By Richard McCuistian 

Gathering data and analyzing it is the most important phase of troubleshooting.

2000 F 250 Super Duty

19,205 miles

7.3L Power Stroke

Automatic Transmission

4Wheel ABS

Antilock Brake System (ABS) warning light illuminated and the speedometer is inoperative.

“Ever Run Across This?”

             Those of us who have been in the field long enough to enjoy a measure of success become accustomed to the phone calls and e-mails that come from stumped techs who hope we’ve either “run across” their particular problem before or that we might be able to point them in the right direction so they can find it.  I once received such a phone call from Africa!  Shared information makes things better for everybody in the long run, and the world of automotive service and repair is changing at warp speed.  There are guys out there who want to be indispensable and continually refuse to share their knowledge, but they aren’t doing themselves any favors; what goes around comes around, and we all need help sooner or later.

            Technical Service Bulletins (TSB’s), regardless of who publishes them, are written and published around the idea that sharing information expedites repairs and increases productivity.  An experienced technician will research the TSB libraries available on a particular concern before digging too deep under the hood, and in many cases, information that would have taken hours to gather in the service bay is right there in black and white, but there are more than a few cases where no TSB can be found and nobody else can be found who has seen the problem.  These trail blazing experiences can be harrowing at best, especially when fresh customers are rolling in, appointments and deadlines are being met, and tickets are being written.

It goes without saying that some of the repair work we do as professionals is fairly simple.   And while it doesn’t take a rocket scientist to replace spark plugs, brake shoes, and belts, it does require technicians with skill, experience, and aptitude to maintain twenty-first century automobiles, and it’s best to have a specialist in the shop to handle the more complex troubleshooting and repair situations.  A “problem child” job that requires more intense troubleshooting can take days (or sometimes even weeks) to figure out. Since problems of this nature pop up at every shop now and then, they frequently generate the phone calls and e-mails some of us receive. And as much some of us like to help other techs, none of us has all the answers.  Be that as it may, sharing information is to the field of automotive repair is what oil is to bearings.  The whole engine benefits.

ABS Lights and Speedometers

This month’s title deals with a 2000 F-250 with an ABS light concern.  The line technician who first worked on the vehicle first specializes in engines, manual transmissions, and differentials, and having noticed the ABS lamp and inoperative speedometer, he had replaced some components in the rear end, to include the Differential Speed Sensor.  The ABS lamp had been illuminated when he first drew the ticket, and when he was done with his grease and steel job, the ABS/speedo problem was still there.  When I drew the repair order (I was working at the Ford Dealer during part of my Christmas break), the ABS light was shining brightly and the speedo needle was as dead as the proverbial hammer.  My research turned up no obvious TSB’s related to this concern, but I had tracked this concern on more than one occasion to find bad ABS modules.

           The 4 wheel antilock ABS module (mounted on the Hydraulic Control Unit right behind and below the driver side headlamp on the F-250) receives the Differential Speed Sensor (DSS) signal from the rear axle.  The ABS module filters and processes the DSS signal on newer Ford trucks and routes it to the other modules, some of which depend on the Central Junction Box (interior fuse panel) to make the connection.                             

Network Test

 The ABS module is communicates using ISO protocol through Data Communications Link (DCL) pin number 7, sharing the ISO network bus with three other modules on the F-250, namely the Restraints Control Module (RCM), the Generic Electronic Module (GEM), and the “Overhead Console Module” (Dubbed OTC for “Overhead Trip Computer” on the Excursion).  Excursions also have the Parking Aid Module (PAM) sharing this bus.

      Connecting the Worldwide Diagnostic System (WDS) to the DCL, I worked my way through the opening program to the point where the Network test is done and found that the ABS wasn’t communicating over the ISO bus.  Moving to another screen to check the PCM for Diagnostic Trouble Codes (DTC’s) I found a P0500 stored as a memory fault.  There were two indicators that the ABS module was offline; it wouldn’t communicate with WDS and it wasn’t sending vehicle speed information to the PCM.   

 The next step was to see if the ABS unit had the necessary power and ground feeds; no computer will work if it’s not powered up.

    Opening the ABS schematic, and pinout charts, I found that ABS connector pin 14 supplies power to the module, with the power coming through fuse 24 in the interior fuse panel (Central Junction Box), and I decided to begin my diagnosis there. 

