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Peterbilt 379 cruise control
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cheddarbob56
Posted 10/17/2012 11:20 (#2645323)
Subject: Peterbilt 379 cruise control


Southeast SD
I have a 379 Peterbilt extended hood that we just bought this spring with a bad motor. It's got a 6NZ C-15 in it, we went completely through the motor, new head, sleeves, pistons, rings, bearings etc. Anyway - the cruise doesn't want to work on it, it will hold and set up to 1400 engine RPM and after that it won't set or accelerate or anything. Same holds true with using it to set the engine idle with the tractor brakes locked, it will hold the engine to 1400 or anywhere up to that but after 1400 it acts like its turned off. Anyone have any suggestions? Also the jake brake isn't working since we put the motor back together, it worked when we bought the truck but doesn't work on any stages now. Motor runs great and everything else works as it should. Any help would be appreciated. Thanks guys.
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4WD
Posted 10/17/2012 11:31 (#2645335 - in reply to #2645323)
Subject: RE: Peterbilt 379 cruise control


SW Iowa

Sounds like the CAT ECM can't see vehicle speed (MPH), So the ECM "thinks" the truck is always standing still (0 MPH) thus the cruise swithes are only acting like they are in PTO MODE, and the ECM is SET/programmed to 1400 RPM in PTO MODE, thus that is all the RPMs that you can get out of the truck, whether stilling still OR driving=ECM thinks truck is at 0 MPH.


Jakes, did you hook up the wires, to jake solenoids, under valve covers?  

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cheddarbob56
Posted 10/17/2012 11:41 (#2645352 - in reply to #2645335)
Subject: RE: Peterbilt 379 cruise control


Southeast SD
That makes some sense 4WD..... when we bought the truck it "had" a 3406E motor in it and in the past someone had put the C-15 in and I did notice when we got it that the little CAT screen on the LH side of the steering column on the dash doesn't do anything so I wonder if they never switched that unit out when they did the motor swap?

And yes I hooked all the wires back up under the valve covers and the main harness is connected to the valve cover on the outside. I have heard that there is a clutch switch that can go bad that controls the jake brakes, could this be a possibility?
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4WD
Posted 10/17/2012 11:53 (#2645369 - in reply to #2645352)
Subject: RE: Peterbilt 379 cruise control


SW Iowa

By your first description/story, the clutch switch and brake switch must NOT be stuck, otherwise you would never be able to even set those cruise switches to 1400 rpm(=won't work if clutch is pushed downward OR if the ECM "thinks" clutch is pushed; and likewise, you couldn't set 1400 rpms IF the brake pedal was pushed downward OR if the ECM "thinks" the brake is pushed downward.

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Vorderstrasse
Posted 10/17/2012 11:54 (#2645370 - in reply to #2645352)
Subject: RE: Peterbilt 379 cruise control



SCNE
The clutch switch could cause both of your problems.
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cheddarbob56
Posted 10/17/2012 12:03 (#2645378 - in reply to #2645369)
Subject: RE: Peterbilt 379 cruise control


Southeast SD
That makes alot of sense, I didn't think about that earlier. So you're take on it the problem is the motor swap and the ECM not talking to each other.
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4WD
Posted 10/17/2012 12:18 (#2645397 - in reply to #2645378)
Subject: RE: Peterbilt 379 cruise control


SW Iowa

cheddarbob56 - 10/17/2012 11:03 That makes alot of sense, I didn't think about that earlier. So you're take on it the problem is the motor swap and the ECM not talking to each other.



Your jake problem, can't be the motor swap, from 3406E to C-15, BECAUSE you stated that it worked, when you purchased the truck(assuming the C-15 was already installed).


Have you tried, shutting OFF the cruise switches, and just try the jakes, setting still, in the yard, but you have to rev up the engine and let it coast back down to idle, when testing, WITH jake switch ON.
Also, are there jakes switches, on the tranny shift tower???? Are they turned ON???? (Some trucks have those, too)  


Another thing, Did you put the correct mounting bolt, for the jakes oil supply, into the correct holes, that hold down each jake head, onto the rocker arms????  


Edit: Did you set rocker arm shaft, down correctly, to supply oil to jake heads, too?         



