Q: I'm having a problem with intermittent reading on the speedometer of my 2000 Nissan Frontier. Sometimes it reads correctly or jumps around to different readings or no reading. There are no code readings stating bad speed sensor. Should I just replace speed sensor or check other things first?
-- Clyde T.
A: Your Frontier's VSS (vehicle speed sensor) is a two-wire magnetic type signal generator which reports directly to the speedometer unit. The VSS signal is then shared with the PCM (powertrain control module) and is used for cruise control operation. It's understandable there may not be an OBD-II diagnostic trouble code (likely would be a P0501) as the fault is intermittent.
Have you tried checking to see if the fault affects cruise control operation? If an OBD-II generic scan tool is available, does it show consistent vehicle speed as the speedometer falters? My hope is the fault shows up everywhere, steering us towards the source of the signal!
The Nissan test procedure for a suspect VSS of this type is to remove it from the transmission, unplug the two wire connector, and see if it outputs 1/2 volt AC or more as you spin it by hand. This isn't going to help, as the fault is intermittent. Viewing the signal with a scope or graphing multimeter while driving is the best way to check, but there are easier/cheaper things to try.
Wiggling/reseating the VSS connector, inspecting for frayed wires and then checking for differing operation would be a productive diagnostic step. I'm typically not one to throw parts at a car, but with the cost of a replacement sensor being only $15 to $40 and it being easy to change, simply renewing it could be a big bang for the buck diagnostic step.
Worst case scenario: If replacing the VSS doesn't help, removing the IPC (instrument panel cluster) and checking connections would be next. Renewing the speedometer would be the final straw, after verifying a solid VSS signal to it during an episode.
Q: My wife has a 2014 Forester Touring, 2.5 liter. I go to the same shop for oil changes and tire rotation. At 53,000 miles, they said the front brakes were down to 3/10 and the next time I should consider replacing. Next visit, at 58,000 miles, they said the fronts were still at 3/10 but now the rears were down to 3/10. Is this an eyeball measurement? Should I just take their word for it? Original brakes, so should I just replace them anyway?
A: This seems a little odd because front disc brakes typically wear out around twice as fast as the rears do. The original friction material thickness on the Forester pads is 11mm in the front and either 9mm in the rear (or 11mm if equipped with ventilated rotors). The shop's 3/10 description is possibly an eyeball guesstimate, or it could have been measured with a tool, and the 10 could be their estimate of the original thickness in mm, which is pretty close.
Subaru says a millimeter and a half is minimum pad thickness. I would have no problem changing them at 3mm to avoid the risk of an accidental metal on metal condition (inner and outer pads don't always wear evenly).
The next questions is: Will this be a simple pad swapping or is rotor refinishing going to be necessary? I typically lean toward leaving rotors alone unless there's a reason for refinishing such as thickness variations or surface imperfections. Ask also for your choices available for pad quality. I would spend a little extra for high-quality pads with a good blend of quiet operation, superior braking performance and long life. A second opinion wouldn't hurt!
Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif. Readers may send him email at email@example.com; he cannot make personal replies.
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