JoeH Posted June 13 Share Posted June 13 My 2003 MR688S AI350/Allison hd4560 suddenly just dropped it's low idle speed from 650 down to 500 or less. No codes, just had a ton of engine work done at the dealer, see my other "rollover" thread under Modern Trucks. Been running great til now. Firing up computer to check stats, and doing a visual inspection. Any recommendations? Quote Link to comment https://www.bigmacktrucks.com/topic/74265-2003-ai-350-sudden-500-rpm-idle/ Share on other sites More sharing options...
fjh Posted June 13 Share Posted June 13 Yes you have to have the shop set the idle as you have no clutch to make it set on your own! Seen this happen before! Just have them re set the idle! Has to be done with the pro link or laptop! Also make sure to tell them to turn off set idle with switches! If you have the lap top with Mack program do it yourself! key thing thing is turn off set idle with switches! Quote Link to comment https://www.bigmacktrucks.com/topic/74265-2003-ai-350-sudden-500-rpm-idle/#findComment-553687 Share on other sites More sharing options...
JoeH Posted June 13 Author Share Posted June 13 12 minutes ago, fjh said: Yes you have to have the shop set the idle as you have no clutch to make it set on your own! Seen this happen before! Just have them re set the idle! Has to be done with the pro link or laptop! Also make sure to tell them to turn off set idle with switches! If you have the lap top with Mack program do it yourself! key thing thing is turn off set idle with switches! Can I reset it with switches myself?? Quote Link to comment https://www.bigmacktrucks.com/topic/74265-2003-ai-350-sudden-500-rpm-idle/#findComment-553689 Share on other sites More sharing options...
fjh Posted June 13 Share Posted June 13 No Quote Link to comment https://www.bigmacktrucks.com/topic/74265-2003-ai-350-sudden-500-rpm-idle/#findComment-553691 Share on other sites More sharing options...
JoeH Posted June 13 Author Share Posted June 13 Then how did it change in the first place?? Quote Link to comment https://www.bigmacktrucks.com/topic/74265-2003-ai-350-sudden-500-rpm-idle/#findComment-553692 Share on other sites More sharing options...
Popular Post fjh Posted June 13 Popular Post Share Posted June 13 Good question I wasn’t there ! However if the on off fast idle switch gets in advertently turned on off three times it will go to default idle which you now have! If set idle with switches is turned on in the settings!sometimes that setting gets missed at pdi time people don’t think auto most of the time I know I don’t!They can run for years till something glitches ! Loose connection or something happens!vibration even! You set the idle and The last step in the sequence with switches is to step on the clutch! You don’t have one !bud! 3 Quote Link to comment https://www.bigmacktrucks.com/topic/74265-2003-ai-350-sudden-500-rpm-idle/#findComment-553694 Share on other sites More sharing options...
JoeH Posted June 13 Author Share Posted June 13 Throwing this out into the abyss... Idle speed is adjustable by driver, assuming the feature isn't disabled. Punching the right cruise control sequence enters the "reprogram mode", and as you say you need a clutch pedal to set it once you've floated it where you want it. Or you need the Handy Dandy Mack software, which I sort of have but can't utilize because it doesn't recognize the DieselLaptops interface device I own. 1 Quote Link to comment https://www.bigmacktrucks.com/topic/74265-2003-ai-350-sudden-500-rpm-idle/#findComment-553697 Share on other sites More sharing options...