Edited by 4WD 10/17/2012 12:19
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cheddarbob56
Posted 10/17/2012 12:46 (#2645419 - in reply to #2645397)
Subject: RE: Peterbilt 379 cruise control


Southeast SD
Yes I agree that the jake problem is not the engine swap, because it did work when we got the truck. I was meaning the cruise problem is the ECM not talking with the speedo.

No I have no tried to test the jakes just sitting in the yard, just tried it on the road. It has a low, med, high switch as well as an "On/Off" switch right next to it. These are the only two switches in the cab for the jakes.

I didn't notice anything different about the bolts for the jakes. I woudl have to take a cover off again and look it over, I followed the assembly manual we had for the motor and torqued everything as spec'd.

As far as I know I put everything back together correctly. I'll have to double check my manual and take a couple of covers off and double check things.
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4WD
Posted 10/17/2012 13:15 (#2645454 - in reply to #2645419)
Subject: RE: Peterbilt 379 cruise control


SW Iowa

One more item: there is an ECM setting, that limits the jakes so they won't work/turn on BELOW a certain MPH, so IF these happens to be set at 5 MPH and your CAT ECM can't read MPH, then the jakes will never get turned on, because the ECM never sees a MPH reading above 5 mph( or whatever it is programmed to).


This setting isn't programmed very often, but it might be on yours, AND it would explain BOTH problems.


This setting, is so truck drivers don't operate their jakes as much, in towns/cities at slow MPH (more quiet=less complaints to trucking companies' home office, about noise in town/cities)  


So, maybe if you fix the MPH reading problem, it may fix your jake issue(depends on programming)     



Edited by 4WD 10/17/2012 14:00
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cheddarbob56
Posted 10/17/2012 14:03 (#2645511 - in reply to #2645454)
Subject: RE: Peterbilt 379 cruise control


Southeast SD
Thanks so much 4WD for all your help and suggestions. I'm hoping I can get a chance to look at it here as soon as I get done hauling grain. Thanks again. I'll try to post what I end up finding out.

Chris
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1466driver
Posted 10/17/2012 14:18 (#2645535 - in reply to #2645370)
Subject: Re: Peterbilt 379 cruise control


Interstate 35 North ia
4WD is correct on the clutch switch , If the clutch switch was bad you couldnt bump up your rpms just sitting still . To test it set up your rpms and then push the clutch , truck will go back to idle .
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cheddarbob56
Posted 10/17/2012 15:44 (#2645642 - in reply to #2645535)
Subject: Re: Peterbilt 379 cruise control


Southeast SD
Yes it does that, when sitting idling if I hit the clutch it backs down to regular idle speed so I would agree that he is correct.
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JD fanatic
Posted 10/17/2012 18:15 (#2645814 - in reply to #2645335)
Subject: Re: Peterbilt 379 cruise control


mw
That is exactly the problem, also the fan will stay engaged all of the time.
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masseyboy
Posted 10/17/2012 22:47 (#2646482 - in reply to #2645814)
Subject: Re: Peterbilt 379 cruise control


Not sure on your cruise control situation. Does the speedometer work? Also as for the Jakes some of these engine ecm's are programmed to latch. This means you have to step on the brake pedal before the Jakes will engage. Some trucks come in programmed this way and it drives me nuts. Thought the purpose of a Jake Brake was to help keep you from using the brakes as much?
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masseyboy
Posted 10/17/2012 22:49 (#2646485 - in reply to #2645814)
Subject: Re: Peterbilt 379 cruise control


Not sure on your cruise control situation. Does the speedometer work? Also as for the Jakes some of these engine ecm's are programmed to latch. This means you have to step on the brake pedal before the Jakes will engage. Some trucks come in programmed this way and it drives me nuts. Thought the purpose of a Jake Brake was to help keep you from using the brakes as much?
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cheddarbob56
Posted 10/18/2012 17:25 (#2647731 - in reply to #2645454)
Subject: RE: Peterbilt 379 cruise control


Southeast SD
4WD - what would be the cause of the ECM on the motor not talking to the speedometer? I think that is the problem, I can cut the idle setting off with either the clutch or brake while sitting still.

