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

[JKosten]

So I want a high idle for my truck now that we are going into winter.



My friend has a 06 and can use his cruise control to raise his idle for warm-up and go me thinking. After looking at the wiring diagrams and operation of the vacuum controller I have come up with an idea of how to do it for the cost of a couple switches and diodes.



This is from the FSM:



The PCM controls the vent and vacuum functions of the vehicle speed control servo on circuits V35 and V36. Depending on the signal it receives from vehicle speed control switches, the PCM either applies vacuum to or vents vacuum from the servo. Circuit V36 from cavity C4 of the PCM sends the vacuum signal to the servo. Circuit V35 from cavity C5 sends the vent signal.



Based on that info I look at the diagram and would insert a diode inline of the speed control switch output (12v from brake switch) and tie in a switch on the dash to allow the servo to turned on manually and not back feed the factory system. Then use a two way momentary switch (on-off-on) to either raise or lower idle as I choose from the cab.



Also in the FSM:



(4) Check for battery voltage at pin-3 of wiring harness 4-way connector (Fig. 3). This is the 12 volt feed from the stoplamp switch. When the brake pedal is depressed, voltage should not be present at pin-3. If voltage is not present with brake pedal not depressed, check for continuity between servo and stop lamp switch. Also check stop lamp switch adjustment. Refer to Group 5, Brakes for procedures.



(5) Connect a small gauge jumper wire between the disconnected servo harness 4-way connector pin-3, and pin-3 on the servo. Check for battery voltage at pins-1, 2 and 4 of the servo. If battery voltage is not at these pins, replace the servo.



That tells me that the PCM is controlling the ground side of the circuit for the servo and could be easily be done manually.





So what I am not thinking of? (and a safety shut off is not in design, but could be. )



Full size photo: #ad

 

[fest3er]

... Then use a two way momentary switch (on-off-on) to either raise or lower idle as I choose from the cab... .

I played with doing that when I was debugging my non-working CC some time ago. Using a momentary contact switch works, but very imprecisely. Your 'momentary' touch will result in 500-1500 RPM change.

To make it work right, you'll want to integrate a one-shot device (the IC is known as a 555 timer) between the button and the actuator so that when you push the button, a very precise pulse is generated. With experimentation, you should be able to set it so that each tap increases or decreases RPM by 100. It should also be possible to set it up so that holding the button will generate 5 pulses per second after a 1/2-second delay.

You don't need to isolate your system from the PCM. The PCM grounds the solenoid circuit. So set up your controls as faster-button->one-shot->relay->servo-pin-1 and slower-button->one-shot->relay->servo-pin-2. The button activates the one-shot timer. The timer output pulse closes the normally-open relay. The relay connects pin 1 or pin 2 to ground for the duty cycle of the pulse. Hitting the brake cuts power to and releases the servo.

A link that decently explains the 555 timer: 555 Timer/Oscillator Tutorial

Or you could use discrete components. Look half-way down in here at the "Monostable Flip Flop", though the output pulse duration definitely needs to be shortened; 300ms is much too long. If I remember, a 1-5ms pulse is in the desired vicinity.

Thinking about it, it might be easy enough to rip an old wheel mouse apart and use the scroll wheel to speed up or slow down. Now that would be an interesting way to make a semi-automatic cruise control.

Any EEs in here with better ideas?

 

[JKosten]

Sweet. Thank you for the help. It gives me some thing to look forward to this weekend. I really did not want to have to use a cable to set a high idle.



So if I under stand this right if I follow the drawing on the right for a "one shot" I should have a quick precise pulse?



See this page Circuits Page 9







Thank You

 

[fest3er]

Give my limited knowledge of circuits, you might be better off with the discrete circuit on the left; it should work at 15VDC, but the R-C timing will be different. (The Schmitt trigger IC is almost definitely limited to +5VDC, though if you search, you may find some that'll work at +15VDC. )

In either case, you'll have to adjust the R-C timing circuit to control the pulse duration. Also, remember you're working with close to 15VDC, not the 5VDC in the drawing. You might want to search for 15VDC monostable flip-flops. There's *gotta* be a circuit already designed that'll work.

And, thinking on it, the inverted output (normally high, pulse is low) might just work best. The output of the flip-flop is 'high' voltage, close to that of the supply voltage. If this is connected to one side of the relay's control and the other side of the control is connected to B+ (15VDC), then the normally-open relay will stay open; it will close only when the pulse is generated. What I can't say is if enough current will flow to operate the relay.

If you're willing and able to play with such stuff, go for it! The parts really don't cost that much.

Anything fairly close would certainly beat the adjustable squeegee handle I've been using for 12 years.

 

[JKosten]

Sorry Neal, I was looking at the 555 Timer Diagram below it which look like it would work since they discuss 12v relay control.

555 timer Mono stable (one shot) circuit

The two circuits below illustrate using the 555 timer to close a relay for a predetermined amount of time by pressing a momentary N/O push button. The circuit on the left can be used for long time periods where the push button can be pressed and released before the end of the timing period. For shorter periods, a capacitor can be used to isolate the switch so that only the initial switch closure is seen by the timer input and the switch can remain closed for an unlimited period without effecting the output.

