“Reboot” or Replace?By Richard McCuistian .jpg "The Truck (1).jpg") In a world of space-age computerized components, “situation ethics”, “lemon laws”, and Jekyll-and-Hyde customers, it’s sometimes hard to decide whether to simply “reboot” a component or replace it. The Work Order
97 F15098,350 miles“Speedometer Inoperative, Overdrive Light flashing, CHECK ENGINE Light on.”
A Dead Speedo Needle
It was a clean, high mileage pickup with three mildly alarming problems. A knowledgeable glance at the repair order indicated that all three concerns were closely related.
In the old days, if an unscrupulous vehicle owner wanted to rack up a few thousand unrecorded miles on his vehicle, he just disconnected the speedometer cable and drove around not knowing how fast he was going for a few months.
Well, this F150 was already a high mileage vehicle, and nothing had been disconnected, but the speedometer was completely dead. .jpg "Dead Needle (2).jpg")
The Powertrain Control Module receives its speed signal directly from the speed sensor, as does the Cruise. The PCM depends on the speed signal for transmission shift strategy, and since the PCM promptly illuminated the “Check Engine” light and began flashing the “OD” (Overdrive) light on the PRNDL stick, the signal obviously wasn’t present there. Related symptom: The Cruise Control was inoperative as well.
A Short History Lesson
Vehicle Speed Sensors are as old as cruise control systems, and electronic speedometer heads have been around for almost two decades on some of the more expensive cars, but Ford pickup speedos remained cable driven through 1991. Ford affectionately acronymed their first electronic pickup speedometer as “PSOM “ (“Programmable Speedometer Odometer Module”). Certainly everybody in the car repair business has seen the analog speedometers with Liquid Crystal Display (LCD) odometers on 92-96 F Series pickups. These are only “programmable” in the sense that they can be reset (up to 3 times on 92 models, 6 times on 93/96 models) to accommodate different tire sizes, but that’s another story.
In 1997, the PSOM was almost completely phased out (the Super Duty clung to the old body style for one more year, still sporting the PSOM) and Ford pickups got a new design from the ground up. Along with rest of the redesign, F150 speedometers morphed once more, this time into a slightly different type of fully electronic unit, but without the LCD odometer. Before the end of the decade, there were two types of speedo heads available for trucks and full-size SUV’s, one with an old wheel-type odometer (still electronically driven), and another with a “Hybrid Electronic Cluster” (HEC), which boasts a vacuum fluorescent odometer, but still uses an old-fashioned needle for vehicle speed indication. The unit I drew this repair order on had the wheel type odometer (see photo.)
To Analyze the Symptoms…
Any effective repair of an electronic speedometer concern demands an understanding of the stuff behind the dash and under the truck. The PSOM’s on 92-96 model years received Vehicle Speed information from the Differential Speed Sensor in the rear axle. This sensor was hard wired to the ABS module but the PSOM intercepted this signal directly from the sensor and reshaped it before sending it out to the PCM and the Cruise Control Module. The 97 F150 still has the Differential Speed Sensor in the rear axle for Antilock brake operation but the Speedo gets its signal from an old style Cruise Control type sensor driven by plastic gears on the transmission output shaft and mounted on the extension housing, and the signal is hard-wired to the PCM and Cruise.
(Note: By 1999, the Anti-Lock Brake system would become the clearing house for vehicle speed information, aligning and defining the Differential Speed Sensor signal before sending it out to the Cruise, the Cluster, and the PCM.)
The Inevitable Code Scan
The scan tool threw me a P0500 code, which was just what I expected under the circumstances, and a quick look at the live data display showed a zero reading for the Vehicle Speed .jpg "Diff Speed Sensor (3).jpg") Sensor. The Check Engine light and the flashing OD indicator, when both activated, generally indicate that a problem has arisen that will affect both fuel/emissions and transmission strategy. The PCM obviously needs to know vehicle speed to properly handle transmission shift strategy, and it also does some pretty interesting things with the fuel and emission system based on vehicle speed. For example, the PCM will shut the injectors down if the truck is driven above a certain speed. Also, the OBDII part of the PCM’s program uses the speed signal to time its drive cycle tests.
(NOTE: The PCM parameters can be modified when reflashing a PCM to reflect a different tire size or axle ratio. I encountered a 99 F150 with an inaccurate speedometer a few weeks back because a dealer somewhere up the road had swapped Powertrain Control Modules with another truck, and since the axle ratio and tire size were different, the speedo was inaccurate to the tune of about 15 mph! That repair required a simple parameter modification using the PCM reprogramming function, but once again, that’s a different story.)