Fuse 24 was fine, but when I accessed the ABS module connector (it was necessary to remove two 6 mm screws and pull the fender liner down first), and turned on the ignition switch, I found no power at ABS module connector on pin 14. 

 The module would obviously be sound asleep without power.  I had discovered the problem, but now what?  Harness circuit 533 to pin 14 needed some 12-volt fire, and it was my job find out why it wasn’t there.  

Backtracking

  Following the schematic from ABS connector pin 14 back toward the power source, I found that circuit 533 traveled through bulkhead connector C1050, which is the center member of a trio of big square connectors located between the master cylinder and the fender, below and behind the Battery Junction box.  The Battery Junction box would have to be removed to access the connector.

         Finding the C1050 pinout chart in the shop manual, I located the male pin (#30) on the ABS module side of the harness.  With my test light monitoring pin 14, I touched power to the pin in C1050, to be rewarded by a nice healthy glow from the test lamp bulb. The harness between C1050 and the module appeared to be fine.  Taking a shot of the hard-to-see C1050 mating connector with my digital camera, I blew the picture up and looked for a damaged or pushed back female pin, but it seemed fine.  It was time to re-connect C1050 and move inside the cab to the part of the harness leading to the Main Junction Box (Interior Fuse Panel). En route to the Junction box is Splice #235, and as I researched S235 in the shop manual, I found that the same circuit also feeds the turn signal flasher via the splice and another circuit. A quick check of the turn signals revealed the fact that they were as dead as the ABS module.  I already had an idea that the Junction Box was at fault, but I wanted to be sure.

       The Junction Box connector in question was C242b.  According to the wiring schematic, pin 3 should have been feeding power from the innards of the Junction Box through C1050 all the way to pin 14 on the ABS module, but the connector pinout disagreed, and a real-world inspection of the connector itself disagreed with both illustrations.  According to the pinout chart for C242b, pin 3 is supposed to be vacant. Pin 2 is supposed to be feeding a Speed Control circuit, but the wire leading from pin 2 happened to be White/Violet, the very color of wire I was looking for. With C242b disconnected from the Junction box, I powered up pin 2 with my jumper and saw the test light come alive once again.  It wasn’t the first bad Junction Box I had ever seen, and it wouldn’t be the last.  A new Junction Box restored power to the ABS module, communication returned, and the Speedometer came online.

Similar Concern: Different Cause

    Friday found me helping my old cohort Ryan with a similar problem on a 2000 Ranger.  He had exactly the same symptoms; ABS light illuminated, Speedo inoperative.  He had also discovered a lack of communication, not only with the ABS module, but with virtually every other module on the ISO bus. 

           The modules on the Ranger ISO bus are GEM, RCM, and the Keyless Entry, or Remote Antitheft Personality (RAP) module, none of which would communicate with the WDS unit through the DLC.  Interestingly, the other modules operated just fine.  Ryan and I discussed the idea that one of the other modules might be interfering with bus communication while otherwise working properly.  It was a nice theory, but we disconnected the GEM, RCM, and RAP modules just to see if the ABS would wake up and talk, but nothing changed. 

       Measuring the DSS signal coming into the ABS unit from the rear end sensor, we found a nice normal 500 millivolts of AC voltage at about 20 mph, but no signal leaving the ABS module on the Gray/Black wire to feed the modules requiring vehicle speed information.  To make a long story short, we found pin 7 in the DLC had been spread by some overzealous troubleshooter, effectively preventing the whole ISO network from communicating with WDS.  Once we straightened the female pin out, we found a defective ABS module.  It had all its powers, grounds, and inputs, but the ABS module innards were cold and dead.  

Concluding Thoughts

             Not even an $11,000 scan tool can give much information when a module won’t talk or when the network goes dark.  On the F-250, the ABS module wasn’t even fired up, a condition that rendered the scan tool ineffective; “Joe Tech” had to reason things out on paper and find the problem the old fashioned way to find the causal component.  

          On the Ranger, communication was actually normal between modules on the ISO bus (except for the bad ABS Module) but a poor DLC connection and a string of error messages on the WDS screen used up most of a morning and part of an afternoon before we thought clearly enough to have a look at the connector.  For this Automotive Mechanics Instructor, it was a fine end to a really memorable week.