Engine/Transmission (1994 - 1998) High idle from cruise control

[Mhannink]

ok so the fsm says "the speed control electronic circuity is integrated into the pcm" cerb help me out. is that what you encountered with the first gen? if so then i might have the same problem you did that being its a pwm signal to the servo. the way i read it though is that the switching is done in the pcm and that sends out a voltage signal to the servo.



the way i read the fsm is the pcm monitors all inputs and outputs and adjusts the servo according to those inputs. what i get from that is the inputs will vary, the pcm takes them in and then sends a voltage signal to the servo to adjust it according to the inputs. that makes me think my idea will work. i do want diodes for safety but may not be needed. what i might do tomorrow if i have time is power the pins myself with the computer disconnected. it will saftely give me an idea of if im headed in the right direction or not. i will also get an idea as to the amount of control needed. we will see.



as to the vss and overrideing it, it is a pulse generator on the trans. its not a twisted pair of wires so i would think that it is pwm to the pcm. if you did jump it to get the cruise to work the problem would be the speedo and odometer. you would rack up miles real fast whenever you go to high idle.



just so everyone knows i enjoy and even want the thoughts, ideas, opinions, and even the opposition to my ideas. it keeps all of us on our toes and exploring all the alternatives to a problem. it definately keeps me thinking about what i am doing and helps out with finding the best solution. just be ready ya'll for me to defend my ideas, and be ready to defend yours as i will try to pick them apart. it is nothing personal i just think it helps expose all angels.

 

[Mhannink]

I go away a for a day or so, and this thread takes off. Sweet. I have not had any time myself to poke at this system, but I might tomorrow after reading all of these posts.



I will use my PowerProbe III to give a quick pulse to either pin 1 or 2 and see what happens. I just really want to have a warmer truck in the morning.



And a side thought: if a short pulse is needed, can't i just hijack an unused circuit from my remote start (Python 1401) that can do momentary pulses at low amperage?



Thanks everybody for getting interested in this topic Oo.

what is funny is it a year almost to the day that it suddenly takes off. i am guessing by your post that you have not worked on this past the point of last post before my first one? i was hoping some of the original people might have some input based on experience.



what is interesting is i went back and looked at your pic of the wiring diagram and compared it to my fsm. they are different. what year was yours from? mine is for 97 which is obd. i looked at my cousins truck early on and there are some physical differences. maybe the 94 and 95 are different from the 96 and 97.



i dont know how well it will come out, but i took a pic of mine with my phone and uploaded it so we can compare. maybe it will help

 

[JKosten]

what is interesting is i went back and looked at your pic of the wiring diagram and compared it to my fsm. they are different. what year was yours from?

I have a digital copy of a 96 fsm.

And has it really been a year since I asked the question? #ad
I think it may have something to do with the frosty windows...

 

[mwilson]

What if you could control the vacuum some other way for the high idle, never mind the rest of it???



I have a ton of stuff going on but I will attempt to take my Mighty-Mite this weekend and just try to see what it takes for vacuum directly to the servo to get 1200 rpms for example. Maybe it will just keep taking off or be too unsteady, but I am going to try it.



If it was steady perhaps we could McGyver a regulated second vacuum loop... ... . that would actuate with a solenoid and dash switch??????????????



Mike.

 

[fest3er]

When you control the servo from outside the PCM and the PCM is not actively controlling vehicle speed, then the PCM will simply not see the change in the servo; it cannot know what the servo is doing. If you manually set the servo to position X, that position will not change until you change it, until the servo is powered off, or until you engage vehicle speed control via the PCM.

If the servo is in good working condition, it is more stable than your foot. Once a servo position is set, it will change for only the following reasons:

• The vacuum solenoid is opened (increasing RPM).
• The vent solenoid is opened (decreasing RPM).
• The CC On/Off button is pressed (PCM disconnects power to servo).
• The brake pedal is pressed (interrupting power to the servo).
• The engine was cold (frigid) and is warming up.
• The properties (quality) of the fuel being injected changes significantly.
• The properties (quality) of the intake air changes significantly.
• Fuel supply is exhausted.
• The engine loses mechanical cohesion (it falls apart or explodes).
• The servo is faulty.

If you place your foot on the go pedal and don't move it or if you put a throttle stick between the seat and the go pedal, you will do almost exactly the same thing the servo does: hold the P-pump's fuel rack in a specific position. This is true whether or not the PCM remains connected to the servo. And because the vehicle is not moving, the PCM will not adjust the servo in any way unless you click the CC On/Off button which will remove power to the servo.

 

[Nyoest]

Ok, ya got me with the odometer reading. I'll be in the truck.

