New Holland backhoe all-wheel steering fault

Rusty Owner/Technician Oakham, Massachusetts Posted   Latest   Edited  
Question
Heavy Duty
Chassis
Circle Steer

This 2000 LB-115 backhoe tractor (Unit S/N … is stuck in circle-steer mode. The axles are not, and will not synch. It appears that circle-steer is the default mode as it defaults to circle steer with the control ECU unplugged. The selector switch functions and shifts the mode correctly on the display but the indicator remains blinking indicating that it cannot complete the mode change. I'm told that the fuse for the brake switch and AWS system has blown intermittently but today it would not blow and the ECU has power and ground.

This module has been replaced (updated) some years ago prior to being purchased by the current owner.

I'm handicapped as I lack a legible and understandable wiring diagram. I have some S/I and was able to identify what each of the 8 and 12 pins in the 2 ECU plugs are for. It seems as though we have no B+ to the steering solenoids above the rear axle. When I test for power on the control side of the solenoids on the 4 "EV" circuits in the 8 pin plug, they are all "dead".

My assumption is that I'll be hunting for a chaffed or corroded wire that feeds the solenoids but I'm baffled as to which of the 12 or so wires in the harness going to the solenoid body is the suspect circuit. Knowing that, I can apply redundant B+ and see if I'm on the right track. Then we did deeper or run an overlay. The harness is extremely difficult to follow and examine.

I do have the translator so I can translate the foreign color-letter designations to English colors but I can't read the diagram as its too fuzzy and there's no continuity between pages. (diagrams are PDFs e-mailed from a cooperative dealer but they are claiming the originals are just as bad) I'm hoping someone here has access to something understandable.

Happy trails,

+4
Chris Diagnostician
Lansdale, Pennsylvania
Chris
 

Rusty,

Scott just sent me over here. Give me 15 minutes or so to get on lunch and I'll see what I can do to help you out. 

+2 Ð Bounty Awarded
Rusty Owner/Technician
Oakham, Massachusetts
Rusty
 

Take your time. I'm into other things now. I'll go back to the machine anther day when I have better info.

I appreciate any assistance but again, no rush on my end

0 Ð Bounty Awarded
Chris Diagnostician
Lansdale, Pennsylvania
Chris
 

Rusty,

Shoot me an email at … - I have everything you should need.

Thanks

0 Ð Bounty Awarded
Chris Diagnostician
Lansdale, Pennsylvania
Chris
 

I'm going to post up the process Rusty and I went through, and he can add on when he has the opportunity. He was able to get the problem resolved.

I had the factory SI for the machine and so Rusty and I begin examining the diagrams and the theory of operation. Through theory of operation, some schematic reading, and a good dose of logic we were able to drill down to a test plan that seemed it would work.

SI states theory of operation of the 4WS as "when changing steering mode, the light for the current mode is extinguished and the light for the new mode starts to flash. When movement of the steering wheel is detected, the light for the new mode remains steady". As Rusty had said in the original post this mode light would remain flashing.

There was also no output to the steering solenoids. Further reading of the SI led us to the fact that there are 2 axle alignment sensors, one front and one rear. These are direct feedback sensors to the steering control unit that tells it to activate the steering solenoid valves when the axles are aligned in the straight forward position.

The output to the steering solenoids will only be disabled if there is an open or short detected at anyone of the 4 solenoids, or if there is no signal from the alignment sensors (which sensor is priority is dependent on which mode you are shifting to - Crab to 2WS or 4WS etc.) 

With the previous blown fuses it seemed possible that a solenoid was intermittently shorting and blowing the fuse before the module had a chance to shut down the high side driver (HSD). We determined Rusty could manually actuate the solenoids to determine wiring and solenoid integrity as a possible step.

We also agreed that the alignment sensors were a vital portion and so needed to be checked for proper operation as well. 

Rusty got back to work on it when he had time and found there was no input from the front alignment sensor. He found "no input to the ECU. All the solenoids are 5 ohms measured at the ECU and the common wire is indeed ground."

Upon further inspection he found this. The pigtail for the front alignment sensor was destroyed, this could also lead to the fuse blowing (if I recall the schematic correctly, don't have it in front of me). The lack of signal from the alignment sensor was not allowing the machine to see the criteria it required to shift steering mode. 

I am sure Rusty will chime in and give a more comprehensive account of the diagnostics as he did all the hard work. Just wanted to put up a resolution for now.

+5 Ð Bounty Awarded
Rusty Owner/Technician
Oakham, Massachusetts
Rusty
   

This machine is fixed. A replacement front "alignment sensor" solved the problem. As you can see in the photo Chris put up, the wiring was a mess where the 3 wires entered the sensor body. It wasn't obvious until some diagnostics let me to suspect that the sensor circuit was at issue. Once we stripped back the harness jacket, the broken and shorted wires became obvious.

Where I took a diagnostic detour was testing circuits at the unplugged AWS module. The SI that the tractor's owner had gotten from a cooperative dealer seemed to suggest that I should have found B+ at the control side of the 4 steering solenoids. I saw zip so I went looking for a wiring issue beneath the tractor toward the solenoid body. Once Chris Groff got me straightened out with some good, readable, schematics and some education, it became obvious that the 4 solenoids shared a common ground (rather than B+) and were POWERED by the AWS module. An impedance check of the 4 control circuits showed ~ 5 ohms each which is correct.

Chris refocused me on other possible causes of the issue and I went back to the machine to recheck all circuits at the module and I discovered that the rear alignment sensor would kick out B+ when the rear wheels were straight ahead but the front would not. Rear sensor resistance was 3.8K and the front was 4.6K. Examining the front sensor showed nothing unusual. I loaded the sensor power and ground circuits at the connector and did an ohms test of the feedback circuit and found no problems so I suggested to the owner that he source a new sensor which fixed the machine.

These sensors are a simple Hall effect proximity sensor that sends B+ to the module when a "tie rod" disc is at the sensor tip. that way the AWS module "knows" when either axle is aimed straight ahead and it can switch between front steer, crab steer, and circle steer modes.

The factory wiring diagrams were a challenge for me initially till I understood the foreign symbols and the translation for the indicated color codes to English and wire thickness to wire diameter in mm. :-)

Again, I big Thank You to Chris Groff and Scott Brown for helping me get aimed in the right direction.

+2 Ð Bounty Awarded