vehicle speed senor
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Senior User
Joined: Jan 2005
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vehicle speed senor
1987 f-150 fuel injected straight six. My speedometer hardly ever works will just bounce occasionally. When i pull the codes i get one for insufecient vehicle speed sensor. Seeing how the only speed sensor i can find is right on top of the speedo connection on the trans, i am guessing the two are related. Now to my question, hom much does the computer use this input? And it going to afffect gas mileage? Or make other un forseen problems? If not i will ignore it. I all ready have a code for egr not opening(vac line unhooked), power steering switch(unplugged because it was stuck and made crappy idle) And the air injection system(which the previous owner removed) so whats one more code.... So any one know??
Later chris
Later chris
Senior User
Joined: Jul 2003
Posts: 299
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From: central valley California
Do you have maf or speed density system..most likely you have SD system so it would affect you some what as that the comp bases calculations for the sensor...
if I am correct the 87 had the sensor on the differental
if I am correct the 87 had the sensor on the differental
Lead Driver
Joined: Nov 2002
Posts: 9,557
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From: Concord, NC
The VSS used by the computer is the one piggy-backed on the speedometer cable. It shouldn't have much affect on how the computer runs the engine. It possibly will affect how the computer acts when you are coasting down from speed, but I don't think it would make a big difference in economy or driveability. The speedo and the VSS code are definitely related. It could be something as simple as replacing the gear on the end of the speedo cable where it goes into the transmission. Or it could be more complex if the gear in the transmission is worn out and not turning the speedo gear. Only way to tell is to pull the speedo cable out of the transmission and inspect the gear on the cable.
Cargo Master
Joined: Sep 2004
Posts: 2,691
Likes: 6
Yah, and if it's not the gear then it's the VSS itself. I had one go bad on my 98 Taurus and the sucker ran like crap. It wouldn't shift at the right times. $60 for a new sensor, about 30 minutes to replace it and all is now good.
Post Fiend
Joined: Mar 2002
Posts: 19,402
Likes: 38
From: Bonita Springs FL
The computer doesn't look at that sensor at all. You have a mechanical speedo, and there were no VSS's on Ford trucks until 1992.
My guess is that either someone swapped the computer with one from a newer truck, or you didn't read the codes correctly. Did it give you two or three digit codes?
My guess is that either someone swapped the computer with one from a newer truck, or you didn't read the codes correctly. Did it give you two or three digit codes?
Posting Guru
Joined: Jun 2004
Posts: 1,781
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From: Central NY
Originally Posted by andym
...there were no VSS's on Ford trucks until 1992.
Correct me if I'm wrong.
Popa Tim
Postmaster
Joined: Aug 2004
Posts: 2,908
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From: West Mansfield, Ohio
You are 100% right popa. There is still a sensor on the pumpkin from like 87 to 91 for the Rear ABS. From the 92-96 years, that same sensor was also used to indicate speed, and the cable was eliminated.
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Post Fiend
Joined: Mar 2002
Posts: 19,402
Likes: 38
From: Bonita Springs FL
Ok, I did some research. All 1987 to 1996 trucks used the same sensor in the rear axle. In 1987 to 1991, it was used for the rear-antilock brakes. In 1992 to 1996, it was used for both the rear ABS and as a VSS.
So I was kind of right.
At any rate, it won't fix the original poster's problem - the computer doesn't look at the sensor for anything other than rear wheel lockup.
Edit: Or, like handyman says...
So I was kind of right.
At any rate, it won't fix the original poster's problem - the computer doesn't look at the sensor for anything other than rear wheel lockup.
Edit: Or, like handyman says...
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Lead Driver
Joined: Nov 2002
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From: Concord, NC
There is a separate VSS from the ABS sensor on '87-'91 trucks. It is mounted piggy-back on the speedometer cable and is used for cruise control and for input into the engine computer. It will set a code in the computer if it is faulty. However, if the speedometer itself is not working correctly, then the VSS might not be the problem. I would fix the speedometer first, and re-check for the VSS code. The VSS isn't used for much in terms of engine tune, but it is used in instances such as coast down to keep the engine from stalling. The sensor in the rear axle is not the problem here, and the VSS might not be the real problem either. The ABS sensor is only used by the ABS computer, which is separate from the engine computer.