JoeH Posted June 14 Author Share Posted June 14 The wealth of knowledge here seems to never end. Thank you @fjh! You pointed me in the right direction to sniff this document out! Quote Link to comment https://www.bigmacktrucks.com/topic/74265-2003-ai-350-sudden-500-rpm-idle/#findComment-553698 Share on other sites More sharing options...
fjh Posted June 14 Share Posted June 14 8 minutes ago, JoeH said: The wealth of knowledge here seems to never end. Thank you @fjh! You pointed me in the right direction to sniff this document out! All good bro! You bet lots of knowledge here we all learn stuff here I never stop picking up stuff! 2 Quote Link to comment https://www.bigmacktrucks.com/topic/74265-2003-ai-350-sudden-500-rpm-idle/#findComment-553699 Share on other sites More sharing options...
fjh Posted June 14 Share Posted June 14 Sorry I wasn't More clear at the beginning of the post about the switch on off scenario thing is all it takes is vibration and a loose plug or wire to trigger this situation after years of operation! Quote Link to comment https://www.bigmacktrucks.com/topic/74265-2003-ai-350-sudden-500-rpm-idle/#findComment-553713 Share on other sites More sharing options...
JoeH Posted June 14 Author Share Posted June 14 Never ending. Dealer computer can't program idle speed because VECU isn't picking up that the parking brake is set, even though the dash indicator light comes on saying the parking brake is set. 🤣 Brought the truck home to dig into this myself.... Quote Link to comment https://www.bigmacktrucks.com/topic/74265-2003-ai-350-sudden-500-rpm-idle/#findComment-553716 Share on other sites More sharing options...
fjh Posted June 14 Share Posted June 14 😡 so maybe check and see if the clutch switch wiring is under the dash and is just tied up and jumped over! Quote Link to comment https://www.bigmacktrucks.com/topic/74265-2003-ai-350-sudden-500-rpm-idle/#findComment-553717 Share on other sites More sharing options...
JoeH Posted June 14 Author Share Posted June 14 Lol, I thought about that, but the shop foreman didn't think that would work as it's programmed for an automatic. I'm not opposed to trying it but I don't want to fry anything. The task at hand is to find/repair the "VCU Park Brake Switch" circuit. It's wire VJ1-10-0.8. I found the wire near the VECU, but I need to find where the VCU park switch is. So the dash instrument cluster has a parking brake lamp that functions, it works off the park brake sensor on the park brake knob. However, according to my wiring diagrams for the truck there should be a VECU park sensor. Any suggestions on where it'd be on an MR688? Quote Link to comment https://www.bigmacktrucks.com/topic/74265-2003-ai-350-sudden-500-rpm-idle/#findComment-553729 Share on other sites More sharing options...
JoeH Posted June 14 Author Share Posted June 14 Also the switch function requires parking brake to be set anyway, so this is a moot point. Gotta fix/hotwire VJ1-10-0.8. any recommendations on lying to that circuit? I'll post a picture of the schematic. Quote Link to comment https://www.bigmacktrucks.com/topic/74265-2003-ai-350-sudden-500-rpm-idle/#findComment-553730 Share on other sites More sharing options...
JoeH Posted June 14 Author Share Posted June 14 17-A-0.8 is split down from 17-A-1.0, which comes from Fuse 17, 10amp. This fuse supplies several functions. Quote Link to comment https://www.bigmacktrucks.com/topic/74265-2003-ai-350-sudden-500-rpm-idle/#findComment-553731 Share on other sites More sharing options...
JoeH Posted June 14 Author Share Posted June 14 Worth noting, while the park brake dash lamp works, it does not operate the brake lights. Quote Link to comment https://www.bigmacktrucks.com/topic/74265-2003-ai-350-sudden-500-rpm-idle/#findComment-553732 Share on other sites More sharing options...
