EWB555 Posted August 7, 2011 Share Posted August 7, 2011 I was driving my 95 RD688S Tri Axle Dump 400E7 12 Speed this weekend, comming up a long pull loaded down to about 1400 RPM, and was getting ready to drop a gear when all the sudden it felt like I either let off the fuel, or the engine went into some sort of derate mode and just stopped pulling. There was no sudden noise but the sound of the engine changed, (as it would if I let off the fuel). All vitals were normal, water temp just above 180, pyrometer had climbed to about 900 (Which is normal for a long pull). Truck stayed running, just didnt have the pulling power it usually does. Got the load off the truck, it seems to feel ok empty, went back got another load (headed the same place) and I had to consistantly use a gear lower then I needed on the previous load (up until the whatever happened that killed the power). I noticed the pyrometer never got above 800 the rest of the day, so thats an obvious sign of less fuel/less boost? Any ideas what to check first? I was gonna change the fuel filters, it's been about 10,000 miles. Also there is no smoke, everything is the same as it was, it just doesnt pull when you hit the hills loaded. Appreciate any help, this thing just seems dead, I was having flashbacks to the 78 R model 300 6 Speed I drove when I first got my liscense. Quote Link to comment Share on other sites More sharing options...
slpwlker Posted August 7, 2011 Share Posted August 7, 2011 Wouldn't hurt changing your filters. Maybe aircleaners to. I got to change my fuel filters on my Cat this week also. Got down to 75 pounds of pressure and starting to hurt my pulling. Usually around 90 to 98 psi Quote Link to comment Share on other sites More sharing options...
fjh Posted August 8, 2011 Share Posted August 8, 2011 There is a wire that goes to the torque limit swithch on an 18 speed trans it may be shorted it will cause the engine to have half the boost of normal.it is usually routed down and around the same area as the speed sensor pu wire.U claim to have a 12 sp and not used but even so the wire will still be there check for it. Quote Link to comment Share on other sites More sharing options...
EWB555 Posted August 8, 2011 Author Share Posted August 8, 2011 Appreciate the info guys. I am def going to start with the easy stuff, ( will change the filters tonight).In regards to the torque limiter-I just put a shifter in this truck 2 weeks ago, and I was going to change the speed sensor while I had the floor pan out and easy access to the wires, as the speedo reads incorrect every once in a while. I unplugged what I thought was the speed sensor connector, only to realize it went to another sensor on the top passenger side of the tranny. Is that the torque limiter? I plugged this back in, but I was thinking since I messed with the wires I could have broken one. I only see 2 sensors on the transmission, I was going to replace the speed sensor, but It didnt look like the one Mack had given me so I left it alone.The other thing I was thinking of is I had the fuel pedal unbolted to pull the floor out, maybe that could have gone bad and its not letting it get full throttle? Again I appreciate any thoughts anyone has on this. Quote Link to comment Share on other sites More sharing options...
fjh Posted August 10, 2011 Share Posted August 10, 2011 The torque limit wires would have to short together to cause this problem unpluged the unit becomes in effective. Quote Link to comment Share on other sites More sharing options...
EWB555 Posted August 17, 2011 Author Share Posted August 17, 2011 UPDATE-I installed a boost gauge over the weekend. Truck was building 30-32 PSI of Boost Monday Morning, drove it about 10 miles, then all it would build was 22-24 PSI the rest of the day. The truck idled for about 45 minutes before I left the house so its not like it was cold. The only thing I can think is the computer is cutting it back for some reason,I have checked the wires on the transmission and see no evidence of a short.Also the speedo acts up intermittantly (But it was perfect All day Monday) and the Cruise Control works for about 5 minutes then kicks out-Could any of these symptoms be related and make the computer cut the power? Quote Link to comment Share on other sites More sharing options...
rhasler Posted August 17, 2011 Share Posted August 17, 2011 The speedometer can cause the engine to derate. Quote "Mebbe I'm too ugly and stupid to give up!" Link to comment Share on other sites More sharing options...
EWB555 Posted August 27, 2011 Author Share Posted August 27, 2011 Put in the new speed sensor today took her out for a test run, everything seems to be ok. Used the cruise control for about 3 miles and it stayed engauged, boost pressure was good. I guess I'll find out for sure next full day, as sometimes it takes a few hours for it to go into derate mode, but the fact the cruise worked makes me hopeful. Quote Link to comment Share on other sites More sharing options...