 

[cerberusiam]

When you control the servo from outside the PCM and the PCM is not actively controlling vehicle speed, then the PCM will simply not see the change in the servo; it cannot know what the servo is doing. If you manually set the servo to position X, that position will not change until you change it, until the servo is powered off, or until you engage vehicle speed control via the PCM.

I hear what your saying but I don't think you can activate the servo motor without using the same control solenoid the PCM uses. The servo motor is really a seperate entity that reacts to how the control solenoid is set for vacum apply and vent.



By default, the sontrol solenoid vents all vacuum form the servo motor for safety. Any interuption cancels CC and the control solenoid returns to essentially an off position via spring pressure.



To achieve a set point the control solenoid is feed a PWM signal that sets and keeps it where it is needed to maintain speed, that is purely under control of the PCM.



You can feed voltage to the solenoid and the servo motor will react but you have to have some way to achive a set point to maintian a balance between vacuum and vent. Wihtout that the servo will just go to the limit and stay there.



Achieving a set point is not just voltage, it is a modulation of the voltage pulse to maintain a solenoid in a set position. That is typical automotive design implementation becuase it is effective.

If you place your foot on the go pedal and don't move it or if you put a throttle stick between the seat and the go pedal, you will do almost exactly the same thing the servo does: hold the P-pump's fuel rack in a specific position. This is true whether or not the PCM remains connected to the servo. And because the vehicle is not moving, the PCM will not adjust the servo in any way unless you click the CC On/Off button which will remove power to the servo.

The control cable is free floating either at the throttle end or the servo end depending on design. Working the throttle is not the same as activating the servo. Throttle operation is totally independent of CC operation.



If you try to activate the CC control valve externally to the PCM it will see that operation and may not like it.

 

[JKosten]

So I went out and finally looked at the cruise control system.

Condition: engine running, transmission in neutral, clutch disengaged, p-brake engaged, and cruise control light on.

I back probed both V36 and V35 (the tan and light green wires) at the PCM and they both had 13. 8 vdc.

Based on that info I believe that the PCM is supping current to both sides of the solenoid in order to hold it at a neutral position.

I want to try putting a 2 pole on-off-on momentary in line from the PCM to vehicle speed control servo in order to disconnect power from one side in order to activate the servo. then once momentary is released it *should* hold it at set rpm until something deactivates the system, like a brake input.

Any thoughts?

 

[cerberusiam]

I back probed both V36 and V35 (the tan and light green wires) at the PCM and they both had 13. 8 vdc.

Do the same thing with the cruise engaged at different speeds and see what your voltage is.

 

[JKosten]

That will take a little more work... And some test leads into the cab. Will get to it soon.

 

[fest3er]

... To achieve a set point the control solenoid is feed a PWM signal that sets and keeps it where it is needed to maintain speed, that is purely under control of the PCM. ...

Then my '98 has a very non-standard servo.



I'm not guessing here. I actually did this:

• I unplugged the connector from the servo
  
• I applied +12VDC to pin 3.
  
• I applied GND to pin 4.
  
• I momentarily applied +12VDC to pin 1
  

The result is that the engine RPM increased and stayed there. The engine speed decreased only when I applied +12VDC to pin 2 or when I disconnected pin 3.



The servo performs one function, and one function only: to set the gas pedal (throttle, fuel rack) to a certain position and hold it there. It does that extremely well when it is in proper working order (e. g. , no leaks).



I think you keep jumping beyond this simple 'throttle lock' (which is what most of us are talking about) to a complete, closed-loop, sensor-based control system that will maintain engine RPM regardless of the load on the engine. In that, you are absolutely correct. But you may have the PCM's part wrong. I would expect that the PCM supplies a narrow pulse to pin 1 to increase fueling when the vehicle has fallen below the set speed, or it supplies a narrow pulse to pin 2 to decrease fueling when the vehicle has risen above the set speed. The PCM nearly constantly increases and decreases fueling to maintain the vehicle's speed; 'nearly' because there is probably a +/- 1/2 MPH tolerance programmed into the system.



It makes no sense for the PCM to keep the vacuum *and* vent solenoids both open at the same time. If the vehicle is rolling along a road that is perfectly smooth and perfectly tangential to gravity, and the air is uniformly dense, the PCM should open the vacuum solenoid to increase fueling until the vehicle has reached the set speed. After that, the PCM should send no signals to the servo because the vehicle's speed won't change.



The function of a servo is to go to a position and stay there until commanded to go to another position. My experiment with the servo on my '98 proved to me that my servo works in that fashion: once the servo has moved to a position, it stays there without further control input. The position will change only when either solenoid is commanded to open, when power is removed, or when the servo develops an air leak.



Using the servo in this fashion is nearly identical to using a throttle stick. It is a fixed, invariant setting. It has no closed-loop feedback control system. Momentarily apply B+ to pin 1 until the unloaded engine's speed is where you want it, then walk away. Hours later the engine will be at that same speed if it was warm to begin with. If the speed does change, then the servo has a air leak and should be replaced.



