Hi, I'm new here. I need some help with an issue that's driving me nuts. I've had my '98 Ranger XL 2.5L 4x2 Automatic for almost 20 years now. Last week I started noticing my O/D light flashing, then the spedometer started bouncing around, especially when I let off the accelerator. The truck also started shifting hard and/or at the wrong time. A lot of times the speedo would drop to 0 when I let off the gas.

I used Forscan and the only applicable code it pulled was P0503 - Vehicle speed sensor (VSS) -intermittent/erratic/high input. I figured the ABS Speed Sensor went bad so I replaced it easily enough but that didn't solve it (tried a second sensor too in case the first replacement part was faulty). The connector didnt appear corroded/worn but I replaced it too with no luck there either. I don't see any wear on the wires for as long as I could trace them before they joined with others in a bundle that runs behind the fuel tank. I checked the connections to the PCM under the hood but they looked fine too. Is my PCM faulty? I'm at a loss for what else could be wrong. I'm pretty sure that's the only VSS on my truck and it runs right to the PCM.

Thanks in advance.
Mike

 

Welcome to the forum

1998-2000 Ranger did use the rear axle ABS sensor as a VSS(vehicle speed sensor), but when it fails the ABS light would also come on along with erratic speedometer function and Computer code P0503

The rear axle ABS sensor outputs 20,000ppm(pulses per mile)
Ford speedometer, computer and cruise need 8,000ppm

What Ford did was to tap both Rear axle sensor wires and run those two wires to the GEM Module behind the radio
The GEM Module is programmed with tire size, it converted the 20,000ppm to 8,000ppm and sent that out to speedo/odo, computer and cruise(if so equipped)

I assume the ABS light works, it comes on with key on, a bulb test, if so then rear axle sensor is OK, if ABS light stays off while driving
If so the problem will most likely be in the wiring at GEM module or GEM module itself

You will need to pull out the radio to access GEM and its wiring
Diagram below of the wiring
You can check the 4 fuses in cab fuse box, seen on diagram

Pin 1 is the Speed signal OUT after its conversion, that wire splits and runs to speedo, computer and cruise, since 2 of the 3 are effected check to make sure that grey/black wire is in good condition
On that same connector pins 9 and 18 are speed IN from rear axle ABS
Pin 18 is actually a ground(low) signal from rear axle sensor, this is the common ground for vehicle, this is why only 1 wire OUT(pin 1) is needed at the speedo and computer, these devices are already grounded

You can use a long jumper wire to test continuity between rear axle wires and the GEM wires(9 and 18) to make sure the taps/splices are OK, so GEM is getting the speed IN signal from rear axle sensor
If these wires are OK then problem is most likely in the GEM module, assuming pin 1 wire looks OK not frayed or loose

 

Thanks for the reply and the diagram! The ABS light comes on for the bulb test but then stays off. I'll pull the radio and check the GEM and wire continuity tonight. So do the wires in pins 9 and 18 on the GEM connector run straight to the ABS Speed Sensor on the rear differential? I thought they ran through the PCM? My Haynes manual doesn't mention the GEM but has the attached diagram of the PCM and I thought pins 33 and 58 on the PCM came from the VSS. It makes more sense if it runs straight to the GEM though since the wires connected to the ABS Speed Sensor are red/pink and green/blk as noted on your GEM wiring diagram.

I may be getting ahead of myself, but if I test continuity and the wires between the sensor and GEM pins 9 and/or 18 aren't OK and I can't find the fault, can I run my own separate wire(s) between the the sensor and GEM? If the wires are continuous and the gry/blk OUT from the GEM is OK, then I assume the GEM would be the problem. Do I need to do anything special to replace the GEM (reprogram, etc.) or could I get one off eBay or from the scrap yard? Would it have to be specifically from the same year/engine/drivetrain as my truck? Thanks

 

1998 thru 2000 Rangers are unique in the speed setup

rear axle ABS sensor's 2 wires run to the ABS module, Ford spliced into them, not sure where, but not at the ABS module or sensor
So pins 9 and 18 at GEM are those 2 spliced wires
Pin 18 would also have continuity to Ground as well, as its grounded at ABS module

You know the rear axle sensor and it's 2 wires to the ABS module are OK, because ABS light stays off while driving, you can confirm this by unplugging rear axle sensor and going for a drive, should see ABS light within a few miles, its not instant, it will go off after plugging sensor back in

PCM gets its speed signal from the GEM, same as speedo/odo, pin 1 on GEM


If you had 4 wheel ABS(4WABS) then its possible the converted 8,000ppm signal comes from the 4WABS module, Ford did that in Explorers in these years, but rarely found on Rangers
But you have RABS(rear ABS) in a 1998 Ranger most likely, so moot point

 

Alright, thanks for the info.

I pulled the radio and the GEM and checked the gray/black wire from pin 1. It was tight and didn't appear to have any frays or damage for as long as I could see. Photo below. I checked continuity between both pins on the ABS sensor and pins 9 and 18 and got a continuous signal on each, respectively.

