Recalibrating late-model speed sensors for tall or small rubber
“Do the math” is one of today’s most over-used clichés. Truth is, if
most of us were disciplined enough to do the math, we’d be Microsoft
millionaires—and possibly paying others to fix our trucks.
Along the same
lines, most of us are good at putting performance Tires
on our trucks. But we’re too dumb or lazy to estimate how far off our
speedometers become after adding big meats. For example, if the stock tire
diameter is 30 inches and we bump up to 33s (a 10-percent increase), does that
mean that the speedo is now 10-percent slow? So, when the needle reads 55, are
we actually traveling closer to 61 mph? “Sorry, officer, but I don’t have any
clue how fast I’m actually going.”
Once again, modern technology saves us
from ourselves. An Icelandic invention called TruSpeed from Superlift offers a
straightforward way to correct the speed signal in many late-model,
computer-controlled trucks. TruSpeed intercepts the speed signal between its
source (such as the ABS sensor-circuit) and the PCM, modifying the pulse waves
so that the truck’s computer receives accurate information. Each quarter-turn of
the TruSpeed’s dial changes the speed signal by 1-percent for highly accurate
calibration. It also stores settings for two different tire sizes.
In
addition to controlling the speedometer reading, the vehicle’s speed sensor can
impact these other systems: anti-lock brakes, auto-trans shift points, fuel
delivery, timing and other emissions-related functions. Fixing the speed signal
to register actual speed and distance traveled restores these functions to their
stock parameters. Maintenance, resale and warranty issues are also affected:
Taller-than-stock Tires
cause the odometer to register fewer miles than actually traveled, and smaller,
low-profile rubber will make the speedometer spin faster than it
should.
TruSpeed
installation only involves four wires and can be done by anyone with basic
skills who can also solder. For calibration, the TruSpeed instructions provide
approximate settings based on the difference between new and stock tire
diameters and/or axle gears. Calibration should be verified by using roadside
mile markers, a GPS, a radar system or by pacing with another
vehicle.
These photos show a TruSpeed installation on a Chevy Duramax
(TruSpeed applications are available for many 1992-newer Chevy/GMC, Dodge, Ford, Jeep
and Toyota trucks). The main installation differences among vehicles are the
locations of their fuse box and speed-sensor wires.
Our leisurely
installation took about two hours. A nearby speed-survey radar trailer helped us
dial in and verify speed calibration. A few passes were necessary since our
initial “dial in” was off—this truck’s stock Tires
were long gone and we guessed at their diameter. Once we zeroed in on the
correct setting, the speedometer was incredibly accurate.
Mount the TruSpeed close to the fuse box if possible. (The unit is
water-resistant and can be mounted under the hood if necessary.) On this
Duramax, the under- column area works well. |
Refer to the instructions to help find a fuse that's hot in START and
RUN (the 10-amp cruise control fuse for this truck). Use a test light to
confirm power and then disconnect the truck’s batteries. |
Pull the fuse with the appropriate tool. Install the supplied tapper
connector and press the fuse back into place (far left, second fuse from
the top here). |
Crimp the supplied spade connector onto the TruSpeed's red wire. This
connection mates to the tapper in the fuse box. |
Route the TruSpeed’s green and yellow wires through the firewall and
into the engine compartment to the PCM. (We used another wire to fish them
through the firewall grommet.) |
Sensor wire locations vary among vehicles. This Duramax’s speed-sensor
wire is in the PCM, which is under the hood on the driver’s side. Release
the mounting clips to free the PCM. |
Refer to the TruSpeed instructions and isolate the correct sensor wire.
Cut it. |
Strip the factory speed-sensor wire. |
Slide the kit’s heat-shrink tubing over the wires, then crimp-connect
the TruSpeed's yellow and green wires to the truck's speed-sensor,
following the instructions. |
Solder each crimped connection for added insurance. |
The kit’s heat-shrink tubing provides a third line of defense. (A heat
gun is recommended, but the wife’s hair drier will work in a pinch.)
|
Superlift recommends wrapping electrical tape over the heat-shrunk
connections before returning the PCM to its mount. Then, use the kit’s
zip-ties to secure the green and yellow TruSpeed wires away from hot and
moving parts. |
Crimp the supplied eyelet connector onto the TruSpeed's black wire.
This ground lead secures under an existing bolt that’s grounded to the
vehicle. |
Use the supplied hook-and-loop fastener to mount the TruSpeed unit.
Secure all excess wires with kit’s zip-ties. |
Refer to the instructions and then use the calibration key to dial in
the approximate speed sensor signal. Fine-tune the speed signal (each
quarter-turn equals 1-percent correction) using a GPS, a radar, or by
pacing alongside a vehicle that has an accurate speedometer. |
From the street speed indicator, we could determine how accurate our
speedometer actually was. After a couple of tries our speedometer was
pretty accurate. |
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