HELP: Procedure for testing speed sensor
Thread Starter
|
Tuned
Joined: Feb 2003
Posts: 426
Likes: 4
HELP: Procedure for testing speed sensor
My truck is an '87 F250, 300 cid, with a T-18 engine. What I'm asking about here is about the vehicle speed sensor that is on the tail of the transmission with the speedometer cable (my speedometer is mechanical). I am not asking about the speed sensor that is on the differential and is for the ABS system.
All my manuals are of little help on how to test this sensor. When I disconnect it and check it for ohms, I get about 200 ohms which I think is okay. Where I need help is with the wiring. There is a red wire and a green wire. Can anybody tell me where these go and what they do so I can test them? Are they positive or grounds? If they're positive, how much voltage should show up on them?
I will appreciate any help.
Thanks
All my manuals are of little help on how to test this sensor. When I disconnect it and check it for ohms, I get about 200 ohms which I think is okay. Where I need help is with the wiring. There is a red wire and a green wire. Can anybody tell me where these go and what they do so I can test them? Are they positive or grounds? If they're positive, how much voltage should show up on them?
I will appreciate any help.
Thanks
Senior User
Joined: Sep 2004
Posts: 484
Likes: 0
From: SC lowcountry
88 manual allows 200 to 350 ohms between pins 2 and 3 of 8 wire connector # 1 is on left looking at connector with latch on top/key on left. pin 2 is light green/ pin 3 is light blue; this reads resistance of speed sensor in Borg Warner 13-56 Electronic shift transfer case. Hope this helps.
Thread Starter
|
Tuned
Joined: Feb 2003
Posts: 426
Likes: 4
Thanks for your reply, but I think we must have a different setup. I have only a two-wire connector to the VSS. My ohms reading on the sensor itself seems okay. Where the two wires go and how I can check them is the mystery for me. I'm hoping somebody can fill me in.
Thanks
Thanks
Senior User
Joined: Sep 2004
Posts: 484
Likes: 0
From: SC lowcountry
Sorry, the info above is taken at the connector to the Elec shift control module and would check the wire continuity to the sensor - info is good for the two pins into just the sensor - wires should still be light green and light blue at sensor connector coming to transfer case. Hope this helps.
Edit - shift control module is located in passenger side cowl panel in front of door lower hine behind kick panel. All ths iinfo is for 88 model there could be some difference to the 87 and is for electronic shift only; the manual shift doesn't use the speed sensor in this shop manual.
Edit - shift control module is located in passenger side cowl panel in front of door lower hine behind kick panel. All ths iinfo is for 88 model there could be some difference to the 87 and is for electronic shift only; the manual shift doesn't use the speed sensor in this shop manual.
Last edited by 95X15; Mar 15, 2005 at 09:04 AM. Reason: location of shift module
Post Fiend
Joined: Oct 2003
Posts: 6,214
Likes: 13
From: New Jersey
That particular sensor is essentially a switch - it shorts between the two wires to provide a pulse, and the ECM has a pull-up resistor to 5V.
So you have to test it in a funny way.
Take two safety pins, and push them through the wires, one safety pin per wire, don't short the two wires together. Push the pins about 2-3" apart, so they don't touch each other or anything under the truck.
Then attach your voltmeter using alligator clips, run the wires underneath through your open door, to the voltmeter on your lap. Start the truck, then roll back a little bit forward or back. The display should change from either 5V (or 0V) to random numbers in the 2.5V range, then when you stop moving, settle down at either 5V or 0V depending if the sensor is open or closed.
If it stays at 0V or 5V when you mvoe the truck, its bad.
So you have to test it in a funny way.
Take two safety pins, and push them through the wires, one safety pin per wire, don't short the two wires together. Push the pins about 2-3" apart, so they don't touch each other or anything under the truck.
Then attach your voltmeter using alligator clips, run the wires underneath through your open door, to the voltmeter on your lap. Start the truck, then roll back a little bit forward or back. The display should change from either 5V (or 0V) to random numbers in the 2.5V range, then when you stop moving, settle down at either 5V or 0V depending if the sensor is open or closed.
If it stays at 0V or 5V when you mvoe the truck, its bad.
Thread Starter
|
Tuned
Joined: Feb 2003
Posts: 426
Likes: 4
Thank you for taking the time to detail the test procedure. I have several manuals for this and similar trucks. They all let me down on how to test the VSS. Your instructions were clear, and I was able to follow them exactly. The voltmeter showed no change when I was driving forward and backward, so it looks like the sensor is bad.
Thanks again,
Thanks again,
Lead Driver
Joined: Nov 2002
Posts: 9,557
Likes: 44
From: Concord, NC
I would try it again with the setting on continuity, in case there is no power to the VSS. The VSS wires go to the computer eventually, pins 3 and 6 if I remember correctly. On my '87, there is a joint in the VSS line where it connects to the computer harness on the drivers side wheelwell under the hood. I want to say one wire is green and the other is black.
Trending Topics
Senior User
Joined: Oct 2004
Posts: 482
Likes: 2
When you are done with the test put some silicone RTV sealer on the wires over the pin holes. If water, and salt if you are lucky enough to live in a salt area, will get in through the pin holes and corrode the wires.
FTE Stories
Ford Trucks for Ford Truck Enthusiasts
3 Best / 3 Worst Parts of Modern Ford Ownership
Brett Foote
10 Amazing Upgrades That Solve Common Ford Truck Owner Headaches
Pouria Savadkouei
Every 2026 Ford Engine Explained
Brett Foote
10 Ugly Ford Trucks That We Still Kinda Love
Joe Kucinski
10 Things Every Truck Owner NEEDS (2026 Edition)
Michael S. Palmer
Rezvani's Latest Post-Apocalyptic Monster Is a Ford F-150 Raptor Underneath
Verdad Gallardo
Top 10 Most Expensive Ford Trucks Ever Sold on Bring a Trailer
Joe Kucinski
2027 Ford Super Duty Buyer's Guide (Every Model, Engine, & Package)
Brett Foote
Top 10 Ford Truck Tragedies
Joe Kucinski
Post Fiend
Joined: Oct 2003
Posts: 6,214
Likes: 13
From: New Jersey
Originally Posted by meck1
Thank you for taking the time to detail the test procedure. I have several manuals for this and similar trucks. They all let me down on how to test the VSS. Your instructions were clear, and I was able to follow them exactly. The voltmeter showed no change when I was driving forward and backward, so it looks like the sensor is bad.
Thanks again,
Thanks again,
Glad I was helpful... but that doesn't mean by itself the sensor is bad. What was the voltage reading? If it was 0V it might mean that one of the two wires are either shorted or open circuit. Set your VOM to "ohms" and try spinning the sensor again, and see if anything changes. If not, its dead. If you do get alternating open/closed readings, then its the wiring to the ECM, or to ground, and a bigger PITA to troubleshoot. Give it a try and report back.
Thread Starter
|
Tuned
Joined: Feb 2003
Posts: 426
Likes: 4
Follow up on the VSS testing-- Checking ohms across the VSS, with the truck setting still, I got a reading, but less than what I originally reported in my first post. The ohms was around 150. When I hooked it up and drove forward/backward, there was only slight change. I think the sensor is, in fact, dead (or near death) and have already replaced it.
I thank everyone for your help.
I thank everyone for your help.