65 to 75 mph, the VCC goes from normal lockup (as per the climate control indicators, to UNLOCK. The odd thing is, the computer is telling it to. And I can't figure out why. 4th gear remains ok. It really isn't enough throttle to really NEED to kick out of VCC lockup, it disengages almost like you went 3/4 throttle, but didn't. Fast disengagement. Not the soft disengage I expect from the PWM VCC.

    ECM readings:
    TPS, normal range, no anomolies watching the voltage as the ECM sees it (angle) from idle to WOT.
    Coolant temp, 92C to 94C
    Vacuum, normal, nothing odd, not low.
    Digital speedo and ECM agree on speed, no errors there
    Fluid ok

    Even if I slow down or speed up, it stays unlocked. It will typically stay like that for 10 minutes or so, then for no apparent reason, lockup again. Flat terrain except the mild mild grade getting on the freeway, then it goes to a 2 mile gentle downhill, still no lockup. If I slow WAY down, like 30, then speed up normally, it will lockup ok. Just that acceleration from 45 to 50 mph when it is locked to 65 it unlocks and stays that way.

    Again, this happens typically when I have driven the car for already at least 45 minutes, 100% repeatable for the same speeds and location. That is the puzzling part. I can even downshift to 3rd, and it won't lockup. It is like there is something TELLING the ECM to unlock, but I can't determine what it might be.

    Thoughts? 173500 miles, synthetic in for the last 100K, no odd noises like fluid restrictions. I don't think the trans has anything to do with it because the ECM seems to be commanding the VCC Off at this time.

    Thanks!
    Tom

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  3. #2
    highonchai's Avatar
    highonchai is offline Cadillac Owners Fanatic
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    Re: VCC Problem

    What about Diag. trouble codes? is it spitting out any of those?

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    N0DIH's Avatar
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    Re: VCC Problem

    Not a one. The last one was some time ago (like 6+ months) was the PRND switch. And it seemed like only 1st gear had an issue, but hasn't come back since, so I ignore it. It is a silent one anyway....

    Tom

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    carnut is offline Cadillac Owners Master
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    Re: VCC Problem

    Is the TV cable adjusted by the book? Maybe the cable is adjusted short allowing late shifts. Assuming you have a service manual, release the retainer and see if the cable housing extends, or have a trans shop verify proper adjustment. If it shifts late it will also delay VCC.

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    Night Wolf's Avatar
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    Re: VCC Problem

    OT.....

    but what is the VCC? I know int he service manual it tlks about it a little... and how it is speical to Cadillac, makes the drivetrian smooth etc.... what I mean is, what exactly does it do?

  7. #6
    BeelzeBob's Avatar
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    Re: VCC Problem

    I think that it may just be operating normally.....if the PCM is commanding the VCC to disengage (as you are indicating by the icon on the climate control panel) then there is a combination of throttle angle, vehicle speed, load, etc... that is requesting the VCC to be disengaged. The VCC control algorithm is not just a simple on/off arrangement with speed or gear. It is not simple at all.

    There are many factors taken into account for VCC operation. Since releasing the VCC or disengaging it will appreciably improve performance it is common to see a VCC disengage when opening the throttle. This allows the open torque converter to allow an engine speed flare and some slight torque multiplication in the converter to improve part throttle acceleration performance without resorting to a down shift. You do not have to go very deep into the throttle to get a VCC release under some conditions so likely that is what is happening accelerating onto the expressway.

    There are also many delay timers that inhibit VCC reapply after a load induced release or a trans shift. This is to avoid a double bump on shifts and to cut down perceived shift business...i.e...if you had a VCC load release due to a throttle movement and then it reapplied immediately when you lifted your foot it would have an additional bump and would feel busy. Also, many times a load release is followed by another load release due to traffic and such so the delay will prevent the VCC from reapplying only to have to release again.

    Without writing a book it is almost impossible to describe all the parameters that govern the VCC apply and release. If the VCC is operating according to the icon on the climate control panel then it is OK. If there are no codes causing the PCM to disable the VCC then the PCM will decide based on it's VCC control algorithm and the operating parameters that it is monitoring and apply the VCC at the proper times to maximize fuel economy while cruising and minimize any driveability pertubations caused by the VCC applying and releasing.

    Was this something you noticed watching the PCM parameters on the diagnostics or is it a driveability issue?? If it is just something that you noticed playing with the diagnostics then I wouldn't worry about it... You will go nuts trying to second guess the PCM on the VCC control...LOL. That is why we do not put the instructions on operating the onboard diagnostics in the owners manual.

    Understand that there is a difference in perceived throttle movement/throttle opening and whether the engine is unthrottled or not. At low RPM like 4th gear and 45 MPH you do not have to open the throttle very far to unthrottle the engine. In otherwords, even 1/3 mechanical throttle opening is enought to allow enough air to enter the engine at low RPM so as to unthrottle the engine completely. You can tell this by watching the PCM parameter for MAP in the onboard diagnostics while you drive. If the MAP is approaching 95 or so (or whatever it is when you do hold the throttle wide oepn) then the engine is unthrottled...i.e...it is running at max power. If you crowd the throttle lightly at low engine speeds and the MAP goes to 95 (or whatever it is when you hold your foot to the floor) then the engine cannot make any more power and the VCC will release to allow some additional acceleration without a downshift.

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    Re: VCC Problem

    Quote Originally Posted by Night Wolf
    OT.....

    but what is the VCC? I know int he service manual it tlks about it a little... and how it is speical to Cadillac, makes the drivetrian smooth etc.... what I mean is, what exactly does it do?