In the idle state, the output at pin 3 will be at ground and the relay deactivated. The trigger input (pin 2) is held high by the 100K resistor and both capacitors are discharged. When the button is closed, the 0. 1uF cap will charge through the button and the 100K resistor which causes the voltage at pin 2 to move low for a few milliseconds. The falling voltage at pin 2 triggers the 555 and starts the timing cycle. The output at pin 3 immediately moves up to near the supply voltage (about 10. 4 volts for a 12 volt supply) and remains at that level until the 22 uF timing capacitor charges to about 2/3 of the supply voltage (about 1 second as shown). Most 12 volt relays will operate at 10. 4 volts, if not, the supply voltage could be raised to 13. 5 or so to compensate. The 555 output will supply up to 200mA of current, so the relay could be replaced with a small lamp, doorbell, or other load that requires less than 200mA. When the button is released, the 0. 1uF capacitor discharges through the 100K and 2K resistors. The diode across the 100K resistor prevents the voltage at pin 2 from rising above the supply voltage when the cap discharges. The 2K resistor in series with the 22uF cap limits the discharge current from pin 7 of the timer. This resistor may not be necessary, but it's a good idea to limit current when discharging capacitors across switch contacts or transistors.

Thank you for the quick responses.

Jesse

 

[fest3er]

Duh. Of course you were. It was late. Yes, that circuit looks like it'll work perfectly. Just gotta get the R-C right: a cap and a pot to adjust the pulse width. In fact, I might just build one myself.

 

[JKosten]

Duh. Of course you were. It was late. Yes, that circuit looks like it'll work perfectly. Just gotta get the R-C right: a cap and a pot to adjust the pulse width. In fact, I might just build one myself.

Do you think I can use the resistor values that are listed and what size pot? I am not familar with the term R-C. What is that?

Also if you build one could you send me a parts list if I don't build one first. (If I get it is in the day or two I will take pics and post them)

 

[JKosten]

I was looking the 555 circuit a little more and just realized: Can it be used to switch a ground or do I have to use it in conjunction with a relay to control the ground?

 

[fest3er]

Hmmm. The 555's output is normally low, which is opposite of what's needed. I believe the trick will be to have the timer drive a transistor. With the timer normally low, the transistor is off. When the button is pushed, the timer generates a high-voltage pulse which turns on the transistor for a moment, which grounds the servo pin and tweaks idle speed.



R-C is a resistor-capacitor network. Varying their values changes the duration of the pulse. Here's a calculator. It looks like maybe a 1uF capacitor and a 100-10000 ohm potentiometer may do the trick. The pot will allow you to dial in the desired pulse width. (That is, with each button push, do you want a 50RPM change? 100RPM change? 500 RPM change?)



I hope to get an quick answer from an EE friend as to which transistor to use and which capacitor and pots to use to adjust the pulse width, as well as any other prudent design changes needed for use in a noisy automotive environment. If I'm lucky, he might even draw up a schematic. If I have to, I'll remind him he owes me. Many times over. For all the computing assistance I've given him over the years.

 

[JKosten]

So just wondering if this has gone any further?

 

[Mhannink]

i was just looking at this my self, did a search and found this thread. im going to play with it later today. has anyone tried it and maybe have any more info as to how it works.



for the moment i am going to try just a momentary switch like the op, i will see how controllable it is that way. unless someone has some more input.

 

[love-horsepower]

hi idle warm up

some one make a kit or good plans i would love this on my truck!

 

[Mhannink]

i could not get to it tonight, i am planning on playing with it tomorrow. i have a 3 position momentary-off-momentary switch. im going to try just taping the wires and grounding with the switch and see how much control i have over it. ill let you know how it goes.

 

[fest3er]

I expect that the fastest you can possibly tap the switch will be too much. To give a SWAG, a 5 Hz clock with 1ms pulse width might give you adequate control.

 

[Mhannink]

what is swag?

 

[fest3er]

what is swag?

Scientific Wild Arse Guess.

 

[Mhannink]

well i played with it today, and it doesnt work, at all. i had a wire tapped into the vacuum control at the servo. start engine, turn on cruise, ground wire, and the cruise light goes off. i had to shut down and restart the truck, cruise worked afterwards just fine. i'm gonna keep playing with it. i ordered a powertrain diag. manual. the service manual says thats where to look for many complete diags so ill see if there is more info in there.



found a thread today that said someone by the name of harold b has a high idle kit. i googled it and nothing looked promising. ill keep looking and see what i come up with. i think it is possible.



i had another thought i am chasing down is did these trucks have a engine speed control for the pto? maybe that can be used to do the same thing.

 

[MLee]

The earlier big block GM's had a deceleration solenoid on the carburetor linkage that when let off to stop, this solenoid would keep the idle up so the engine would not die when stopping. They had a vacuum, and a 12 volt operated one. I know cause I have both. Anyway I used the 12 volt on mine and adjusted it for 1200 rpm. Just flip the switch, tap the pedal, 1200 rrs, turn off switch rs drop cause the solenoid turns off. Mounted it off the front gear housing and it pushes on the throttle linkage. used a keyed 12 volt source.

 

[MLee]

for the PTO, probably a manual pull on twist knob was used.

 

[cerberusiam]

Prior to Jan 1998 any feasible speed control was a manual implementation. There is no computer to control rpms that has fine enough control to work very well. An electronic one that is really a servo and cable is about the only option.