What and where is the servos - 1999 GMC Jimmy

A hard brake pedal is usually caused by an air leak from engine to brake servo (brake booster) or a defective servo/booster itself.

Have a look in your engine bay. The brake servo is usually at the rear, on the engine wall. It's circular, has the brake master cylinder attached. There should be a black hose/tube .. check that tube for security and splitting. You may have to change the servo/booster.

Here's some generic images of what it looks like:
brake servo Google Search

It sounds as if you may have a leak on the servo, this will make the brakes go hard as the air leaks out. This can also make it hard to start as the mixture is affected by the extra air getting in. Without servo assistance on brakes the pedal will be hard. To test start the engine if you can with your foot on the brake, if it starts and the pedal does not go down, the servo is not working. Obviously you need the engine running for this, but you can also check the airline going to the servo for holes, normally this goes from the inlet manifold to the servo. This image shows the servo with the hose leading out, you can follow it to the manifold, make sure the engine is not hot before touching anything. typical brake servo connection on car engine Google Search

5R55W -2002 Ford Explorer - Servo Replacement Saga. - Ford ...

https://www.explorerforum.com/forums/showthread.php?t=380199

Oct 18, 2014 - 19 posts - ‎4 authors
I have a 2002 Ford Explorer XLT. ... My plan was to replace both Overdrive and Intermediate servo's with a new pair that was ... Lifted truck off the ground. ... With both snap rings removed, I removed the servo caps. .... Elite Explorer members canchange the forum "styles" to eliminate most ads, including the ...Missing: intermed

blown out servo cover - 5r55e - Ford Explorer and Ranger Forums ...

www.explorerforum.com/forums/showthread.php?t=346555

Feb 19, 2012 - 15 posts - ‎4 authors
blown out servo cover - 5r55e Explorer & Ranger Transmissions, Transfer ... 02-18-2012, 02:08 PM ... Got underneath and discovered that the OD servo cover had ...Once I get the cover back on I could just top off the fluid and hope for the ... EliteExplorer members can change the forum "styles" to eliminate ...Missing: intermed

Hi,
This may be similar to what I just fixed on an '00 dodge:
http://www.dodgeintrepid.net/showthread.php?t=89711

Accessing the Integrated Diagnostic System:

Procedure for Automatic Temp Control (ATC) equipped vehicles:

With the engine ON and the vehicle stopped set the temp to 75*F. Next hold down the Floor, Mix and Defrost buttons until the ATC display begins to flash. While the display is flashing, the system will run it's tests and calibrations.

When the display stops flashing, the tests and calibrations are complete.

The ATC display can only display one error code at a time. Any stored codes can be scrolled through by pressing the Panel mode button once for each code. DO NOT press any other button or you will have to start the test over. Note the codes and refer to the list below to determine what system is malfunctioning.

List of Climate Control DTCs:
23: Blend door actuator feedback failure (bad servo)
24: Mode door actuator feedback failure (bad servo)
25: Ambient temp sensor failure
26: ATC In Car thermister failure (ATC only, bad ATC head or ATC head disconnected while battery is connected)
27: ATC in car sensor failure (ATC only)
31: Recirculation door actuator stalled (stuck door or bad servo)
32: Blend door actuator stall failure (stuck door or bad servo)
33: Mode door actuator stall failure (stuck door or bad servo)
34: Engine temperature message not received (ATC only, bad ATC head or bus failure)
35: Evaporator sensor failure
36: ATC control communication failure (bad ATC head or bus failure)
37: Blend door actuator output shorted to battery (bad servo or wiring fault)
38: Blend door actuator output shorted to ground (bad servo or wiring fault)
39: Mode door actuator output shorted to battery (bad servo or wiring fault)
40: Mode door actuator output shorted to ground (bad servo or wiring fault)
41: Recirculation door actuator output shorted to battery (bad servo or wiring fault)
42: Recirculation door actuator output shorted to ground (bad servo or wiring fault)
43: Common door shorted to battery (probable wiring fault, could be a servo that failed in an odd manner)
44: Common door shorted to ground (probable wiring fault, could be a servo that failed in an odd manner)
45: A/C control blend door input open or shorted with respect to ground (MTC only, wiring fault or bad servo)
46: A/C control blend door shorted to battery (MTC only, wiring fault or bad servo)
47: A/C switch failure (MTC only)
48: A/C control mode door input shorted to ground (MTC only, wiring fault or bad servo)
49: A/C control mode door input shorted to battery (MTC only, wiring fault or bad servo)
50: A/C control electric backlite (EBL) switch fails (MTC only)

51: System voltage too low for calibration!!! (check battery cables, battery and alternator!)

