Hello I am working on a 1998 Mazda B4000 4x4 manual transmission with 3 error codes.

P0500 - Vehicle Speed Sensor A
P1151 - Lack Of H02S21 switches - Sensor Indicates Lean
P1506 - IAC Valve Duty Cycle Higher then expected. ** I'm pretty sure I fixed this one. The ideal was just turn up and I already replaced the IAC valve and checked for vacuum leaks before I found that it was.

Right now I am focusing on P0500 but help on the others Is very welcome.

For P0500 I have replaced the ABS sensor that is on the rear diff since I did not find a speed sensor on the transmission. After which made when ever I go above 10 miles an hour pegs my speedometer gauge at max and makes the truck lurch. (which i assume is the computer trying to fix things and slow down. ) Before I replaced it I had no speedometer at all.
I am going to try and return the sensor and see if I can get another one to see if it was just a bad part. If any one has seen this issue before and has advise I would greatly appropriate it.

 

Yes, 1998-2000 Ranger/B-series used the rear axle ABS and GEM Module for the Speed signal, but just those 3 years

Rear axle ABS sensor is still connected to ABS module, so if the yellow ABS light is not on then the rear axle ABS sensor is OK, assuming ABS bulb works, should come on with key on then go off

Hopefully it is just a bad new sensor

The GEM module has 2 wires spliced into the 2 ABS sensor wires, ONLY 1998-2000 GEMs have this
ABS sensor outputs about 20,000ppm(pulses per mile), Ford computers, cruise and speedometers need 8,000ppm
The GEM is used to convert the 20,000ppm to 8,000ppm then sends it out to computer, cruise and speedo/odo on 1 wire, its spliced off to the 3 devices
GEM actually has stock tire size in memory to do the conversion, and it can be changed, doesn't need axle ratio since pulses are axle rotations not drive shaft rotations

ABS sensor generates its own AC Voltage, a Sine Wave, 0.5vAC up to about 8vAC at high speed
Only half the AC sine wave is used, the + half, so one wire on the ABS sensor is actually a ground
This is why only 1 wire is needed from the GEM to each receiving device, because devices already have a ground

No speedo signal(and I assume odometer is also not moving, they share the 1 wire)
P0500, no computer speed signal
And if so equipped, no cruise

If ABS light is working and not on when driving then not the Rear Axle sensor

So its the 2 wires to the GEM, or the GEM itself, or the 1 wire out of the GEM

The GEM is behind the radio
GEM can have up to 4 connectors but each has a different number of pins/slots for wires
On its 18 pin connector are the speed wires you can test
Pin 1 is a Grey/black stripe wire that runs to computer, cruise and cluster(speedo), same color at all devices

Pin 9 is the + side in from rear axle sensor
Pin 18 is the - side in from rear axle sensor, the ground

If you get a long piece of wire you can test continuity between the 2 wires on the rear axle sensor and pins 9 and 18 at GEM connector to take that off the table

 

Sorry for the confusing wording.
Before the part was installed the speedo was stuck at 0.
After the part was installed when ever I go above 10 miles an hour the speedo reads 100mph.
Thank you for responding. I am picking up the part from napa tomorrow so hopefully it was the part and I can update then.
Also Ive replaced the gem already so I don't think that will need to be replaced again, but you never know.

 

Does the yellow ABS light come on with key on?
Is the ABS light ON when driving?

What year is the new GEM?
First few numbers
F87 = 1998 Ranger/B-series
XL5 = 1999 Ranger/B-series
YL5 = 2000 Ranger/B-series

Only 3 years that have the speed interface
Also needs to be for 4x4 as the GEM is also the 4WD controller from 1995 thru 2000

 

XL5T-14B205-EC
The listing I bought it from said was for a 4x4.

I just replaced the sensor and its not maxing out at 100 but it is back to staying at 0.
Also I have no ABS light on. So it was not the sensor to begin with. dang

 

I would pull the GEM and test the 2 wires that go to the sensor first, not sure where they are spliced to the rear axle sensor but the splice could go bad

If you have 4 wheel ABS the wires maybe spliced at the module in engine bay

 

So I tested the continuity from pin 9 to the wire from the speed sensor on the diff and I do have continuity.

Does that mean I am good from the sensor to the gem input? So I should be looking where pin 1 leads?
Thanks

 

Yes, and pin 18 should also have continuity to other wire on the rear axle sensor, either from a splice to that wire or a common ground point

Pin 1 on the same GEM connector goes to instrument cluster, Pin 1 on 12 pin connector, same grey/black wire

It also goes to Pin 58 on Computer, grey/black wire
And Pin 3 on Cruise Control, grey/black wire

 

Hello, I have hit a bit of a road block. Im not quite sure where check. So far I have eliminated,

• The speed sensor
• the wiring from the sensor to the GEM.
• The GEM itself has been repalced

I'm not sure what else to check. I was thinking making sure the dash connections are ok, but would the p0500 code go off if the speedometer connection is disconnected?
Any advice on what I should check out first would be appreciated .

 

Did you clear the P0500 code and then go for a drive and get it to come back?