EWB555 Posted August 30, 2011 Author Share Posted August 30, 2011 Ok, truck ran great for 1 load today then back to derate mode. Anything else that can cause this??? Quote Link to comment Share on other sites More sharing options...
Lmackattack Posted August 30, 2011 Share Posted August 30, 2011 dont you love electronics...... Sounds like your boost is good at one moment and then cutting back the next moment... wich tells me its a electronic/sensor issue some how cutting the power back. but that could also be a poor air or fuel flow issue...Now Im not on top of all the electronic stuff that well but what I have seen before is the following.... hope this helps...trans torque limiterhi water temphi oil temphigh egt tempbad air filterbad fuel filtersbad fuel hose or kinked hosehot out side ambent tempslow water leveltiming out of specclogged fuel pick up tubemelted turbo bladesfuel cut off sticking closed/limiting flow...have also seen throttle pedals go bad and not allow full throttel.most of these can be spotted when "plugged in" to a Computer at the dealership...Trent Quote Link to comment Share on other sites More sharing options...
EWB555 Posted August 30, 2011 Author Share Posted August 30, 2011 (edited) Appreciate the info. If there is one thing I'm certain its definately the computer derating the engine. I've had it happen to me loaded, where it cuts the boost back from 30-34 to 25 Max. I get to the plant, dump the load, then shut the truck off for a sec, and its back to over 30psi when I'm back on the road.-It may stay that way 30 minutes or a few hours until she cuts back again.-The torque limit sensor is that on the passengers side top of the transmission? Its the only other electrical plug I can see other then the speed sensor which I just replaced. I checked the wiring and it seems good. All the other things you have listed I think I can almost rule out. Water temp is never over 180- Highest I've seen the pyrometer is just under 1000 i believe after a long pull, usually around 6-700.I may have to break down and have the mechanic look at it with his computer, rather then throw more parts at it. Speed sensor made the most sence to me since the speedo acted up every now and then, and the trip odo is way inacurate (still is even with the new sensor)It's frustrating because the truck runs really good, when the computer isnt cutting it back. I've only had the truck 3 months, and other then this issue the truck is in exellent shape for a 95. I want it to be perfect, but dont want to go broke either. Edited August 30, 2011 by EWB555 Quote Link to comment Share on other sites More sharing options...
Lmackattack Posted August 30, 2011 Share Posted August 30, 2011 i dont know for sure where your torque limiting is done. It could be off of a air line from the range selector to a electronic sensor on the trans... im more old school and have not had much trouble shooting on the newer stuff.. Quote Link to comment Share on other sites More sharing options...
fjh Posted August 30, 2011 Share Posted August 30, 2011 A derate will throw a code if the check engine light is coming on there should be a logged code.If not its likely something the puter dosent know about that but thinks is normal like the puff limit switch!Having said that the throttle pedals have been known to glitch once and awhile with out throwing a code.And also the fuel pump pins at the plug can give ya 50 fits at times ,try unplugging the big fuel pump plug and put it back together1/2 a dozen times and see if this changes the operation.The key off and back to normal thing makes it sound like a derate. Quote Link to comment Share on other sites More sharing options...
GaryMac Posted August 31, 2011 Share Posted August 31, 2011 Just some thoughtsI like mechanical MACKS and CATSLast year i heard of a bloke with a Cummins electronic with similar problems.They checked everything with no luck, then started to renew the components,i.e. sencors, throttle pedal etc etc including the total on engine wiring loomwith no success. With the progress bill for this work fastly approaching thetrucks value things looked bleak. Finally the problem was found, being ainput from the vehicles brake system. Quote Link to comment Share on other sites More sharing options...
shortstack Posted September 1, 2011 Share Posted September 1, 2011 I wonder if the sensor you mentioned on the trans is the reverse light switch? I'm not too familiar with your engine as far as electronics but my AC 460 did something similar before and it was the boost sensor had gone bad but the check engine light did come on. Replaced it and it ran great. Trey Quote Link to comment Share on other sites More sharing options...