Gears and Stuff: The Visual Inspection
Since the speedometer was the only part of the cluster that was inoperative and it shares a fuse with some other components on the cluster, a quick visual inspection of the speed sensor was in order. The vehicle speed sensor on the extension housing was securely connected to the wire harness, and there was no evident damage to the wires. Sometimes a dog or a rat will chew on the wires and cause problems, but I didn’t see anything like that here. The pins were clean and tight on their terminals. Next, by removing the speed sensor, I made a visual inspection found a nice healthy looking driven gear on the sensor.
Reaching into the hole with my finger, I checked for the drive gear on the output shaft to be tight. Sometimes, particularly on 4 wheel drive pickups of this vintage, the gear will be dancing around loosely on the shaft, but this one didn’t have that problem. Measuring the resistance of the sensor, I found 280 ohms, a figure which was within the 200-300 ohm range indicated as normal in the Ford shop manual. Reinstalling the sensor, I left the underside of the vehicle and moved my troubleshooting focus to the instrument cluster itself.
Removing the Cluster
.jpg "Removing knob (5).jpg") Popping the trim cover off between the cluster and the steering column is simple, and with this cover removed, it’s quite easy to see which screws have to be removed to get the trim panel around the cluster out of the way. There is one screw behind the headlight switch, however, and it’s important to know how to remove the switch. Turn the headlight switch knob to the on position, then pull it out to turn on the fog lights (it’ll do this even if it isn’t equipped with fog lights). This exposes a small square aperture (see photo) where a pocket screwdriver can be inserted to release the catch holding the knob, which comes off quite easily.
Turning the knob over (see photo below), insert it back on the switch and turn it to the right. This releases the switch, which can then be pulled out of the dash quite easily. (I’ve encountered more than just one or two vehicles with mangled headlight switches where frustrated mechanics had used force to remove the switch.) Behind the headlight switch is a hidden screw which helps secure the cluster trim panel.
Checking the Signal
This speedometer has a fairly intricate little microprocessor built into it, but there wasn’t much I could do to check that. What interested me at this point was whether the sensor was actually generating the speed signal and whether the signal was reaching the cluster connector or not.
.jpg "Reversing knob (6).jpg")
The VSS signal runs through a wire coded gray with a black stripe. This wire travels from the sensor to the speedo head, but the PCM (pin 58), and the Cruise Control Module also receive a parallel hookup. The PCM also receives a connection from the ground side of the two-wire sensor on PCM pin 33.
Since nothing anywhere was seeing a signal on the gray/black wire, (the Cruise was inoperative also), I disconnected the instrument cluster and got it out of the dash and out of the way. I then connected my digital voltmeter to the gray-black wire leading to the cluster and clicked it over to DC volts. Sometimes the fuse panel will go bad and start bleeding voltage from other circuits into the VSS signal wire, effectively killing the AC VSS signal. I’ve had to replace a few junction boxes (fuse panel assemblies) for this malady. As it turned out, there was no DC voltage, so I clicked my meter over to AC volts. Jacking up the rear wheels with a floor jack (jack stands are always a good idea) I fired up the power plant, dropped the PRNDL stick into Drive and spun the wheels up while watching the speed signal on my meter. A stickler might want to connect an oscilloscope at this point, but I was satisfied with the alternating current I saw increasing with wheel speed (see photo).
.jpg "Removing Switch (7).jpg")
Next I decided to reconnect the cluster and re-check speedometer operation. The microprocessors we have all over the space-age vehicles we drive nowadays sometimes have the annoying tendency to come back to life when they are disconnected and reconnected, and this speedometer did just that. Spinning the wheels back up, I saw the needle sweep up like a champ, and speed signal was everywhere now. It was a classic case of a microprocessor “reboot” that brought about the unexpected resurrection of an otherwise dead speedometer.
Decisions, decisions
So what now? On one hand, the speedometer might work just fine from now on. On the other hand, it might quit before the customer got out of the parking lot, no matter how far I drove it to verify my “repair.” So far, I had a smooth hour invested in diagnosis, which the customer would gladly pay for, but should I sell him an instrument cluster (the speedometer head isn’t available separately) or let him try it for a few days? On the one hand, he might be really pleased to get out with such a cheap repair, but some customers get rather unsettled if the problem returns after a few days. We’ve all seen customers who will be really nice on the day they pick up the vehicle and pay the bill, only to turn nasty later on, no matter how clearly things are explained when the repair order is closed out.
In this case, I chose to let the customer try it for a few days. At the time of this writing, it’s been over a week and we haven’t heard from him. Hopefully, the “reboot” took care of his problem. Time will tell.
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Learning From My Experience 
