Hi all, this is my first post here so I'm very sorry if I mess something up but here goes: I have a 1998 ford ranger with the 4.0 v6 and 4wd, the spedometer will work absolutely fine until I turn on either the running lights or the fog lights, and then the speedometer will stop working, or stop and then bounce back up then die again. I have replaced the vss sensor on the rear differential and checked the connections going into the cab underneath. Anyone have any ideas?

 

Welcome to the forum

No, nothing obvious comes to mind

On the Rear axle is an ABS sensor, but in 1998-2000 Ranger it also goes to GEM module behind radio, GEM converts it to Speed signal and sends it out to speedo/odo, cruise and computer
In general, if rear axle ABS sensor goes bad the ABS light will come on as well as no speedometer

The Speedometer is electric, and the Odometer, do both go off?
They share the one speed signal wire from GEM

Does Cruise control work when speedometer is off?
Can you read OBD2 codes?
If so see if you have P0500 code, that means Computer also has no speed signal

If cruise works and no P0500 code then problem will be in the instrument cluster
If no cruise and you have P0500 then problem is in GEM

Park lights do turn on the instrument clusters back lights, but separate circuit in the cluster
Adjust dimmer wheel and see if that effects speedometer, park lights on

 

I don't have an abs light on the dash lit up currently, when the spedometer dies the odometer stops working as well. My truck also doesn't have cruise control. I can check codes later today and see if I have the P0500 code.
I'll see if the dimmer affects the speedo as well. Thank you for all of your help!

 

so I checked the codes yesterday and no codes showed. I figured out i just have to have the dimmer switch turned down lower than half and the spedometer works great! Thank you for all your help.

 

So it is a Cluster issue

1996 to 2003 Ranger or Mazda B-series instrument clusters are direct Plug and Play, with tach or without, wire is there for tach, just FYI
Black face or White face doesn't matter

 

Guessing this may be a ground issue. Your instrument lights add a lot of current that has to flow to ground. If the ground is poor / high resistance between instruments ground and the actual chassis ground, then the electric current from the lights will cause the instrument 'ground' to float at some voltage higher than '0'. Dimming the lights causes less current to flow, reducing the voltage offset.

Recommend you pull the instrument cluster and inspect the connectors (forget which one has the instrument ground - maybe more than one). Clean both the plugs and the electrical contacts on the flex plastic backplane with a good electrical cleaner. Recommend Deoxit (available on Amazon) which won't damage electrics and cleans corrosion.

If that doesn't work, replace all the light bulbs with LEDs which draw much less current. Note dimming won't work as well with the LEDs, but should work well enough (had LEDs in the dash lights of my 1989 F150 and they worked fine).