Attention: TDR Forum Junkies 
I have a 2004.5 Auto. The cruise control works on flat ground, but when I hit/hold "accel" or come upon a hill the cruise control disengages and will not operate again until the truck it turned off and back on (one key cycle).
To clear the air, the 2004.5 Automatic trucks have a vacuum pump and vacuum operated cruise control solenoid that pulls a cable to control throttle position and thus vehicle speed. Manual transmission trucks of this year do not use a vacuum system.
To date:
1.) I've checked my vacuum solenoid, it holds vacuum - I bench tested it by applying 12V to the proper solenoids and then used a vacuum pump to actuate the vacuum diaphragm to approximately 50% of the total stroke. The diaphragm held vacuum for 20 minutes with no loss of vacuum. - Its good.
2.) I detached the vacuum line from the CC solenoid and pulled vacuum on the line back toward the vacuum pump and reservoir, this system held vacuum for 30 minutes with no loss of vacuum. - FYI - Like many on this forum, my hard plastic line that connects to the solenoid had rub spots but mine had not yet rubbed through.
3.) I instrumented and watched operation of the vacuum solenoid control signals; specifically voltage at: the Vacuum, Vent, and Dump solenoids, and plumbed in a vacuum gauge to monitor system vacuum and did a road test.
Observations:
A.) Vacuum level barely registered on my gauge (which is a 0 - 30" gauge), I'd say it registered almost 1" of vacuum and barely moved the needle; which leads me to my first question: How much vacuum should this system pull? I feel this level of vacuum is too low but I don't know the system. The FSM does not state what constitutes an acceptable vacuum level.
B.) Since power is supplied to the cruise control solenoids through the brake switch, I monitored the voltage after the brake switch to see if my switch may have been faulty and inadvertently de-energizing the solenoids. This is the violet wire with white tracer (VT/Wh). During the road test, performed on flat road, it held speed until I asked for more by pressing and holding the accelerate/resume button. It began to accelerate, then kicked out and stopped working, the voltage on the violet/white wire went from 14V to 0V de-energizing all of the solenoids and dumping vacuum in the diaphragm - thus cruise control is disengaged. After this occurred the cruise could not be re-engaged by pressing Set or Resume. "Cruise" text is still displayed on the instrument cluster. I turned cruise control off then on, and it will still not work or set speed. I have to stop and key cycle the truck to get cruise to re-engage and the same problem repeats when I hit Accelerate or I come upon a hill and more throttle is needed to hold speed.
C.) I did NOT monitor voltage on the Violet/Yellow (VT/YL)wire that connects from ECM to brake switch, but will tomorrow because I suspect the ECM is de-energizing the VT/Y wire which would de-energize the VT/Wh wire. note: there are two VT/YL wires in this system, the Source wire connecting ECM power to the brake switch, and the Solenoid wire which connects the Vacuum solenoid back to the ECM.
How this system works:
The power to control the vacuum solenoid (located under driver-side battery), and thus all of your cruise control functions, is sourced/supplied by the ECM on a VT/YL wire. The ECM supplies 12V to this wire when the cruise function is turned on. This wire connects to the vacuum solenoid through the brake switch. When it gets to the brake switch it lands on a contact of a normally closed switch (pedal up = switch closed). On the other side of that normally closed switch a VT/Wh wire leaves and goes to the vacuum controller. This VT/Wh wire connects and applies voltage to one terminal of each of the three solenoids: Dump, Vent, and Vacuum. The second terminal of the Dump solenoid is always connected to ground through the black wire; thus, when the VT/Wh wire is energized the Dump solenoid operates (plunger moves) and "plugs" a "vacuum dump hole." If the Dump solenoid happens become de-energized, by brake switch opening or turning off the cruise from the steering wheel control, it dumps vacuum and the cruise stops applying throttle (fail-safe type design). The other terminal of the "vacuum" and "vent" solenoids are connected back to the ECM. At the ECM these two solenoid "ground-side" terminals are controlled by intermittently grounding them which allows current to pass through the solenoid(s) operating the plunger. This architecture allows the ECM to "pulse" or intermittently ground these solenoid terminals which tunsthe solenoids on and off as necessary. This adds or subtracts vacuum from the vacuum diaphragm which increases or decreases throttle respectively.
My theory - yet to be proven...
If there is not enough vacuum to operate the vacuum diaphragm properly, due to vacuum leak or poor vacuum in general (i/e/ bad pump), the system calculating an error and de-energizes the entire cruise control power system. I suspect that if the ECM is "calling for more throttle" by pulsing the "Vacuum" solenoid but does not see the throttle position sensor position changing enough, or at all, it determines the "call for more throttle" input is not making the desired "increased throttle position" output and de-energizes the VT/Y wire because that constitutes an error. After this error the computer does not allow the cruise control to be re-engaged until the key is turned off for at least 10 seconds and then back on.
Next Step:
I will check the rest of the vacuum system tomorrow and the voltage on the VT/YL wire when the cruise malfunctions.
I don't think the brake switch is the cause, the VT/YL wire test will confirm.
I don't have any leaks between the pump and vacuum diaphragm
I don't have a good schematic of the vacuum system, will trace out all lines tomorrow.
