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Thread: How to: High Idle Switch (Using Cruise Control Servo)

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    How to: High Idle Switch (Using Cruise Control Servo)

    Okay, so for a while I have been thinking this over in my head how I was going to create some type of high idle switch. I originally thought of using vacuum but couldn't come up with a good way to do it. Then I thought of using a small solenoid at the throttle. Then I thought how about using the cruise control system to do this. I started searching around the internet a way to do this with SPDT relays. You can do this on any vehicle that uses the same servo as my ZJ (93) which apparently is alot. This will let you keep your cruise control working like normal when the switches are not in use.

    You will need:
    4 SPDT relays
    1 on/off switch
    1 inline fuse
    1 indicator light
    1 2 way switch
    Wire (I used 12g but thats just because i had it laying around in bulk rolls, the factory wiring is 18g, Which when wiring at the servo will make your life way easier if you user similar gauge wire. I used two different colors to easily identify the ones from the servo and the relays, grounds from one another.)
    .250 female disconnects
    Self tapping screws
    A piece of sheet steel to make a bracket out of
    Soldering gun, Solder
    Electrical tape
    Wire strippers

    Other than the wiring that I had I bought everything at Canadian Tire but you could get it anywhere that sells this type of stuff. Make sure you get true Single Pole Double Throw relays they are needed for properly isolating the circuit.)

    You will also need this wiring diagram. Courtesy of mallcrawlin. (ZJ's kickass)


    Once you turn the power on, the led lights up and the relays energize. To increase idle you tap the momentary switch up and to decrease you tap down.

    This is part one I will finish it within the next few days when I do the in truck wiring for the switches.

    The first thing I did was make my bracket. Then I found a spot to mount it at the highest point I could find that was pretty protected from water. I decided on my passenger side fender at the front beside the battery. It was about the only open space I could find to mount it out of the way, that wasn't to low or I knew it was going to get wet. It was also close enough to the servo that I wouldnt have to run a ton of wiring.

    Bracket made with relays attached

    The spot where I chose to mount the bracket and relays to. Test fitting it.


    I then took the relays out and proceeded to start wiring all of the wiring that could be done outside of the truck on the work bench. (Relay to Relay, Ground wires, etc). I then test fitted it again to make sure the wires weren't going to hang to low.

    The four wires joined together with the tape on the end are to be wired to the switch. You can also see one of the relays connect to each other. The other four are ground wires.


    Next locate the plug on the servo and unplug it. The black is ground. These are the wires you will need to tap into so you can make this work. Cut the three wires leave enough on both ends that you will be able to splice wiring into both. Wire as diagram provided shows.

    The plug unplugged with the ground and 3 other wires seperated.


    Spliced together. Soldered, then shrink wrapped all connections, then I taped the wires coming from the plug at the servo together and the wires from the loom going to relays together.


    Plugged back into the servo.


    All of the relays connected, grounded. Relays connected to servo. (Note I also used the factory battery ground location as my ground. Since I had to undo the bolt I decided I would also change my battery ground cable since it looked like it was the original one from 93.)


    At this point I reinstalled my battery as there is only 3 wires that I still have to connect. I will have to remove the battery again when I wire the switch to connect the last 3 female disconnects to the relays. But that is part two in a few days.


    Stay tuned for my part two when I wire the switches.
    Last edited by BlueZJ; 09-14-2009 at 12:42 AM.
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    Quote Originally Posted by igotafrigginjeep View Post
    I have been thinking about this for a while...but haven't yet added OBA or winch so the need isn't there yet...

    If I could make 2 suggestions...

    I would suggest the you get some di-electric grease (available at CT as Spark Plug Boot Grease - I think it is made by Permatex) and coat all the terminals on the relays and the connectors you have crimped on the wires. This will help prevent corrosion.

    Also...

