vehicle speed sensor question / GEM module
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Tuned
Joined: Oct 2002
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From: Southern Connecticut
vehicle speed sensor question / GEM module
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
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
Elder User
Joined: Oct 2005
Posts: 786
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From: Reno nv
it is mostly square wave, on,off but the leading edge is 5th order rounded. frequency is determined by rotational speed of differential unless you have 4wd abs.
the PCM actually compares the signal with a look up table and outputs a new signal to whatever. If you have access to an AE and some handheld scanners, you can modify the ratio. There are tire ratio tables somewhere out here on the net, mine are for my SD250. Whoever did the installs should fix their programming problem. Ford can do it but it won't be cheap.
you didn't specify how long the driveway is. I cant get above 5 mph on mine.
pop off the electrical connector on the VSS and while it will set a DTC/SES light, it will go into default mode and should allow you to drive it somewhere. You might lose the speedo, abs and possibly the airbag.
the PCM actually compares the signal with a look up table and outputs a new signal to whatever. If you have access to an AE and some handheld scanners, you can modify the ratio. There are tire ratio tables somewhere out here on the net, mine are for my SD250. Whoever did the installs should fix their programming problem. Ford can do it but it won't be cheap.
you didn't specify how long the driveway is. I cant get above 5 mph on mine.
pop off the electrical connector on the VSS and while it will set a DTC/SES light, it will go into default mode and should allow you to drive it somewhere. You might lose the speedo, abs and possibly the airbag.
Thread Starter
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Tuned
Joined: Oct 2002
Posts: 370
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From: Southern Connecticut
thanks for the response. I dont think we have tried driving it with the VSS disconnected, i should try that first to see what happens.
All the work was done by my father and I. The truck suffered a serious voltage spike and fried a number of things. I did double check all the parts we changed to make sure they were the correct ones. It was taken to Ford for a PATS re-program,(no problem) and then i took it back for a possible GEM calibration reprogram, and i got the run around at 2 dealers. I am tired of them charging me $85 just to look at it, and then do nothing.
I have seen those universal speedo adjustable calibration boxes, and i would consider buying one ($80 - 100) if i knew it could work.
Thanks
Mark
All the work was done by my father and I. The truck suffered a serious voltage spike and fried a number of things. I did double check all the parts we changed to make sure they were the correct ones. It was taken to Ford for a PATS re-program,(no problem) and then i took it back for a possible GEM calibration reprogram, and i got the run around at 2 dealers. I am tired of them charging me $85 just to look at it, and then do nothing.
I have seen those universal speedo adjustable calibration boxes, and i would consider buying one ($80 - 100) if i knew it could work.
Thanks
Mark
Elder User
Joined: Oct 2005
Posts: 786
Likes: 0
From: Reno nv
oops, sorry, on my 6 speed the vss is part of the abs.
the speedo recal boxes won't do it, the problem is within the look up tables. According to the book, I can have anything from a 15" DIA tire up to a (I think) 40" and use a stock table. If I loaded the 40 and ran the 15" tires, it would shift first within 2 or 3 inches. if I loaded the 15 and ran 40's, it would never shift. I can go bigger than 40" but it will take a custom calculated code to get there, as long as the number isn't bigger than (i think) 4400. probably more data than you need right now.
perhaps a hot rod shop that specializes in mustangs/Ford racing programs could re-flash you or alter the tables. maybe even a tire shop?
I understand the bit about Ford stealerships, have sort of the same problem with my local. OTOH, If I paid them 85 for a diagnosis, then dagnummit, they ought to be giving me a diagnosis that will fix it, or I'm on the ford customer service hot line kicking betuckus and taking names.
the speedo recal boxes won't do it, the problem is within the look up tables. According to the book, I can have anything from a 15" DIA tire up to a (I think) 40" and use a stock table. If I loaded the 40 and ran the 15" tires, it would shift first within 2 or 3 inches. if I loaded the 15 and ran 40's, it would never shift. I can go bigger than 40" but it will take a custom calculated code to get there, as long as the number isn't bigger than (i think) 4400. probably more data than you need right now.
perhaps a hot rod shop that specializes in mustangs/Ford racing programs could re-flash you or alter the tables. maybe even a tire shop?
I understand the bit about Ford stealerships, have sort of the same problem with my local. OTOH, If I paid them 85 for a diagnosis, then dagnummit, they ought to be giving me a diagnosis that will fix it, or I'm on the ford customer service hot line kicking betuckus and taking names.
Last edited by piotrsko; Mar 2, 2009 at 05:58 PM. Reason: d'oh
Thread Starter
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Tuned
Joined: Oct 2002
Posts: 370
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From: Southern Connecticut
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 to 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.
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 to 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.
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