So what's remaining, the GEM itself? Any way to check that pin 1 wire, other than visual?

Should a GEM off another 1998-2000 Ranger be compatible, and if so, is replacing it as simple as plug-n-play or does it need to be programmed somehow? Below is a pic of mine opened up.

Thanks again for all your help!

GEM connector. Pin 1 gray/black bottom right. Pin 9 red/pink bottom left. Pin 18 green/gray top left.

GEM module open

 

Yes, that's about all that's left, the GEM, yes must be from 1998-2000 Ranger or Mazda B-series, engine or transmission don't matter, power group or 4x4 does

You can plug back in the connectors and go for a drive, see if its working again, sometimes just cleaning the contacts(by unplugging and plugging in) fixes intermittent issues

1998-2000 GEM is programmed for tire size, that's it, and there wasn't much difference in those years
GEM runs power door locks and windows(power group) also ran 4WD system from 1995-2000

You could put an ohm meter on Pin1 wire, shouldn't show 0ohms, then wiggle dash wiring around so see if there is a short to ground, causing the intermittent issue

 

If I don't have power windows/locks does it matter if I get one from a truck with power, or will it just not do anything?

 

No that shouldn't matter, the other way would matter

 

I thought the picture of your GEM looked different, its actually called a CTM(Central Timer Module) since its missing the connections for many power group functions

So try looking for 1998 CTM

Is this the part number on the unit, F87B-14B205-AD

Looks like this in the "package": https://i.ebayimg.com/images/g/QUEAA...rVH/s-l640.jpg

Where a GEM of the same year looks like this: https://i.ebayimg.com/images/g/QUEAA...rVH/s-l640.jpg

 

Yes, my part number is F87B-14B205-AD. I'll see what I can find at local salvage yards from 98-2000 4x2 Rangers or Mazda Bs. If I don't have any luck, I'll try ebay. Thanks - I hope this does it...

 

Yes, check Rangers and Mazda B-series with no Power Group


Only other test you could do which is not really needed is to jack up rear wheels and run engine so wheels are spinning
Block wheels!!!

Set Volt Meter to AC volts, yes AC volts
With GEM plugged in test the red/pink stripe wire with green/black stripe, should see about 1 volt AC, volt level is not that important, just getting the AC volts means the Rear Axle sensor is generating AC volts, which we know it is.
That's how it works, sensor generates its own power in AC volts, but the Frequency is used as a speed pulse not the volts, volts are the by-product of generating the pulses, volts can go as high as 10vAC at say 80MPH

Then test Pin 1 grey/black with green/black or any ground really, and should also see AC volts, the converted signal pulse
If not then GEM is not working to convert speed to 8,000ppm
And should be a steady voltage, no jumping around, it will go up as wheels spin faster but steady at all times, like a working speedometer

 

Alright, strange new development. I plugged the GEM back in and went for a drive just to check and see. ABS light came on as I left the driveway. Speedometer never budged off 0 and the truck wouldn't shift out of 1st as I drove it (slowly) around the block. GEM connections were both tight and my ABS sensor was still connected. Is the GEM totally shot now or did I screw something else up?

 

If ABS light is on then its the rear axle sensor, maybe it didn't get a good connection when plugged back in the last time

 

Ok, yes, the ABS connection wasnt fully clicked in.

I went to the scrap yard and found a GEM from a 99 4x2 Ranger w/ no power group. Hooked it up and took it for a test drive. It seemed maybe better but once I drove a few miles it eventually started reading erratic VSS. I used Forscan to clear DTCs and ran a few of the tests. Went for another drive and it did the same thing. VSS would be good for a while then (usually when I let off the gas) it would start jumping around. It popped up a code P0503 again. Forscan readout of VSS would go from 35 (actual speed) to say 55, then 20, then 40, then 12, etc.

Only thing I can think is there's an intermittent short somewhere. I know continuity on the pins was good when I tested but maybe once I'm driving it changes. Maybe I can change the connector? I grabbed that from the scrap yard too. After that I'm out of ideas and may have to take it to the shop. I have a road trip in a month I need to take it on.

 

The ABS module would care about whats happening because its just looking for a sudden 0 RPMs, locked up rear wheels
So although a long shot it might be the tone ring inside the differential, what the sensor reads

If possible hook up Volt meter, set to AC volts, to the 2 wires at the GEM, and go for a drive, should have steady AC volts increasing and deceasing with speed
That would show tone wheel is not the issue
GEM doesn't need to be plugged in for vehicle to drive, just won't have wipers or interior lights

 

Is the tone ring what DTC P0503 is referring to when it says this DTC could be caused by "Worn or Damaged VSS drive gear on transmission output shaft"?