You could have fixed a wiring issue and now just have a speedo issue

Test pin 1 wire to see if its shorted to ground, shouldn't be 0 ohms to ground, should be some resistance thru speedo, computer and cruise connections
If it is 0 ohms then you will need to unplug each one to see which one has the internal short

You can jack up rear wheels and start engine put in gear and test signals
Speed signals are AC Volts
Set volt meter to AC volts
Input voltage to pins 9 and 18 should be 1.5vAC to 15vAC(high speed), volts go up and down with wheel speed, but 20k pulses per mile on rear axle sensor

When GEM is plugged back in pin 1 output should be .5vAC to 8vAC, only 8k pulses per mile
If the grey/black wire on pin 1 is shorted out then no voltage
Or new GEM is bad, try old GEM just to see

 

Hello, Just to update you, I have testing pin 1 and was not getting any continuity to ground (I used pin 18 in case that matters) .
So I checked pin 1 and pin 58 on the PCM and I was not getting any continuity.

Given that I am going to try and follow the wire from the GEM to the PCM.

The thing that is bugging me is that when I had the faulty speed sensor and my speed was going all over the
place I thought that meant the wiring was good or how else could it get a signal.

Here is what I used in case someone else needs it.

 

That wiring diagram is only for 1995-1997 Ranger/B-series
1998 to 2000 were different
And then 2001 and up were different again

1997 was last year for a VSS(vehicle speed sensor), after that it was ABS sensor and then OSS sensor

Rear axle(ABS) sensor======================GEM pin 1-----------(grey/black wire)-----------------computer pin 58
So correct pin number at computer

Yes, pin 1 on GEM, grey/black wire, should have direct connection to pin 58 grey/black wire

Also to Pin 1 on instrument cluster C215, same grey/black wire
And to pin 3 on cruise control module, grey/black wire

The GEM Pin 1 grey/black wire splits into 3 wires each goes to one of the 3 devices
Computer, pin 58
Instrument cluster, pin 1, connector C215
Cruise control, pin 3

The cruise diagram shows it, but a bit hard to read, lower right, grey/black wires

 

Thank you for the clarification. I just reset everything with forscan and when for a long drive and I still have p0500 and some other codes that Ill look at later.
I was able to check the pins on the instrument cluster, cruise control and pcm and they all have continuity to the GEM pin 1.

My next step will be to try what you said about jacking up the back wheels and testing the GEM on pin 9 and 18. And if they are in the range of 1.5vAC to 15vAC I will know for sure that the gem is getting the proper signal.

Then I will test the gy/bk wire when the gem is plunged in ( probably from the cruise control module and a ground since they are easy to access) which I want to be around .5vAC to 8vAC.

I'm not sure about how to measure pulses per mile for them. Ill take a look at my multi-meter and see if there is any data like that on there.

Let me know if I am missing something anything but other wise I'm going to see where I put my jack stands.
Thanks again.

 

The Pulse per mile is the AC Sine Wave, in this, and most vehicle VR(variable reluctance) setups, only the positive half of the sine wave is used, which is why 1 wire is grounded

Sine Wave looks like this: https://www.investopedia.com/thmb/vk...f4d65c15af.jpg

There is a + half and - half as the AC voltage changes polarity, which is why its called Alternating current
In speed setups only the + half peaks are counted, and the peaks get closer together as speed increases, so more Pulses Per Mile

You need an oscilloscope to see the actual sine wave signal

Good SHORT video on testing VR sensors, Google: Variable Reluctance Sensor Testing
3min video

 

Ok I just got the measurements from the gem pin 9 and 18. In first I was getting 0.017vAC and it was reacting when I pushed in the gas. The multi-meter also said APO and T-RMS. I don't know if that effects anything.

Is this considered a short? Or maybe I got 3 bad speed sensors in a row?

 

And no ABS light?

Yes, that voltage is too low

APO? Auto Power Off?

TRMS is True root mean square, turn it off, you are not measuring a controlled Sine Wave like home electric power

 

That is correct no ABS light. I also don't have a check engine light on only reason I know its still off is because of forscan. But they still turn on when I put the key in so I know the lights work.
I do not think I can turn off T-RMS on this meter. Ill see if I can borrow a different one tomorrow.

 

Sorry about the delayed response. I decided to check if I got another faulty speed sensor and have it replaced again. testing it in first my speedometer reads im going 40 - 90mph and bounces around in that range. Its hard to read the acv from the gem it appears and disappears.

Something I noticed when i had the back wheels up is the left side spins freely but the right side does not. It twitches forward sometimes but its basically not moving.

I tested the voltage on the cruise control and was getting a pretty solid 5.5ACv

Any Help would be appreciate I am not sure what is going on to the back wheels.

 

If you have a limited slip rear axle then clutches may be worn down
Limited Slip means its LOCKED all the time, so it you spin one wheel the other wheel should spin the same way, as the axles are always locked together
"limited" slip means you added a friction modifier so the clutches can "slip" a little when going around a corner

OPEN axle means the EASIEST wheel to spin will get all the power from driveshaft
If you spin one wheel the other wheel will turn in the opposite direction, because they are not locked together
If one axle is harder to turn then that wheel may not turn at all when opposite wheel is spun
Could be failing wheel bearing or differential issue