To be honest, I could be mistaken. Pins 1 and 2 could be 'floating' at B+, waiting for the PCM to momentarily ground them, thus completing the circuit and commanding that solenoid to open its valve. I don't remember exactly; it's been a couple years. But I do know that once I set the RPM above idle, it stayed there with no further control input.

 

[cerberusiam]

Then my '98 has a very non-standard servo.

Maybe not non-standard just different. There were at least 2 different setups used and not sure there was any rhyme to which trucks got what.



The one I tested just went wide open with 12 volts then back to off when the power was removed. There was no set on it. Figured that had to be a function of the PCM and since the PWM controls were so popular it was that type.

 

[Nyoest]

Since its the topic of the day, can any one tell me how to modify the cruise for a higher horse truck? When I use the cruise it applies at an almost alarming rate, hits target speed and defuels. It drops speed so here we go with some more 1. 5 g acceleration action. And then it gets real fun on hills. Doesn't work too bad loaded though.

 

[fest3er]

The one I tested just went wide open with 12 volts then back to off when the power was removed.

How *long* did you have 12V applied to the pin? 0. 5 to 1 second is enough to go wide open (on mine).



You are correct in that a narrow pulse is used to control the solenoids; it doesn't take long to make a large change in position.



Nyoest, have you tried the 'retrain' trick: hold the pedal steady (real steady), and set/cancel CC 15 times (I think) fairly quickly. If that doesn't work, you'll need a circuit that delays the onset of each control pulse by a few hundred microseconds. Alas, even that's beyond my ken.

 

[JKosten]

Since its the topic of the day, can any one tell me how to modify the cruise for a higher horse truck? When I use the cruise it applies at an almost alarming rate, hits target speed and defuels.

I have noticed the same thing with mine on the highway, and since the DD doesn't slip and the engine mods are there, it lurches when the cruise control kicks in or you accelerate. Not sure how to fix that...

 

[Mhannink]

So I went out and finally looked at the cruise control system.



Condition: engine running, transmission in neutral, clutch disengaged, p-brake engaged, and cruise control light on.



I back probed both V36 and V35 (the tan and light green wires) at the PCM and they both had 13. 8 vdc.



Based on that info I believe that the PCM is supping current to both sides of the solenoid in order to hold it at a neutral position.



I want to try putting a 2 pole on-off-on momentary in line from the PCM to vehicle speed control servo in order to disconnect power from one side in order to activate the servo. then once momentary is released it *should* hold it at set rpm until something deactivates the system, like a brake input.



Any thoughts?

your results are what i was thinking would happen. it would mean that the wire is grounded through the computer. when i tried that though it did absolutely nothing except turn the cruise off (had to shut the truck off to restart the cruise).

Then my '98 has a very non-standard servo.



I'm not guessing here. I actually did this:

• I unplugged the connector from the servo
  
• I applied +12VDC to pin 3.
  
• I applied GND to pin 4.
  
• I momentarily applied +12VDC to pin 1
  

The result is that the engine RPM increased and stayed there. The engine speed decreased only when I applied +12VDC to pin 2 or when I disconnected pin 3.



The servo performs one function, and one function only: to set the gas pedal (throttle, fuel rack) to a certain position and hold it there. It does that extremely well when it is in proper working order (e. g. , no leaks).



I think you keep jumping beyond this simple 'throttle lock' (which is what most of us are talking about) to a complete, closed-loop, sensor-based control system that will maintain engine RPM regardless of the load on the engine. In that, you are absolutely correct. But you may have the PCM's part wrong. I would expect that the PCM supplies a narrow pulse to pin 1 to increase fueling when the vehicle has fallen below the set speed, or it supplies a narrow pulse to pin 2 to decrease fueling when the vehicle has risen above the set speed. The PCM nearly constantly increases and decreases fueling to maintain the vehicle's speed; 'nearly' because there is probably a +/- 1/2 MPH tolerance programmed into the system.



It makes no sense for the PCM to keep the vacuum *and* vent solenoids both open at the same time. If the vehicle is rolling along a road that is perfectly smooth and perfectly tangential to gravity, and the air is uniformly dense, the PCM should open the vacuum solenoid to increase fueling until the vehicle has reached the set speed. After that, the PCM should send no signals to the servo because the vehicle's speed won't change.



The function of a servo is to go to a position and stay there until commanded to go to another position. My experiment with the servo on my '98 proved to me that my servo works in that fashion: once the servo has moved to a position, it stays there without further control input. The position will change only when either solenoid is commanded to open, when power is removed, or when the servo develops an air leak.