    There is a clutch inside the torque converter that locks the pump and turbine elements together so that the torque converter will no longer slip. This improves fuel economy as it gets rid of the inherent slippage of a fluid coupling. The clutch is a mechanical connection just like a clutch in a manual transmission car. It has a clutch face and clutch material and a spring damper system to soften the engagement of the clutch when it applies. This is typically called a TCC for torque converter clutch.

    One problem with a TCC is that the driveline is no longer isolated from any load pertubations from the engine or loads telegraphing back up the drive axles from the road surface itself. This manifests itself as driveline roughness called chuggle. Chuggle is what you feel when you drive a manual transmission and have the trans in too high a gear at low speeds and try to accelerate...that roughness or light bucking that you feel. That is chuggle. Since the torque converter can normally slip this never happens with an automatic. But with the TCC locked the unit is just like a manual trans with a solid clutch connection. Chuggle is not acceptable in a luxury car. Especially a Cadillac. And, unfortunately, the transverse FWD arrangement, whether in a Cadillac or other car, tends to drastically accent or multiply the chuggle effect.

    To eliminate it, the TFWD Cadillacs have a special TCC unit that utilizes a viscous coupling inside the torque converter that serves as the TCC. This is called a Viscous Converter Coupling or VCC. It works just like a TCC except that when applied the viscous coupling in the TCC mechanism allows the unit to creep ever so slightly to isolate the driveline from any load pertubations and minimize the chuggle effect.

    The VCC unit itself is a sealed unit that consists of interlocking rows of concentric rings or tall ridges with a very very viscous silicone fluid filling the void between. The power is transmitted thru the silicon fluid and the unit can slip or creep by shearing the silicone fluid. This fluid is VERY "thick"...like the proverbial molasses on a freezing day. So it takes a LOT of force to shear it and cause one side of the VCC connection to slip relative to the other. So the unit, when applied, locks the torque converter 95% or so. Enough that you get the full advantage of a locked torque converter for fuel economy improvement yet you get the smoothness associated with an open torque converter as the slight slippage isolates the drive line effectively.

    Type in "VCC" and "TCC" into the search feature and read the past posts I put up on this for more info.

  9. #8
    N0DIH's Avatar
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    Re: VCC Problem

    It is something I have been looking into because I am very aware of the operation of the car (GM ECM's are sort of a hobby). I know when typically it always is or should be in locked up mode. For some reason on this particular road and conditions, is unlocks fully, (no indicator light), and you feel it unlock, which I expect it to with this throttle, but the problem is settling down to 65 to 75 mph on cruise with a slight uphill then a LONG gentle downhill SHOULD by all means be locked, it won't, just won't. It stays that way for a LONG time, around 10-12 miles, then after a long moderate uphill, just locks up like normal.

    I call it strange operation for this. It is almost like it kicks out of closed loop to open (but still shows closed) loop and doesn't return for a while. When it locks, you can feel the PWM of the ECM to gently tighten up, but at this particular time when it unlocks, it is a little more abrupt than "normal" driving on any other area. If I approach the same area at slower speeds (like 40 when I have the typical slowpoke in front of me), it will stay locked longer, but as soon as I go to pass, it unlocks, and doesn't return. Almost like the area is a steep incline, but doesn't appear to be.

    No, I know it isn't operating properly. But I can't seem to pin down WHY the PCM doesn't want to command it. Cruise, 65 mph, very light load (I can look at the MAP readings next time) even, where at almost anytime, it would be locked up/engaged fully.

    I guess what I would want to know is:
    1. Does the light in diagnostics come on based on a complete loop of the power through the VCC? Or is it independant and only shows when the VCC is commanded from the PCM, and doesn't even know if the VSS is actually present or not.

    2. Is there anything in the A/D lineup that would be an override to to tell the VCC NOT to lockup at all, like Engine overheating, or trans overheating, or another input not in the normal TPS/Coolant temp/vehicle speed/eng speed/MAP/3rd gear engaged switch/4th gear engaged switch ? What other inputs have an input on the VCC lockup speed?

    Maybe that is it? My 85 Cutlass was the OI code trans which included a oil pressure switch that the other models did not have that would tell the ECM that the trans is "NOT in 3rd gear". There was a separate 3rd gear switch. But the ECM would not allow TCC at all. After digging up the oil pressure switch (again, unique to OI code THM200C trans), and determining the input was a logic low to the ECM in all but 3rd, and logic high in 3rd, I determined I could cut the wire and all would return to "normal". It did. I suspect this was to prevent the trans from getting TCC locked up without the trans in 3rd gear or to early in 3rd gear (It was a 2.14 geared car). Most ECM's/PCM's and other A/D input lines are "floating high", which means they have a pull up resistor like a 100K to +5v or +12v. So when you cut the input it will "float" high. This prevents non logic inputs from getting into the A/D's on the ECM/PCM, preventing "confusion".

    I'll be digging in the book this weekend.... If there is a pressure switch in addition to the 3rd gear and 4th gear switches, that would explain a lot. With age the trans pump may put out less pressure or an internal leak will develop and the oil pressure related switches will engage/disengage at different speeds.

    Tom

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    Night Wolf's Avatar
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    Re: VCC Problem

    Thanks Bbob! now i know exaclty what it does...

    ... I noticed that chuggle int he Olds, when it is in OD and shoduln't be etc.... now that you mentioned that, I have never had that happen in the '93, so it is doing its job!

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