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120 Cruise control not working if lights are on and towing a trailer?

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  • 120 Cruise control not working if lights are on and towing a trailer?

    Hi all

    1st post here. Have been signed up a while and had nothing to contribute as there seem to be plenty with far more knowledge than myself on the subjects I've needed to view. I've used the Search function but not found any info specific to the following issue:

    I have a 2006 Prado 120 VX (Spanish).

    It came with factory fit cruise control which has always worked perfectly (lights or/off) - before I took it to Toyota!
    Not long after purchase I had a tow bar fitted. I would have worked out how to do this myself but Spain is very restrictive on altering the cars here without authorised certification.
    Rather than deal with this hassle I took to a Toyota Main Dealer (Cumaca Malaga) for them to do it.

    I've used it a few times to drive to race tracks towing my track bike, however the few trips I did in the year were all in the day... no need for lights and the cruise control worked.
    Anyway, I went to Aragon which in the north of Spain last year and ended up driving through the night.... which is when I found the problem...

    When the cars lights are on AND it's connected to a trailer the cruise control will not engage.
    If you drive with lights off, the cruise control works.
    If you drive without a trailer, the cruise control works with lights off and on.
    Apart from the above problem, all the lights work correctly in any situation.

    Unfortunately, by the time I found this problem (I had use of a van, so most of the time would use that for track trips) I'd gone past the 1 year guarantee from Toyota and they were not interested in helping - without paying.

    So... any ideas? Clearly (?) it's a wiring cock-up of some kind but not sure where to start. Toyota said they will look at it, but based on the above I'm not inclined to trust their competence to fix it without ripping me off.

    I'm asking now as have removed most of the interior while I'm upgrading the audio system and fitting an ARB rook rack so seems a good time to have a go at this as well.

    Thanks in advance!

    James

  • #2
    I really have no idea why your Prado is behaving this way. My only thought is that one of the many ECUs is displaying some obscure malfunction. You may be able to check if the vehicle is showing any fault codes Eg via Techstream or and aftermarket CAN bus interface.

    As an amusing aside, you mention fitting an ARB Rook Rack. I can visualise you driving around with a family of rooks perched on your new roof rails!

    Comment


    • #3
      I would have another towbar installer look at it, sounds like it has to do with the towbar wiring.

      Comment


      • #4
        When you say it doesn't engage, what do you mean? Does the cruise turn on, but not set? Does the 'cruise' light light up? Does it flash?

        Do you have any warning lights on - check engine / TRC / ABS?

        Does it work OK if you have the lights on and dont have anything plugged into the towbar socket? If so, can you plug a tester into the towbar socket and check the wiring is OK?

        Comment


        • #5
          Thanks so far. I’ve picked up an OBD diagnostic tool so will try that.

          Comment


          • #6
            Frog1520...
            The cruise control lights up on the dash when you push the button, but when you try to set the speed to maintain it, it doesn’t.
            no warning lights of any kind.
            Yep, it works fine when nothing is plugged into towbar socket. Wasn’t aware of a tester for that, will have a look online and order one.

            one good thing is I’m getting a good understanding of the car, pulling parts off it all over the place.

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            • #7
              Have you tried a different trailer?

              Comment


              • #8
                I think checking the codes is the way to go. I assume the speedo is working OK?

                Is your towbar socket just a 7 pin job? Struggling to think how that might affect it, as its really only lights on there....

                Unless there is a bad connection some place. A bad speed sensor (P0500) can cause the cruise not to set. See what the codes say, but please do report back because I'm curious about it now!!!!

                Comment


                • #9
                  Piggy... same issue with different trailers.

                  frog.... will check when home in a few days. Speedo works fine, absolutely no issues/warnings elsewhere...but maybe using OBD will show something.
                  it’s a 7 pin.
                  ive found a 120 Wiring PDF (via google) which will hopefully help out.

                  Will report back.... I hate getting to the bottom of a thread with no actual answer/conclusion to read.

                  Comment


                  • #10
                    Here's a strange question, but does your vehicle have parking sensors? If so do you disable them with the switch in the back when you're towing?

                    My parking sensors had a fault that caused the speed wire between the speedo and the ECU to malfunction, and the cruise wouldnt work. I did get a check engine light and a P0500 code though.

                    (The parking sensors are connected to the speed wire because they only come on at low speeds)....

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                    • #11
                      When you turn on the lights (brake not applied), do the brake lights (full bright) turn on at the trailer, as though you've stepped on the brakes?
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                      • #12
                        When you turn on the lights with no brakes applied, do the brakes (full bright red) come on at the trailer as though you've stepped on the brakes?
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                        • #13
                          Another thought - your brake lights and tail lights arent switched in the towing connector are they?

                          Comment


                          • #14
                            Originally posted by amts View Post
                            When you turn on the lights with no brakes applied, do the brakes (full bright red) come on at the trailer as though you've stepped on the brakes?
                            I think this will be they key to solving this. I does sound like a feedback through the trailer lighting circuit energizing the brake circuit telling the cruise that the brakes are on.
                            If you have the trailer connected and engage the cruise control, then turn the lights on does the cruise disengage?
                            the fact that it does it regardless of the trailer suggests that it may be in the trailer plug socket on the vehicle where maybe somehow lights and brakes are incorrectly connected to each other.

                            Comment


                            • #15
                              Quick update.... nothing new.
                              been busy trying to find work as made redundant last year due Covid19 so been away from home and had no time to fix this issue yet.
                              Hoping this year turns out better than last.

                              anyway...


                              Originally posted by thebaz View Post
                              I think this will be they key to solving this. I does sound like a feedback through the trailer lighting circuit energizing the brake circuit telling the cruise that the brakes are on.

                              If you have the trailer connected and engage the cruise control, then turn the lights on does the cruise disengage?
                              YES. Exactly this.
                              works fine with lights off and trailer connected.
                              when you turn lights on with trailer connected, cruise disengages.



                              the fact that it does it regardless of the trailer suggests that it may be in the trailer plug socket on the vehicle where maybe somehow lights and brakes are incorrectly connected to each other.
                              To be clear (if I wasn’t before). Without towing a trailer.... cruise control works perfectly with lights on or off.
                              I’m going to be working on it this week so when I find out the problem I’ll update.

                              thanks for the pointers so far!

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