Using the servo in this fashion is nearly identical to using a throttle stick. It is a fixed, invariant setting. It has no closed-loop feedback control system. Momentarily apply B+ to pin 1 until the unloaded engine's speed is where you want it, then walk away. Hours later the engine will be at that same speed if it was warm to begin with. If the speed does change, then the servo has a air leak and should be replaced.



To be honest, I could be mistaken. Pins 1 and 2 could be 'floating' at B+, waiting for the PCM to momentarily ground them, thus completing the circuit and commanding that solenoid to open its valve. I don't remember exactly; it's been a couple years. But I do know that once I set the RPM above idle, it stayed there with no further control input.

sounds like it does need power. i am going to go play with it now and see what i can find. i will be back later.

 

[Mhannink]

ok so i think i have it figured out.



i went out and started playing with the servo. long story short i ended up removing is and doing a partial disassembly. for starters, cerb dont take this personal but there is no computer or pwm circuitry. there is just 3 coils in side. one is power for the whole thing, and there is one each for each solenoid.



ok so the problem. what i did is powered up the main coil, thats pin 3, and i grounded pin 4, doing that i got an audible click. next i powered pin 1, i heard a click but nothing happened. servo could still move freely. so i powered up pin 2, again nothing. found out they have to be grounded (good thing less to worry about with the computer). so i scrificed myself and applied a vacuum to the vac side of the servo and started ground things out. what i found is in order to move the servo pin 1 and 2 have to be grounded. this closes the vent and open the vac and allows the servo to move. if pin 1 is grounded it will draw air from atmosphere through the servo. pin 2 is grounded it closes the vent and holds position.



so for high idle to work im thinking this way, and im doing this on the fly while i type so i might screw it up. pin 3 and 4 can be left alone, when you want high idle turn on the cc. pin 2 needs to be on a toggle switch to ground. this will force it to hold closed while idled up. pin 1 will be a momentary switch to ground. this will allow you to adjust vacuum in the servo. to go from high idle to normal operation you throw pin 2 toggle switch to open and it should work like normal. this will also cancel high idle too. thats how im seeing it, correct me if i am wrong, but i think that will work. this is all fine and good in theory, but now i am going to go back to my truck and see if i cant get it to work in reality.

 

[Mhannink]

ok so i put it all back together wired up pin 1 and 2 and drum roll please... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... it works, kinda.



ok so here is our next challange, the computer does not like me playing with it like this.



first with what i did

i hooked up two wires, one to each pin 1 and pin 2, with the truck running i grounded the vent side (pin 2) and held it there. then i touched the vacuum side (pin 1) to ground as well. the first couple of times it just turned the cruise off. every time i had to shut the truck off and restart before the l. e. d. would come back on. throughout all my playing with it, the cc would shut off sort of at random while i was grounding wires. granted i was going to battery which might make a difference on how it works, but it did work and did raise the idle.



now for the controllablity, its not as bad a some thought, in fact it is quiet easy. when it did work i could hold the wire there and it would rev all the way up, but it was not much trouble/difficult to bump it up only a few rpms. i dont know how many cause i couldnt walk around to look at the tach, but judging by ear it was right about where i would want it to be. so personally i dont think a timer device as originally disscused will be needed. however the problem of it wanting to intermittenly work is another issue. anyone have any ideas as to why the cruise is just shutting off sometimes and not others?

 

[Mhannink]

alright, i took a chance and powered the servo off the battery, it does back feed and turn the l. e. d. on the steering wheel on, im not too worried though as there is a resistor inbetween, i dont think the computer will have an issue. at least it does not seem to have a problem yet.



so to get consistent function the servo has to be powered off the battery. this let me play with the high idle without restarting the truck. i have a video but i dont know how to upload it. the main problem with controling the high idle is taking up the slack in the cable, thats what makes it tough to set it consistently. a "one shot" device would make it easier, but its not needed, just a little practice.



here is what i am thinking for wiring. i would like to run the power to pin 3 through a relay, on the control side of the relay i want to hook the wire off of pin 2 through the relay and to the switch. that way one toggle switch will switch both power and ground. then the momentary switch will ground the wire from pin 1 on one side and pin 2 on the other side. if i remember my electrical lectures correctly then when the toggle is switched it will ground pin 2 through the resistor, and switch power to pin 3 with the relay. my only concern is the extra load of the resistor will drop power in the servo and it wont be able to hold it closed. the momentary switch will ground both pin 1 and 2 giving you up and down control. if the relay idea works then when pin 2 is grounded through the momentary it will be an easier path to ground and will work. if the relay idea doesnt work then we will have to find something else. my reason for the relay is i do not want to mount 3 seperate switches just for high idle. 2 im ok with.

 

[JKosten]

Really cool findings. Have you thought about a diode between the computer and where you tap into the circuit to protect the system and prevent the cruise control from seeing the external power input?