I'll try the voltage test you suggested as well as the ohm meter on pin 1 to check for a short to ground. If I can figure out how to de-pin the connectors I'd like to swap them out for the "new" ones I just got too. The gray/black wire from pin 1, where does it lead to after it leaves the GEM? to the instrument cluster? Maybe I can pull that and check that entire wire...

 

Yes, that refers to possible tone wheel or sensor issue

Picture here of rear differential: http://www.supermotors.net/getfile/1...tonering_3.jpg

Tone wheel is what "looks like" a gear on the back side of the Ring Gear
Its just a press fit so can come loose although very very rare

Tone ring on its own: https://cdn11.bigcommerce.com/s-wh2t...320330.jpg?c=2

ABS sensor(most VSS sensors) is a magnet, each time a tooth on the tone wheel passes by it it causes an AC voltage pulse, this is called a Variable Reluctance sensor, used for crank sensors, cam sensors and front wheel ABS sensors

I do not know where Ford split the GEM Pin 1 Speed Out signal, on diagram below it shows S229(splice 229) is where it splits but that location is unknown to me, it would be in the 1998 Ford Ranger shop wiring diagrams

 

If the tone ring were loose it seems it would be a constant issue and not intermittent, right? My last test drive, it was working just fine until after a few minutes and I got up to 50-60 mph. Makes me lean toward an electrical issue I think. That tone ring looks tough to fix...

 

Test/watch the AC Voltage right from sensor while driving(pins 9 and 18), should be steady and go up and down with speed, if so then not a tone ring issue

 

Ok, I tested voltage and pin 1 resistance.

With GEM unplugged, pin 1 to ground was showing no continuity. I shook the dash wiring and still no continuity.

AC voltage with GEM plugged in on pins 9 and 18 started at 0 then worked up to about 1 volt at 35 mph and stayed pretty steady even when speedometer and transmission started behaving erratically.

AC voltage with GEM plugged in on pin 1 and 18 (ground) was erratic. It increased to about 6v at 35 mph but bounced around from 4.5 to 5 to 2.5 and even 0 at the same time the speedo was bouncing and truck was trying to downshift. I even swapped in my old GEM and it did the same thing.

 

That's odd the ABS sensor voltage wasn't changing or higher at higher speeds

Only thing left is to cut the Pin 1 wire, so you can re-splice it, and then test it again and see if AC voltage is stable, if so then one of the 3 units, speedo, computer or Cruise is causing the issue

If its still unstable then you have 2 bad GEMs, long odds but not 0 odds

 

No, the ABS sensor voltage (pins 9 and 18) was changing, it started off at zero and slowly worked up to about 1.0v at about 40 mph. Was around 0.75v at ~25mph.

I cut the Pin 1 wire and re-spliced it with a new terminal crimped wire I got off the scrap yard truck. Didn't see anything wrong with the original pin 1 crimp terminal. I even removed all the other pins from the connector and switched out the connector itself with the one from the loaner and re-inserted the pins. felt like I was picking a lock, lol.

Went for another drive measuring voltage on pins 1 and 18 and it still wasn't stable. Jumped again from around 6.6v (which is what seemed to be the "correct" voltage at ~35mph) down to almost zero and everywhere in between. It seemed to only behave erratically when I let off the gas.

I guess this GEM is bad too? in the exact same way? I ordered another one off ebay. It's just almost unbelievable that both GEMs would have the exact same fault. Are GEM issues fairly common with Rangers?

...Forgot to mention earlier I checked voltage on connector pins 1 and 18 without the GEM hooked up and it was reading a pretty constant 0.02v. The truck wouldn't shift out of 1st gear and the speedo didn't budge though but I guess that's expected if the GEM isn't hooked up.

 

The 1998-2000 are the only ones that use this circuit, I wouldn't say common but is does come up, and these 3 years are pushing 20+ years old now

There is an after market solution usually used on V8 conversions, seen here: https://www.dakotadigital.com/index....od/prd1192.htm
It does the speed conversion instead of the GEM, also adjustable for tire size
So you are not without options, and you have all the wiring IDed so simple install for you

 

Thanks. I really hope that's the case and the GEM replacement solves the issue. I'll find out Thursday when the part is supposed to arrive.

Good to know there's an aftermarket solution in case none of these GEMs are able to last this long and the GEM is in fact the culprit. I'm guessing you leave the GEM installed and just run VSS speed in, ground, and pin 1 speed out to this device instead. I've been considering trying to convert my Ranger to run on 15" tires instead of my current 14s so maybe this thing would come in handy anyway. I have no idea what else that conversion would require though and would need to research. 15" tires are just so much easier/cheaper to find.

 

Yes, that's all there is to it, speed signal IN, and speed signal OUT, and of course 12volt key on power and a Ground, so its all at the GEM

And it looks like Dakota Digital added Bluetooth and an APP for that converter, so you can adjust it/calibrate it from your phone, never seen that before???

Wheel size doesn't matter, tire size does, 14" or 15" wheels using 29" tires are the same to the speedometer