Thanks for all your help so far.
Chris
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4WD
Posted 10/18/2012 20:16 (#2648071 - in reply to #2647731)
Subject: RE: Peterbilt 379 cruise control


SW Iowa

Normally, on those later trucks/engines, The OEM manufacturers choose to send the MPH signal, from the rear sensor on the rear of the tranny, up to those engine's ECM, THEN the engine's ECM would send a precalculated signal (say 30,000 pulses per mile) up to the OEM dash/speedometer.{note: now, all oem "dip switches, in back of speedometers could be set the same for all trucks, BECAUSE the engine's ECM made the calculations, for different axle ratios and tire size =that was a different number programmed in engine's ECM, too}


Note: This is cheaper, for the OEMs, because they only had to put one sensor, at rear of tranny, instead of two sensor(one for engine; another one for OEM speedometer) Two sensors were common, on older truck with earlier electronic engines.  


Your MPH problem is probably either the MPH sensor, at rear of tranny, or its connector, or its wiring, going up to the CAT ECM
(IF there is only one sensor, on this "converted truck"; which I assume to be the case
Note: Cruise switches go to CAT ECM, so if the CAT ECM had a good MPH signal, then your cruise would probably work, EVEN IF the OEM speedometer didn't work, Because the OEM speedometer is just an output from the CAT ECM(again, I am making an assumption, of how your truck is wired, but pretty sure it is new enough that this applies)  



Question: did you guys remove engine OR engine and tranny during that C-15 repair? If so, maybe something didn't get hook up, by the tranny, OR wires got pulled too hard, trying to remove engine and the connectors' wires got pulled out, OR wiring was left loose and the driveshaft/u-joint ripped it apart. 

Have you guys crawled under truck and looked around?    

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cheddarbob56
Posted 10/18/2012 23:49 (#2648711 - in reply to #2648071)
Subject: RE: Peterbilt 379 cruise control


Southeast SD
The truck is a 99 and the motor is a serial number 6NZ if that helps. We did not pull the motor or tranny during the rebuild, just did a complete in-frame on it, however the cruise never worked since we got it and I couldn't tell you if whoever made the engine swap did just the motor or a motor/tranny all at once. The place we bought it from just thought motor but didn't really know for sure either. We have crawled around under it a little but didn't really know what to look for exactly. Now I have some things I can look at and check. The speedometer seems to be working just fine we've run this truck along with our other truck that we know has no problems and the speeds seem to be withint 5 mph of each other going down the interstate as far as we an tell. I'm shingling a roof this weekend but am anxious to crawl around under the truck and see what I can find out. It's not the end of the world not having cruise control but it sure is nice to have if the option is there :)

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4WD
Posted 10/19/2012 09:08 (#2649080 - in reply to #2648711)
Subject: RE: Peterbilt 379 cruise control


SW Iowa

Now, that you mention that the speedometer works, then I believe the MPH signal is actually getting to the CAT ECM and going up to the speedometer(again assuming only one MPH sensor at rear of tranny). 

Earlier posts make me believe that the cruise on/off works, the set/resume works, the clutch and brake switches work, SO now, I wonder about ECM programming ; because if is programmed to 127 mph at Low Cruise=cruise will never turn ON. (This setting is for companies that don't want their drivers using cruise control  )

Typical cruise programming = low cruise at 20-30 MPH and high cruise at 70-127 MPH (note: 127 MPH is the default=doesn't mean it goes that fast)


You may need to have somone with a computer(with CAT ET program on it) look at this truck and trouble-shoot it; IF SO, make them print out all the ECM parameters, so you have a record of how it is programmed. Also, ask him to program it how he normally sees a truck that runs in your application would be programmed (=get rid of programming from old company specs, that the engine was used for, previously


Note: At same time, with a computer hooked up, have them test jake switches operation, and look at jake programming, too. They can also test the wiring, up to the jakes solenoids to see if the current makes the jake solenoid plungers go up and down(must remove valve covers) AND with valve covers off, then at idle, they can manual push on jake plungers, to see if jakes work (=are they getting oil pressure to them) That procedure throws a little oil around, and is just a little messy.

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