Does anyone have a vacuum specification for that pump, i.e. what is the vacuum level supposed to be? 1" of H2O, which i measured, seems low.
Please advise.
To clear the air, the 2004.5 Automatic trucks have a vacuum pump and vacuum operated cruise control solenoid that pulls a cable to control throttle position and thus vehicle speed. Manual transmission trucks of this year do not use a vacuum system.
To date:
1.) I've checked my vacuum solenoid, it holds vacuum - I bench tested it by applying 12V to the proper solenoids and then used a vacuum pump to actuate the vacuum diaphragm to approximately 50% of the total stroke. The diaphragm held vacuum for 20 minutes with no loss of vacuum. - Its good.
2.) I detached the vacuum line from the CC solenoid and pulled vacuum on the line back toward the vacuum pump and reservoir, this system held vacuum for 30 minutes with no loss of vacuum. - FYI - Like many on this forum, my hard plastic line that connects to the solenoid had rub spots but mine had not yet rubbed through.
3.) I instrumented and watched operation of the vacuum solenoid control signals; specifically voltage at: the Vacuum, Vent, and Dump solenoids, and plumbed in a vacuum gauge to monitor system vacuum and did a road test.
Observations:
A.) Vacuum level barely registered on my gauge (which is a 0 - 30" gauge), I'd say it registered almost 1" of vacuum and barely moved the needle; which leads me to my first question: How much vacuum should this system pull? I feel this level of vacuum is too low but I don't know the system. The FSM does not state what constitutes an acceptable vacuum level.
B.) Since power is supplied to the cruise control solenoids through the brake switch, I monitored the voltage after the brake switch to see if my switch may have been faulty and inadvertently de-energizing the solenoids. This is the violet wire with white tracer (VT/Wh). During the road test, performed on flat road, it held speed until I asked for more by pressing and holding the accelerate/resume button. It began to accelerate, then kicked out and stopped working, the voltage on the violet/white wire went from 14V to 0V de-energizing all of the solenoids and dumping vacuum in the diaphragm - thus cruise control is disengaged. After this occurred the cruise could not be re-engaged by pressing Set or Resume. "Cruise" text is still displayed on the instrument cluster. I turned cruise control off then on, and it will still not work or set speed. I have to stop and key cycle the truck to get cruise to re-engage and the same problem repeats when I hit Accelerate or I come upon a hill and more throttle is needed to hold speed.
C.) I did NOT monitor voltage on the Violet/Yellow (VT/YL)wire that connects from ECM to brake switch, but will tomorrow because I suspect the ECM is de-energizing the VT/Y wire which would de-energize the VT/Wh wire. note: there are two VT/YL wires in this system, the Source wire connecting ECM power to the brake switch, and the Solenoid wire which connects the Vacuum solenoid back to the ECM.
How this system works:
The power to control the vacuum solenoid (located under driver-side battery), and thus all of your cruise control functions, is sourced/supplied by the ECM on a VT/YL wire. The ECM supplies 12V to this wire when the cruise function is turned on. This wire connects to the vacuum solenoid through the brake switch. When it gets to the brake switch it lands on a contact of a normally closed switch (pedal up = switch closed). On the other side of that normally closed switch a VT/Wh wire leaves and goes to the vacuum controller. This VT/Wh wire connects and applies voltage to one terminal of each of the three solenoids: Dump, Vent, and Vacuum. The second terminal of the Dump solenoid is always connected to ground through the black wire; thus, when the VT/Wh wire is energized the Dump solenoid operates (plunger moves) and "plugs" a "vacuum dump hole." If the Dump solenoid happens become de-energized, by brake switch opening or turning off the cruise from the steering wheel control, it dumps vacuum and the cruise stops applying throttle (fail-safe type design). The other terminal of the "vacuum" and "vent" solenoids are connected back to the ECM. At the ECM these two solenoid "ground-side" terminals are controlled by intermittently grounding them which allows current to pass through the solenoid(s) operating the plunger. This architecture allows the ECM to "pulse" or intermittently ground these solenoid terminals which tunsthe solenoids on and off as necessary. This adds or subtracts vacuum from the vacuum diaphragm which increases or decreases throttle respectively.
My theory - yet to be proven...
If there is not enough vacuum to operate the vacuum diaphragm properly, due to vacuum leak or poor vacuum in general (i/e/ bad pump), the system calculating an error and de-energizes the entire cruise control power system. I suspect that if the ECM is "calling for more throttle" by pulsing the "Vacuum" solenoid but does not see the throttle position sensor position changing enough, or at all, it determines the "call for more throttle" input is not making the desired "increased throttle position" output and de-energizes the VT/Y wire because that constitutes an error. After this error the computer does not allow the cruise control to be re-engaged until the key is turned off for at least 10 seconds and then back on.
Next Step:
I will check the rest of the vacuum system tomorrow and the voltage on the VT/YL wire when the cruise malfunctions.
I don't think the brake switch is the cause, the VT/YL wire test will confirm.
I don't have any leaks between the pump and vacuum diaphragm
I don't have a good schematic of the vacuum system, will trace out all lines tomorrow.
Does anyone have a vacuum specification for that pump, i.e. what is the vacuum level supposed to be? 1" of H2O, which i measured, seems low.
Please advise.
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