EWB555 Posted September 5, 2011 Author Share Posted September 5, 2011 I'm not sure where the check engine light is on this truck. I pulled the dash apart today and the light to the right of the speedo had no bulb in it. So I was thinking the previous owner pulled the bulb out before selling the truck. Put a known working bulb in and it did not come on when I turned the key on (I'm not sure it was supposed to). Is the chk eng light normally to the right of the speedo? Also can you check the codes on a 95 by using the cruise control buttons? Quote Link to comment Share on other sites More sharing options...
shortstack Posted September 6, 2011 Share Posted September 6, 2011 I couldn't tell you for sure but I'm sure someone here can. I also thought a '95 might be pre-computer controlled but not positive on that one either. I'm just not much help! Quote Link to comment Share on other sites More sharing options...
rhasler Posted September 6, 2011 Share Posted September 6, 2011 Here's the information on the dash light for VMAC IIVMAC II dash light information .pdf Quote "Mebbe I'm too ugly and stupid to give up!" Link to comment Share on other sites More sharing options...
EWB555 Posted September 23, 2011 Author Share Posted September 23, 2011 rhasler- I appreciate your help but I dont think thats the page for my truck because it talks about the self test, and says their is a 2 second delay before you can start the truck. I can turn my key and the truck starts right away, and if the mack self test is anything like a C15 Cat I should be able to hear relays and selonoids clicking every time I turn the key on and dont start the truck.I still dont know whats up with this thing-I replaced the speed sensor a month ago, and the speedometer was reading inaccurate again for a little while the other morning just like it did from time to time before. Does the speadometer head interface with the computer at all, or does the computer only take the reading from the sensor? I was thinking if the ecm gets info from the speedometer head that could be whats causing the derate?? Quote Link to comment Share on other sites More sharing options...
TOMD Posted September 23, 2011 Share Posted September 23, 2011 computer only takes reading from sensor. check sensor adjustmant. was there any metal on sensor when you changed it? check yoke bolt see if tight. Quote Tom ~ Parts & Service SpecialistWatt's Mack Sales, Inc. 1-888-304-6225 Link to comment Share on other sites More sharing options...
EWB555 Posted October 5, 2011 Author Share Posted October 5, 2011 I cheked the Yolk bolt it was tight. There was no metal on the old sensor but the seal is leaking so there was trans grease on it. Speedo still acts up from time to time, however trip odometer has never been right, reads about 5 miles for every 50, could it just be the speedometer itself? Its really not a major concern, unless its whats causing the engine to derate-in that case it is a major concern. I havent changed anymore parts to fix the derate, as the speed sensor made the most sence due to the speedo acting up, I need to have the computer scanned before "throwing another part at it"The other observation I made is that it seems to go into derate faster when on the highway, I can run 30-50 MPH roads and it seems to go much longer before cutting the power back. Which to me makes me think its still speed sensor related. Quote Link to comment Share on other sites More sharing options...
krismoriah Posted October 5, 2011 Share Posted October 5, 2011 My 1998 E7400 did almost the same thing...lost all power on the hills and the jakes acted up. There is a solenoid on the fuel pump towards the front of the pump..someone had had problems with mine before I got it..there was all kinds of hammer marks on it..then one day the wire shorted out and thats when I had all the problems. Maybe try banging on it to see if it helps you. The part was $500 thru my stealership...but i got a used one for $200 at a local junkyard. Quote Link to comment Share on other sites More sharing options...
Mark Jams Posted May 4, 2019 Share Posted May 4, 2019 Did anyone ever figure this out? Quote Link to comment Share on other sites More sharing options...
Dirtymilkman Posted May 4, 2019 Share Posted May 4, 2019 (edited) 23 minutes ago, Mark Jams said: Did anyone ever figure this out? Not that I heard of, but to me it sounds like something getting sucked onto the fuel pickup tube in the tank. Like a shop rag or something smaller but stiff. That's why when you shut the truck off it'll run normal again. I have seen it happen with the seal from a fuel supplement bottle. Edited May 4, 2019 by Dirtymilkman Quote Link to comment Share on other sites More sharing options...
theakerstwo Posted May 4, 2019 Share Posted May 4, 2019 I had for a while a 95 with a VMAC1 in it.It was a RD Quote glenn akers Link to comment Share on other sites More sharing options...
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