Gareth Posted October 22, 2022 Share Posted October 22, 2022 I have been dealing with an ongoing issue that has proved very difficult to resolve. ****Sorry in advance for the very long post!!!*** I have a 2007 Mack MR with a E7 350 and an Allison automatic. My truck has a crane mounted on it, and my problem is with the high idle when I'm operating the crane. The high idle uses the cruise control circuit so if I am going to use the crane I rev up the engine to 1100 RPM or so. The high idle will work properly for anywhere from a half hour, up to 2 or 3 hours, then it will trip an "electronic malfunction" light and kick down to low idle. The "electronic malfunction" light will turn off and everything will go back to normal if I drive the truck, usually as little as 30 to 50 ft. When my mechanic pulled the codes it said there was an irregular signal coming from the tail shaft speed sensor. (Allison transmission) My mechanic checked the tail shaft to make sure there was no play where it goes into the transmission, we replaced the speed sensor, the pigtail, the wiring harness back to the computer, and I had the computer reflashed at Mack. None of this made any difference. I are at a loss as to what to do. One more thing about the cruise control circuit is that if I set the cruise control while driving down the road it will not turn off when I press the brake, I have to turn off the cruise control switch. My mechanic checked all four of the brakes switches down by the treadle valve and they are all working fine. (I don't necessarily think that this problem is related, but it has to do with the cruise control circuit) Does anybody have any ideas? Quote Link to comment https://www.bigmacktrucks.com/topic/70600-high-idle-problem/ Share on other sites More sharing options...
fjh Posted October 22, 2022 Share Posted October 22, 2022 Sorry Limited experience on My part ! Suggestion ! This is a long shot And a bit of a parts Canon ! But have you tried replacing the set resume switch! If your using an extra set of switches at the crane end you may have issues there Conflicting signal to the ECU! That said ! It really sounds as thou the vehicle ECU has a glitch! It may have something to do with temperature change ! Not sure where the ECU is located on an MR but I believe its on the drivers side behind a kick panel ! The fact that the ECU is not seeing the brake switch is a concern! Try checking that function while its cool in the cab then when warm ! If it works then don't work with temperature change CAB heats up This may indicate you have an ecu issue! Just a thought! 1 Quote Link to comment https://www.bigmacktrucks.com/topic/70600-high-idle-problem/#findComment-513213 Share on other sites More sharing options...
Joey Mack Posted October 22, 2022 Share Posted October 22, 2022 I believe,, The brake pressure switch that shuts the cruise off, is a Normally open switch... Are you sure you have the correct switch. With truck off an no air pressure, you should be able to Ohm the switch and the fluke meter will say "OL".. if there is continuity, then its the wrong switch. 1 Quote Link to comment https://www.bigmacktrucks.com/topic/70600-high-idle-problem/#findComment-513215 Share on other sites More sharing options...
Gareth Posted October 22, 2022 Author Share Posted October 22, 2022 2 hours ago, Joey Mack said: I believe,, The brake pressure switch that shuts the cruise off, is a Normally open switch... Are you sure you have the correct switch. With truck off an no air pressure, you should be able to Ohm the switch and the fluke meter will say "OL".. if there is continuity, then its the wrong switch. Hmmm that could be something to check out.... Quote Link to comment https://www.bigmacktrucks.com/topic/70600-high-idle-problem/#findComment-513231 Share on other sites More sharing options...
fjh Posted October 22, 2022 Share Posted October 22, 2022 25 minutes ago, Gareth said: Hmmm that could be something to check out.... yup anything is worth a look at this point. Quote Link to comment https://www.bigmacktrucks.com/topic/70600-high-idle-problem/#findComment-513232 Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.