    I would suggest that you cover the terminals with something (shrink tube or electrical tape) to prevent anything shorting out. If using shrink tubing, get some with a diameter that is large enough to slide over the body of the relay and cut it long enough to cover the body of the relay and about 2 inches of the wire, heat it up and watch it shrink tight around the relay and wiring. If using tape, start at the top of the body of the relay and wrap around until the whole body and about 2 inches of the wires are covered. This will prevent the terminals from shorting if something was to inadvertently fall against them. It wall also be an extra level of protection against the terminals sliding loose.
    Thanks for the suggestions when I do the switch I am going to finish that up I had planned on covering the relays some how I just hadn't decided yet, I was going to use electrical tape, but shrink tubing is definitely a pretty good idea, I will use that for sure. I never thought about the dielectric grease but again a good idea, thanks for the suggestions.

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    Excellent job. I was thinking of doing this through J1979 (or whatever the on highway SAE communication standard is). Chrysler's diagnostics computers can hold engine idle at various rpm's and I was thinking of making a controller that could do the same, you just select the rpm, but this method is much cheaper and simpler!
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    Quote Originally Posted by EngineerJoe View Post
    Excellent job. I was thinking of doing this through J1979 (or whatever the on highway SAE communication standard is). Chrysler's diagnostics computers can hold engine idle at various rpm's and I was thinking of making a controller that could do the same, you just select the rpm, but this method is much cheaper and simpler!
    It was pretty cheap, it cost me about $70 in parts + the wire I already had. It only took me about an hour as well upto this point.

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    Thanks was trying the way on NAXJA, but never finished it. I don't need the relays as something went funky with my cruise, then I swap columns and no longer have the switches, but need higher idle for OBA.
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    Quote Originally Posted by igotafrigginjeep View Post
    BUMP...any chance of an update?

    Did this actually get finished and how does it perform?

    Thanks...seriously considering it now that I picked up a York...
    I will finish it off possibly tommorow have just gotten really busy as of late and havnt had a chance to spend anymore time on it. I got busy building my snatch point and moving my vents up since I went offroading on sunday and that was more important to finish then the high idle since I dont technically have any real use for it yet other than I just wanted it.



    I'll PM you when I finish it off.

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    Very cool mod!

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    Thanks igotafrigginjeep for finding this thread, I will update as soon as I try this myself. Looks like BlueZJ did a very detailed writeup.
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    Re: How to: High Idle Switch (Using Cruise Control Servo)

    Neat, I'm going to have to add this to my ever growing list of stuff to do.
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    How to: High Idle Switch (Using Cruise Control Servo)

    I wired this up this evening. Unfortunately when I 'tap' the up switch the engine jumps to 3500rpm(with no load). I was kinda hoping for a gradual increase. I'd really like to open a servo up and see how it works. I think you are opening two small valves one to let vacuum behind diaphragm and one to vent to atmosphere. I'll double check the wiring tomorrow.

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    Thanks for the update, would be interesting to see what happens when a load is placed on the engine (eg. a hard winch pull). 3500 isn't so bad if it drops due to engine load when the winch starts working.


    IIRC, you are doing this on an older Jeep (early 90s?), I wonder if it makes a difference for newer models that might have different electronics/parts...
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    Tried it again with the air compressor engaged and surprisingly it brought down the idle to a more useable 2500 rpm.
    I'm amazed how much air that little stock Sanden compressor can put out, especially with a high idle.

    I'm sure I am still missing something, I am sure there is a way to control the servo with some more finesse, the ECM can manage minute adjustments of idle somehow.

    I was hoping someone who has done this modification before might speak up.

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    How to: High Idle Switch (Using Cruise Control Servo)

    I know the hvac selector has an impact on rpms. Not sure how you control your compressor
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    I am using the stock wiring for the air compressor, I just shorted out the two wires leading to the low pressure switch that used to be on the dryer/accumulator. I am not sure what the high pressure switch is set to but I left it in place thinking it might save the compressor if the blow off valve doesn't.

    I'll try just energizing the clutch straight from the battery and see what happens regarding engine rpms. I've used the high idle switch a couple of times now, mostly for charging batteries and I think my problem is I am giving the servo too long of a pulse, either the relay I am using or switch is sticking closed too long.

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