I just purchased a 1998 Ranger with a 2.5l and 4-speed auto and the trans doesn't shift correctly and the speedo doesn't work. Showing a P0500 code. I went to change the speed sensor in the rear end and found it was brand new, apparently the prior owner had already changed it but the speedo doesn't work still, sometimes it will briefly jump up to 10mph while driving then goes back to 0. Trans also does the typical behavior when the speed sensor doesn't work, shifts hard, doesn't want to shift through gears properly, but it does clear up somewhat once it's warmed up but if I get up to highway speeds it will max out around 63 mph (used a speedo app on my phone) and if I try to accelerate further it will just rev up like it's out of gear and it starts slowing down. It looks like someone may have messed with the wiring harness trying to fix it as the looming is all removed for about the first 8 inches out of the ECU and replaced with electrical tape.

I'm considering trying a new ECU to see if this does the trick but are there any other possible causes I may have overlooked? Thanks in advance.

 

Welcome to the forum

In 1998-2000 Rangers the GEM module behind the radio did the speed conversion for speedo, computer and cruise

The rear axle sensor is for the ABS system, Ford hijacked it for those 3 years to do speed as well
So if ABS light is not on then rear axle sensor is usually not the problem, just FYI

The rear axle sensor sends out about 20,000ppm(pulses per mile), Ford speedos, computers and cruise need 8,000ppm

So the 2 wires from the rear axle sensor to the ABS module are tapped(not sure where on a 1998) and 2 wires are run from there to the GEM Module

GEM has several connectors, the 18 pin connector has the Speed system
Pin 9 and 18 are the two wires from the sensor/tapped wires
9 is the signal, usually pink/red stripe, but can be grey/black
18 is a ground, usually Green/black, but can be just black

GEM converts the 20,000ppm down to 8,000ppm and then sends that OUT from pin 1, it runs to speedo, computer and cruise on a Grey/black stripe wire

Could be that part of GEM is bad, or its a wiring issue TO the GEM
OUT wire would have to be unplugged for both speedo and computer to be effected

GEM is a multi-function "body computer", also called the BCM(body control module)
If does wipers, door lock, dome lights, battery saver relay

If other systems are not effected then its most likely just a bad wire

 

Thanks for the info! I'll check into that, hopefully that's all it is, if so I got a good deal. The dome lights and wipers work so that would mean the module itself isn't bad? Also FWIW the cruise control buttons are completely roached as they are on a lot of these trucks. Any possibility that would have any effect?

 

No, outside of cruise not working it shouldn't be an issue to the speed system

GEM has multiple functions, and separate circuits for each, so just one part of GEM can fail and other parts still work

If one of the OUT wires(grey/black) was shorted to ground then all 3 components, speedo, computer, cruise, would lose the signal
So when you have the GEM connector off test if that wire is grounded

Another test is to get a long wire and hook it to 1 pin on the unplugged rear axle sensor connector
Then test for continuity to the GEM wire
Test both rear axle sensor wires that way

One wire will show as a ground, thats OK, the other will will not, that should be the pin 9 wire at GEM

 

Thanks, also this is my first time really delving into electronics on a vehicle, what's a good source of information for how to do the testing your referring to? I have a decent understanding of electronics in general just not this specific area. Thanks again.

 

Vehicles use a 12volt DC electrical system, for something to work its needs both 12volts and 0volts(AKA negative/ground)

Any electronics are board level, inside a module, not something you test in the vehicle, you can only test the electrics powering the electronic part

There are 3 things you check
12volt wiring
Ground wiring
OHMs, the resistance in wiring or a device

GEM module is an electronic part, not sure Ford ever published board level diagnostics for these, its basically a replace, not repair, module

But you should test the wires going TO the GEM
does it have 12volts on the wires it should have 12v on
does it have good ground wires where it should have
Are the signal input wires OK, not broken

Below are the 12v and grounds for the GEM

Set meter for DC Volts and test the wires, put black probe on a good cab ground, metal bolt
Red probe on each wire that should have 12volts
If all the 12v wires test OK
Then leave red probe on a working 12v wire and test the ground wires on GEM connector, if it good you will see the 12v again

Wires can be tested with OHM Meter
Switch meter to OHMs
Touch the two meter probe tips together
Should see 0 ohms because the two probes are "shorted" together
If you put one probe on the end of 1 wire and put the other probe on the other end of the same wire then you should see 0 ohms again(or low ohms) if the wire is good, not broken or corroded

Because the meters probe wires are only a few feet long its hard to test wiring in a vehicle.......................but its really not
You just need to lengthen 1 probe's wire to 20 feet long, clip end of 20ft wire to probe, hook wires other end to the wire to be tested, like VSS sensor wire under the truck
Then test other end at the GEM, should see 0 ohms(or low ohms) if wire is good
If ohm meter doesn't change then wire is broken, if it changes but high ohms then it corroded

Wire to be tested-----------------------------------------------------------------------------------------------------------------------------------
.........................\----red probe--------OHM METER--------black probe---------<>---20ft wire---------------------------------------------/

Unbroken wire would be 0 ohms

You can also test spliced wires, a wire can be spliced at a few places, but each end of that harness should show 0 ohms to each other end and main wire, because its "one wire" split to different devices

 

This is exactly what I needed and doesn't sound terribly difficult. Thanks again