Check that vacuum line to the servo or booster (as it is sometimes called) is attached with no holes in it if that looks good then its probably the servo. the servo is what your master cylinder bolts onto. this happened to mine, to replace simply unbolt master cyl. from serve the unbolt servo by reaching under dash just above brake pedal and pulling carpet down and remove the 2 or 3 nuts or bolts holding servo then pull servo from firewall and replace.

Hi
Sounds like the servo is shot !

Clamp off the vacuum pipe,to the servo with a brake hose clamp, and if noise disappears, you need a new servo unit.

A FixYa would be nice, Thank´s.....

Good Luck !

John.

You cannot bleed a servo. it runs using the vacuum from either inlet manifold, or a seperate vac pump.Ensure pump is working if fitted (end of camshaft) the pipes to the servo are not leaking by removing the pipe from servo and start veh. If there is good suction then its servo failure. hard pedals can also be caused by pressure plate fatigue in the clutch itself.

Code 34 is set when the computer does not detect a voltage to and through the speed control servo. The cruise control computer is built into the PCM, which grounds two windings in the servo mechanism to regulate the vacuum applied to the servo diaphragm.
Note: All Chrysler corporation fuel-injected engines have the computer circuitry and software for the cruise control system installed, even if there is no cruise control installed on the vehicle. If there is no cruise control installed on the care or truck, the computer will interpret this as an open winding and set code 34.
First: turn the key ON, turn the cruise switch ON, and look for +12 Volts on the dark blue/red wire that connects to the servo mechanism. This circuit is fed:

• from the ignition switch through to 10 (dark blue wire)
• from fuse 10 to the steering column (white/red wire)
• through the clock spring to the cruise control ON/OFF switch on the steering wheel (white/red wire)
• back through the clockspring to joint connect "C" (yellow/red wire)
• to the brake lamp switch under the dashboard (yellow/red wire)
• through the firewall from the brake switch (dark blue/red wire)
• to the servo (dark blue/red wire) A break anywhere in this path (broken wire, bad brake switch, loose connector, etc.) will interrupt the 12 volt supply to the servo and set a fault code. Work backwards until you locate the break.
  

Turn the key OFF. Unplug the the connector from the servo and measure the servo coil resistances.

• measure the resistance between the servo pin that connects to the tan/red wire and the servo pin that connects to the dark blue/red wire.
• measure the resistance between the servo pin that connects to the light green/red wire and the servo pin that connects to the dark blue/red wire. The resistance in both cases should be above 0 ohms and less than infinity.
  

Disconnect the battery(s). Unplug the 60 pin connector from the PCM and check for continuity between the pcm connector and the servo connector for the tan/red wire (cavity 33) and the light green/red wire (cavity 53).
If no wiring problems are found, check the PCM connectors for corrosion, and reconnect. Reconnect the battery and test the circuit again. If the problem still exists, the PCM is probably bad. NOTE: Personally, if the truck was out of warranty I would try to find for a used servo before buying a new PCM.

On the passenger side of the trans there are two servos.A servo is a steel piston with rubber lip seals bonded to the steel.The servo piston strokes in or out,applying or releasing a band.There are two servos,each hidden behind circular aluminum covers held into the case with snap rings.The rear servo actuates the 2nd gear band,and the front servo actuates the 4th gear,or O.D. band.The rubber lip seals on the servo pistons are soft and pliable when new,but with hot and cold cycles,and years of use,the rubber hardens and doesnt seal very well,causing loss of gears.It is very possible the servo has hardened,and that's why you are losing 4th.The other possible cause is electronic.This trans,the A4LD has 2 electrical solenoids inside the trans,4th gear solenoid and lockup solenoid.These are the ONLY electronics inside the trans,another possible problem area.I would remove the servos first for inspection,but before removal,take a scribe and scratch into the aluminum covers,"F" on the front one,and "R" on the rear one.These servos are different diameters,and MUST go back where they came out of.On the servo covers you will see cast into the covers 2 letters identifying the servo it requires....AB.....BB.....ZC...etc.These are examples,and yours will likely be AB and BB.If you DIY....use plenty of vaseline on the rubber lip seals,and the servo bores, to prevent cutting the new servo seals and O rings.Good luck.

Servo motor? I disagree. Actually, there are 3 servo motors in the vehicle; an air inlet servo, an air mix servo, and an air outlet servo. None of which control the temperature.
The AC temp is controlled by a thermister in the cooler box/eveporator unit under the dash.