Blazing New Trails

Jan. 1, 2020
One Moody pickup
Blazing New Trails
One Moody pickupVEHICLE: 2000 Ford F-250 Super Duty pickup truckMILEAGE: 19,205 milesDRIVETRAIN: 7.3L Power Stroke with automatic transmission and four-wheel ABSCOMPLAINT: ABS warning light illuminated and the speedometer is inoperative.Technical Service Bulletins (TSBs), regardless of who publishes them, are written 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 before. These trailblazing experiences can be harrowing at best, especially when fresh customers are rolling in, appointments and deadlines need to be 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 21st 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. 
Because 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 as many of us like to help others, none of us have 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 story deals with a 2000 Ford F-250 with an ABS light concern. The line technician who worked on the vehicle first specializes in engines, manual transmissions and differentials. He noticed the ABS lamp and inoperative speedometer while he was replacing some seals and bearings in the rear end and included a new Differential Speed Sensor (DSS) as a part of his repair. 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, the ABS light was shining brightly and the speedo needle was as dead as the proverbial hammer. My research turned up no TSBs, but I had tracked this concern on more than one occasion and found a faulty ABS module to be the root cause.The four-wheel antilock ABS module, which is mounted on the Hydraulic Control Unit right behind and below the driver side headlamp on the F-250, receives the 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.
This ABS communication failure message rendered the scan tool rather ineffective in finding this particular concern. The “OTC” failure message referred to an Overhead Trip Computer, an option not present on this truck.
(Above and below) Circuit 533 travels through pin 30 in C1050, which is one of three large bulkhead connectors. Applying power to the male side of the connector fired up the test light, which was connected to ABS module pin 14. The circuit from the bulkhead to the module proved to be fine.
Above, after removing the two bolts holding the front of the left hand fender splash shield, this connector could be accessed. Finding Pin 14 in the real world is fairly easy with a shop manual pinout chart (right). There was no power at the pin.Network test
The ABS module communicates using ISO protocol through Data Communications Link (DCL) pin number 7. On the F-250, it shares the ISO network bus with three other modules: the Restraints Control Module (RCM), the Generic Electronic Module (GEM) and the Overhead Console Module (dubbed the 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 Network test and found that the ABS wasn’t communicating over the ISO bus. Moving to another screen to check the PCM for Diagnostic Trouble Codes (DTCs), 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 power and ground. 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 Central Junction Box, so 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 6mm screws and pull the fender liner down first) and turned on the ignition switch, I found no power at the 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.
In a situation like this, it’s a good idea to set up a high impedance test lamp in plain sight so that it will continuously monitor the circuit in question. Otherwise, you may repair the concern without knowing how you did it. Nothing is more annoying than having power return to a circuit while you’re manipulating harnesses and connectors. If you don’t know where you were working when power was restored, you may see a hard fault dissolve into an intermittent, which is a revolting development that will ruin anybody’s day.Finding the C1050 pinout chart in the shop manual, I located the male pin (#30) on the ABS module side of the harness. While my test light monitored pin 14, I touched power to the pin in C1050 and was 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 reconnect C1050 and move inside the cab to the part of the harness leading to the Main Junction Box. 
En route to the Junction Box (remember we’re backtracking through the circuit) is splice S235, 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 chart 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 based on the ABS schematic. With C242b disconnected from the Junction Box, I powered up pin 2 with my jumper and saw the test light come alive once again. Aha! 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.The innards of these Ford junction boxes are a laminated design that seems pretty robust. Unless it was spoiled by a water leak at the base of the windshield, I’ve never been able to tell why any have failed. I’ve disassembled some, but the laminated plates and their plastic separators are so complex that I’ve never been able find the place where the circuit went open or shorted inside the box. Any time you have lots of screwy electrical problems on ’97-and-later F-series trucks, check out the Junction Box. I replaced two of them on brand new 2000 Power Stroke diesels because the circuit feeding the backup lamps and the electric vacuum pump went dark. I’ve seen strange speedometer problems where the speedo would read fine up until 30 mph then drop to zero due to a Junction Box circuit shorted internally to power. I’ve seen headlights come on for no apparent reason and stay on for hours, only to find that the Junction Box was shorting out internally. They don’t all fail, but we do replace more of them at the dealership than would seem proper. The replacement needs to be the right one; one size doesn’t fit all.Similar concern – different cause
One Friday I was 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 also had 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 that pin 7 in the DLC had been spread by some overzealous troubleshooter, and it was effectively preventing the whole ISO network from communicating with WDS. Once we straightened the female pin out, we regained communication with the other modules and found a defective ABS module. It had all its powers, grounds, and inputs, but the ABS module innards were cold and dead. ‘Joe Tech’ had to reason things out on paper and find the causal component.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 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.

Sponsored Recommendations

Best Body Shop and the 360-Degree-Concept

Spanesi ‘360-Degree-Concept’ Enables Kansas Body Shop to Complete High-Quality Repairs

ADAS Applications: What They Are & What They Do

Learn how ADAS utilizes sensors such as radar, sonar, lidar and cameras to perceive the world around the vehicle, and either provide critical information to the driver or take...

Banking on Bigger Profits with a Heavy-Duty Truck Paint Booth

The addition of a heavy-duty paint booth for oversized trucks & vehicles can open the door to new or expanded service opportunities.

Boosting Your Shop's Bottom Line with an Extended Height Paint Booths

Discover how the investment in an extended-height paint booth is a game-changer for most collision shops with this Free Guide.