Can a hand throttle be added/emulated on a DBW LQ9?

[zathras]

I just purchased a 1956 GMC 370 Low Cab Forward rollback wrecker that's currently sporting a 427 BBC and the original 5 speed manual with PTO for the hydraulic pump. The rollback controls include a hand throttle that's used to increase the engine speed when using the hydraulics and that works fine with the carb. I'm looking into updating the drive train with an LQ9 and a built NV4500 to enhance the drivability and economy of the truck. I won't be using this commercially, but will need to be able to pick up a project wherever I find one. Since the LQ9 is drive by wire adding cruise control (one of my wants for long distance driving) is easy, but information about being able to control engine speed from the rollback has been impossible to find so far. Does anyone have any experience with something like this or pointers to information? Ideally, this could be done through the ECM with a simple toggle switch, but I'm also wondering if I could modify a second throttle pedal by cutting the pedal off, shortening the shaft and making a bracket or detent of some kind that would physically hold it in position for working the bed.

Any help would be greatly appreciated. I'd like to know before the truck gets here in a couple of weeks as I'm going to strip it down to clean up the frame and get the cab painted and swapping the engines would be much easier with the cab off.

- Glenn

[SMWS6TA]

You can get a drive by cable setup for any LS engine and have a tuner remove or close the DBW software in the tune.


It's a fairly common setup for engine swaps into older cars.

[zathras]

If I can only have one or the other the cruise control will need to give way to the throttle control for the rollback. I find a number of aftermarket cruise control option for the dbw but haven't seen anything for the cable setup. All of my experience with the Gen III and IV engines has been with dbw. How was cruise implemented with the cable actuated throttle bodies?

You can get a drive by cable setup for any LS engine and have a tuner remove or close the DBW software in the tune.


It's a fairly common setup for engine swaps into older cars.

[SMWS6TA]

Here is how mine is setup.

You can see on the throttle body to the right the 2 cables. The inner one is for the throttle and the black one is for cruise control.

If the hydro controls are with cable also I think you could fab up a bracket that will let you control the throttle.

[zathras]

Thanks for the photo. It still looks like it may be an either/or situation since I don't think I could hook up 3 cables (throttle, cruise, hand throttle) like this. I'm still hoping to find an electrical solution to directly actuate the throttle body motor from the back of the truck. The suggestions and discussion are welcome and I appreciate the input.

[pajeff02]

It seems that you would be able to easily tap into the voltage feed to the throttle body for a simulated throttle input. Assuming it operates on a 0 to 5 volt range (need to check this) you can create a switched output of the desired voltage by utilizing resistors between the switch at the back of the truck and the 12 volt source to the switch. There would be some variation with temperature, but it should raise the engine rpm to the desired level when the switch is activated. If there is concern about any back feeding of voltage to the pedal assembly then a diode can be installed to prevent this just prior to the"T" connection in the stock harness.

[zathras]

If I recall correctly, the pedal sends the TAC two signals, one going up from 0 to 5 volts the other going down from 5 to 0 volts. The TAC then does a comparison of those voltages as a form of error checking before sending its signal to the throttle body actuator. If one of the signals is lost or the two signals don't sum to 5 volts (within some acceptable deviation) the TAC knows that something's wrong. That way you don't suddenly find yourself at WOT when trying to back out of the driveway. I think you're on the right track about using some kind of voltage divider - maybe zener diodes rather than resistors - to send the necessary voltage levels to the TAC to get the engine up to a high idle. There would definitely need to be some of allowing only one source to supply signals or the TAC would get all kinds of confused. I'll look into that. Thanks!

[pajeff02]

You are correct -- there are two voltage inputs. I had not finished reading up on it last night when I posted. Have someone hold the throttle at the rpm you desire and measure the voltage at each input with a DVOM.

[LHanded Lead Foot]

You could use a second APP sensor and switch between the two. The second APP sensor would not have a spring return and could use a friction stop with a knob. Having a rocker switch that is capable of switch on-off-on for 6 wires would be perfect, but would require some research.

Otherwise you would have to unplug one, then plug in the other. The switch has to be on-break-on, like a 6 pole double throw as some make the on-off-on type.

These are simply two potentiometers wired opposite of each other. One goes high while the other goes low.
I do not believe they could not work wired in parallel at the lowest resistance & highest would be constant.

Oh my goodness, the possibilities?

[pajeff02]

Wonder if the OP ever got this figured out? Been almost a year since his original post.

[zathras]

Wonder if the OP ever got this figured out? Been almost a year since his original post.

Never did come up with enough information to convince me I could make it work (and I've got a PhD in computer science). Thanks go to the auto manufacturers who think we don't have the right to fix or modify the vehicles we own. I picked up a 454 to replace the tall deck 427 and I'm planning on going the Holley Sniper EFI route with mechanical throttle control which will make adding a hand throttle a no-brainer. But I am grateful to all of you who offered ideas and suggestions.

- Glenn

[pajeff02]

Thank you for the response. It was a damn interesting question.