Vehicle speed sensor / GEM question

[heck914]

I have a long story on a nightmare Ranger, but i guess what my basic question has become is , What is the relationship between the Vehicle speed sensor signal and the GEM module and the PCM???

2000 Ranger, 3.0, automatic 2wd. The vehicle speed sensor is in the rearend.

I am under the assumption that the VSS signal is a square wave digital signal?? It then goes into the GEM, comes out of the GEM and goes into the PCM.
Does the GEM convert this signal? multiply it? divide it?

Does the VSS signal have a specific frequency?

The problem i have is the speedo reads WAY TOO fast. 60 mph by the time you reach the end of the driveway. As a result The shifts are all screwed up, truck is not really driveable. Truck has New VSS, new GEM, new PCM.

Sorry for the specific technical question, any help or insight would be appreciated.

Thanks
Mark

 

[Cobraguy]

I can't speak for a Ranger. But the Explorer has used different strategies over the years. They have taken the VSS signal directly to the different components, they have taken it to the ABS module and had it distributed from there, and they have had is sent directly to the PCM and had it distributed from there. I'm sorry, but I just don't ever recall seeing any of them utilize the GEM to distribute the signal, but it is certainly possible. But since you've changed the PCM and the GEM, I'm wondering if either or both of those need to be programmed by the dealer to set the tire size and gear ratios? Maybe this is one of those cases that require a module programming. I'm just thinking out loud here...

May I ask why you changed the PCM? It is so very very rare to ever see one of those fail. And if they do, it is usually from improper handling by the tech. i.e. static.

 

[heck914]

Thanks for the response.

The PCM was changed for the exact reason you spoke of, there was a serious voltage spike which fried a lot of stuff (my nephews truck, he has changed the story 10 times about what happened).

The PCM was reprogrammed for PATS at the dealer, and then when we got it running and brought it back for the GEM recalibrate(we thought that could be the problem) , we got the run around from them and they charged us $85 and did nothing. I really dont want to spend any more money on this without learning exacty whats going on.

Unfortunately i dont have an oscilloscope so i cant measure and see whats happening.

Mark

 

[Cobraguy]

Can anyone else help us out here? Does anyone know where you go to adjust for tire size, gear ratio, etc on a 2000 Ranger? It's in the ABS module of my 99 Explorer believe it or not. But I don't know about a Ranger. It sure sounds like something isn't programmed correctly.

 

[MrWizard]

VSS and PCM

I have no idea why your speedometer and/or VSS signal is messed up, but here are some sample specs on the Ford VSS signal
(this is for a '94 Ford Explorer):

It's a DC-coupled square wave of roughly Vbatt-0 (i.e. 14V peak to peak when the engine is running). The frequency linearly varies, 2.22Hz/MPH, or
roughly 8000 pulses/mile. The signal runs to the ECM which counts the frequency. As you've noticed, if this signal is incorrect, or the ECM is
not counting it correctly, you really mess up the automatic transmission shifting, the fuel flow when you lift off the throttle,
the cruise control, etc, etc. I don't think the GEM has anything to do with the VSS signal, other than maybe they route
the VSS signal through it, on the way to the ECM.

From 1998 onward they use the ABS rear wheel sensor to send the VSS information to the ECM. Don't know if the signal characteristics
are different from the earlier pre-1998 models.

You didn't mention if the speedometer is reading erratically, or if it is simply wrong by a constant ratio (i.e. reads 60 MPH, when
it should read 30MPH.) You also didn't mention if you're getting a DTC code like P0500, P0501, P0503 (ECM thinks there is a VSS fault,
or VSS is intermittent).
Are you sure the wiring from the VSS sensor to the ECM is good? If not, I would guess the speedo would be reading erratically.
If you look in a Chiltons, or similar, they show the pinout of the ECM.

I'd also make very sure the ECM and VSS sensor are for that model year as well. I would also guess if you have the wrong ECM
programming the speedo would display a fixed, but incorrect reading. If it is completely the wrong VSS sensor, or ECM module,
you'd have a lot more not functioning properly. Again, just guessing but have you checked any ABS fuses to make sure they
are all good as well?

 

[Cobraguy]

Mr. Wizard...good information. But when they changed to the rear VSS sensor, the signal did indeed change. They aren't compatible. But the signal reaches the ECM via the ABS module. That is where it is distributed through out the vehicle. It also means you will have a VSS signal if you lose any of the ABS sensors...as long as one works. I had an ABS light on my 99 and was shocked to find out I only had one front ABS sensor operating. Yet everything in the truck (other than ABS) worked flawlessly. So the ABS module is adaptable. It's also where you go, IIRC, to adjust for tire size and gear ratios.

The GEM indeed gets the VSS signal. It comes directly from the ABS module. Pin 9 of connector 283. No VSS signal is distributed from the GEM, however. The GEM also receives the speed sensor inputs from the transfer case...but that's another story.

Somewhere around 2001 or so, Ford made another change and the VSS started going directly to the PCM where it was 'conditioned for use' and distributed. However, I'm not certain of when that change took place. There is a thread here on the forum that deals explicitly with overcoming that problem with a swap. Good read.

Please keep in mind, this is for Explorers. I still am not certain the Ranger is the same.

 

[heck914]

Sorry about the delay, but i just wanted to give you an update.

We took a chance and hooked up an old style speed sensor in the transmission, (its a rebuilt trans, still had a gear on the output shaft) and tapped the output wire of that VSS into the output wire of the GEM. That GEM output wire goes into the PCM, the speedometer and somewhere else , i cant remember.

The truck runs and drives now, shifts OK, and the speedometer works. No check engine lights yet.

We werent sure it was going to work, because we didnt know if the signals were going ot be similar.

At this point i am happy that we could at least use the truck as is. Thanks for all your help and information.