Hello all...

had a question...I have an 87 F150 2WD that is in great shape and runs well. I recently changed the tires to a 265/75//R15. They look great. However, I have ben noticing some poorer mileage (gone from around Average of 14 to 11.15 to 12) and a vehicle speed sensor code on the EEC-IV system.

I am suspicious that the larger tires are "throwing off" the VSS, and am considering getting a speedo gear appropriate for the larger tires (about one more tooth I hear, although I need to figure that out too). I am thinking that the more appropriate speedo gear will set things right with the VSS, giving the speedo and VSS a more accurate measure of vehicle speed.

This is all based on the premise that the tires are causing this, which I cannot confirm or deny....It puzzles me as to how the EEC-IV system would notice the difference, unless its got some hard coded RPM measurements for different speeds in the computer or some such...

Any ideas on this, like whether these premises are sound?

This truck has once again become a daily driver, and i would like to get the mileage back closer to the 14/17 I used to enjoy (instead of the 11.5/13) I get now....

Also, the truck has the stock 3.55 rear end, that, according to web sites, would become essentially a 3.24 after the larger tires go on..Have I just confused the computer enough to throw off the mileage?

Thanks for any insight...Pertinent specs on truck below...

87 Ford F150 302 - N (EFI) , 3.55:1 Rear Axle, 4 speed E4OD Auto tranny...

 

Does nyone know what the VSS (vehicle Speed sensor) does on a 1987 F150 4x2 with cruise?..

I have scoured the web looking for more information, but its mighty scarce. My haynes manual has electrical schematics that show the VSS going to the ECM, and to the cruise amplifier. I have read on one site for engine swaps that some folks felt that the early VSS's were not used by the ECM for air/fule corrections..Anybody know how critical this thing might be, truck seems to run OK, but it may just be because the ECM is inserting a "default" value to get me by....

Thanks for any info...This is giving me a headache...:-X23

 

Are you calculating speedo error in your mileage readings? If not, that could be a part of you poor mileage, and it's not really that bad. The VSS is used by the cruise control and the RABS (if equipped). The RABS use it to detect a rear wheel lockup, and the cruise uses the pulse wave to maintain speed. The ECM only uses the VSS if you have an electronic speedo. If your speedo is mechanical, then the VSS isn't used for speed measurement. Oh, the ECM does use the VSS and Throttle position to determine shift points with the E4OD.

 

Well, I have done some calculations, and it seems that my speedo error should be around 10%...I confirmed this with a run between mile markers on the interstate...at 65 on the speedo, I calculate that I am actually doing about 71...I add 10% to the mile figure I use to derive MPG.

I spoke to the Ford Dealer about replacing the speedo gear in the tranny, and he mentioned that it would probably be sufficient to get a gear with one additional spline to make up the difference...

I think that is a quick thing to try, and might resolve my VSS signal too....Also, I recall that the VSS code (29) is in memory, not on a KOEO test....I assume this means its not a hard failure but intermittent....

I do have RABS, but I was under the impression that there was a seperate axle mounted sensor for that functionality...the wiring schematics for an 87 do not show a VSS input to the RABS sensor..This may be a difference for the early F150's...On that note, does anyone know of a way to get the RABS to give up diagnostics? It seems to have some sort of test connector....

I do have a mechanical speedo, so the ECM does not need it for that...The big question is whether the software routines utilize the VSS input for anything but cruise...

In any case, I will keep plugging along, and try the speedo gear change and see if things get better.....

I did run across a web site that had a mention of a Cruise control module test, but it specifically said it did not test the VSS sensor....Oh well...

1987 F150 4x2, 302 MFI, E4OD, all stock but K&N filter and suspiciously missing fresh air intake hose.
https://www.ford-trucks.com/user_gallery/displaythumbnail.php?&photoid=4972&.jpg

 

I had an '88 F-150, and the VSS is connected to the RABS system for rear wheel information. The sensor is located on top of the differential housing. That is the only sensor on the rear axle.

You can pull the RABS codes. You have to ground the pigtail test connector and turn the ignition on. The pigtail should be behind the e-brake, but I cannot completely remember.

As I think about it, I never got the VSS code on my F-150. I had a 6" lift and 35" tires on it, and didn't change the speedo gear for quite some time. It's probably from the VSS sensor, at least that's my guess.

 

Thanks Aaron..i guess I am puzzled by the wiring schmetics that show the 87-89 having a separate Anti-lock rear axle sensor, that is different from the VSS... i do take heart in the fact that you added 35" tires and had no VS code...Makes me want to keep looking for what this is about....

I will run the RABS diag and be sure that its coming up clean..Thanks for the tip...

Ronnie

 

 

Hi. I have a 92 Bronco that im doing some work on the RABS b/c my speedo is not working and flopping around. Im also going to replace my 30x9.50x15 to 31x10.50x15 and there is a test that you can run that will reset the speedo to the factory speed settings. Beware though, you can only reset the speedo three times!!!!! then you have to but a new head!!! I hope this helped you out


Scott

 

[updated:LAST EDITED ON 20-Nov-02 AT 02:21 AM (EST)]ftrain
The VSS that's mounted on the t-case where the speedo cable goes in is for the E4OD shift points and for the cruise computer. The ABS sensor in the rear axle wasn't used for those duties until '92. The computer won't detect the tire size change, but that change WILL affect the transmission's performance and your PERCIEVED gas mileage. The only reason the VSS would throw a code would be a short in the wiring or an internal failure. Consider replacing it.


Broncobigtire
It's 6 times. Read this:
http://bbs.off-road.com/ubbthreads/showflat.php?Cat=&Board=offroadfordbroncolate&Numb er=898100&page=1&view=expanded&sb=5&o=&fpart=