JoeH Posted June 15 Author Share Posted June 15 From the vmaciii 8-211 manual. Schematic for MR parking brake blink code 7-2. I don't have the blink code, that's only tripped when park brake is set while moving. Quote Link to comment https://www.bigmacktrucks.com/topic/74265-2003-ai-350-sudden-500-rpm-idle/#findComment-553743 Share on other sites More sharing options...
fjh Posted June 15 Share Posted June 15 Sorry bro I’m going to be of no help on this not familiar with that wiring on these at all! My former suggestion is all I have to offer! If the wires are there! They will have a jumper set the idle and disconnect! We’re the switches turn off when they looked at the settings?will the engine ramp up with the switches? Quote Link to comment https://www.bigmacktrucks.com/topic/74265-2003-ai-350-sudden-500-rpm-idle/#findComment-553749 Share on other sites More sharing options...
fjh Posted June 16 Share Posted June 16 JoeH did you get this nailed down yet? We’re the set with switches turned off when they looked at the settings? Quote Link to comment https://www.bigmacktrucks.com/topic/74265-2003-ai-350-sudden-500-rpm-idle/#findComment-553802 Share on other sites More sharing options...
JoeH Posted June 17 Author Share Posted June 17 They can't change anything til the VECU registers that the parking brake is set. I need to trace wire VJ1-10-0.8 to find the missing parking brake sensor. I found the wire by the VECU, just need to cut a boatload of zip ties so I can follow it. Quote Link to comment https://www.bigmacktrucks.com/topic/74265-2003-ai-350-sudden-500-rpm-idle/#findComment-553828 Share on other sites More sharing options...
JoeH Posted June 17 Author Share Posted June 17 Even if this were a manual trans with a clutch I couldn't switch set the idle without VJ1-10-0.8 receiving parking brake signal. It can enter the reprogram mode without the park brake signal, but can't program a new idle without it. Worst case scenario I just have to plumb in a new park brake switch receiving power from 10amp circuit breaker #17 to one side of a park brake switch, and wire VJ1-10-0.8 on the other side of the switch. Quote Link to comment https://www.bigmacktrucks.com/topic/74265-2003-ai-350-sudden-500-rpm-idle/#findComment-553829 Share on other sites More sharing options...
JoeH Posted June 17 Author Share Posted June 17 Then it's back to the dealer for them to try again. Quote Link to comment https://www.bigmacktrucks.com/topic/74265-2003-ai-350-sudden-500-rpm-idle/#findComment-553830 Share on other sites More sharing options...
fjh Posted June 17 Share Posted June 17 1 hour ago, JoeH said: Then it's back to the dealer for them to try again. 👍 sorry it wasn’t more cut and dried for ya bud! Quote Link to comment https://www.bigmacktrucks.com/topic/74265-2003-ai-350-sudden-500-rpm-idle/#findComment-553837 Share on other sites More sharing options...
JoeH Posted June 17 Author Share Posted June 17 8 hours ago, fjh said: 👍 sorry it wasn’t more cut and dried for ya bud! Wound up being real easy: Double check on my Mack Flash Drive P/N 23-021 (has every wiring harness of every Mack chassis from 2000 to about 2020/2021) to compare the switch circuit/wire codes from the 2000 MR688 donor truck to my 2003 I'm working on. Circuit identical. Found wire VJ1-10-0.8 on the 2000 harness which is on my shelf. Appears to go to parking knob area. Got 2000 MR688 dash panel out of storage box, park knob has 2 sensors on it! Went to my 2003, only has one sensor, but has a plug in other port! Found wires nearby, VJ1-10-0.8 is intact, taped to its pair 17-A-0.8 which has been cut short and the end electrical taped. "Borrowed" a sensor from the 2000 dash park knob which I hate doing, but I used the label maker to leave a note for future use; repaired wire end 17-A-0.8 plugged it all together and ran the test light. System functional, I'll be heading to the dealer hopefully today to get the software side cleaned up! Will post results later! 1 Quote Link to comment https://www.bigmacktrucks.com/topic/74265-2003-ai-350-sudden-500-rpm-idle/#findComment-553843 Share on other sites More sharing options...
JoeH Posted June 17 Author Share Posted June 17 My suspicion is that the wire end and possibly sensor melted a bit, and got deleted. Quote Link to comment https://www.bigmacktrucks.com/topic/74265-2003-ai-350-sudden-500-rpm-idle/#findComment-553844 Share on other sites More sharing options...
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