Just bought a couple days ago... 98' B2500 5 speed 4x2, single owner truck & had been sitting most of its life.

Starts no problem, drives like a dream, but it has some quirks...

Speedometer/Odometer weren't working consistently so I replaced the speed sensor... but nothing changed.

I noticed that the brake lights turn on randomly even with the truck turned off, no keys in ignition, so I swapped out the brake light switch thinking it would fix it... but nothing changed.

Door Ajar light stays on — was able to "fix" it with some WD-40 but I figure it's part of the same overarching problem.

I'm realizing now that it might be a deeper electrical issue... anybody have any advice?

 

Welcome to the forum

No connection on these things

Door switch is a common issue on Rangers, 1995 and up, not to hard to change, time consuming but not hard
Yes, spraying WD40 into the latch can unstick it but its temporary

1998-2000 Rangers use the ABS sensor on the rear axle for speed signal, it can fail but ABS light should also come on, did it?
Speedo/odo share 1 wire from the GEM module behind the radio in the dash, for speed, GEM converts the ABS 25,000ppm to 8,000ppm signal
Not common for GEM to fail on the speed circuit but its possible

If brake pedal is not staying up tight against the brake switch then the brake light will come on
Brake lights, like head lights, have full time power, key off doesn't matter
If brake pedal is not staying up you could have a problem in Master or booster

 

Thanks Ron, you've given me a lot of reassurance with these issues. ABS light never came on before or after replacing speed sensor so I'm assuming it is an issue with the GEM module or circuit leading to it... where would you recommend I go from here?

 

Pull out the radio to access the GEM module

Diagram below, you only need GEM 1

Top wire on GEM 1
VSS OUT(grey/black wire) is the wire that goes to cluster, computer and Cruise(if so equipped), its just split/spliced 2 or 3 times

The IN from ABS sensor is on same connector, pin 9 and pin 18
You can use an OHM Meter on pins 9 and 18 and should see 800-2000 ohms from the ABS sensor
You can use an OHM Meter and a length of wire to test if both wires are connected to the 2 wires at ABS sensor, 0 ohms

ABS sensors are Variable reluctance(VR) type, so they generate their own voltage, AC Volts, 0.5-8volts AC while wheels/axles are spinning
The voltage is not used
AC power comes as a Sine Wave, switching +/- volts, the frequency of the switching is the "signal" pulse, and only the + side is used, so - is grounded to vehicle
Which is why only 1 wire OUT is needed on GEM, VSS OUT, just the calibrated + signal needs to be sent to speedo, and computer because they are already grounded

If GEM VSS was the issue you should probably get one of these OBD2 codes
P0500 Vehicle Speed Sensor Malfunction
P0501 Vehicle Speed Sensor Range/Performance
P0502 Vehicle Speed Sensor Low Input
P0503 Vehicle Speed Sensor Intermittent/Erratic/High

And CEL(check engine light) would come on