rfhaney

I'm getting conflicting info on location for the vehicle speed sensor (VSS).

AutoZone shows two different Vehicle Speed Sensors for my vehicle: This one (Click here) on or at the speedometer cable; and this one (click here) is on the transmission.

Rock Auto shows at least three different speed sensors when I enter "vehicle speed sensor" in the search box after I've selected my vehicle info ID. Two of them are shown under "1988 FORD F-150 5.0L 302cid V8 : Electrical : Speed Sensor".

One entry says, "Mounted on Transmission". Another says, "Differential Speed Sensor Rear".

And another section (keyed to my vehicle ID) shows "ABS speed sensor" associated with the rear brakes.

And this web page ( http://www.oldfuelinjection.com/?p=33 ) says the VSS is inside or on the transmission.

( http://www.oldfuelinjection.com/ seems to be focused primarily on Fords over any other make or model.)

So how do I make sense of this conflicting info?

Purplehazeir

Don't worry about the ABS sensor. That is strictly for brakes.

You may need to get under your truck and look to see if you have a sensor on the side of your transmission or on your differential. While you are down there, see if you can get the transmission tag number, or check your vin. That will help you determine what transmission you have and then you can check with the parts counter at the dealer on location, etc.

rfhaney

Thanks, Kevin.

Here's the latest:

Since I've gotten responses from two different people saying that the vehicle speed sensor (VSS) is on the differential, I decided to jack up the truck's rear end and crawl under and take a look.

I could not get a look from more than one angle, but it looks like this (click here) is probably the appropriate VSS because it has the same sort of "L-head" for the two-pin connector, assuming that the one show here is also two-pin with exactly the same fit.

The only Motorcraft speed sensor I found is this one (click here), but the head is not L-shaped, and besides it looks like it is designed with two different connector heads. Conceivably, one of the two connectors might match my F150 plug, but the "business end" of the speed sensor looks very different from the "business end" of the L-head speed sensor. So it cannot be that both of them are replacements for the one I have.

It also appears that the first one above -- the one with the L-head connector -- is simply a magnetic pick-up head with no moving parts. So it is hard to imagine how it could fail or wear out, etc. So if that is the right part, why would I be getting a 29 trouble code? Also, the warranty info on that link suggests that the speed sensor shown there is an ABS sensor associated with the rear brakes. That too is puzzling.

I also googled for 'Motorcraft 1988 F150 "vehicle speed sensor" ' but could find no Motorcraft speed sensor with an L-head.

And neither AutoZone nor O'Reilly (local parts stores) have either a Motorcraft or a L-head speed sensor on their web sites.

Can anyone positively identify the part I should order? Is it the one with the L-head at the first link above (Raybestos example shown)?

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I also felt around behind the one on my differential housing, and it seems that there is probably an obscured bolt fastening the sensor into the differential housing.

After unplugging the sensor, is this a simple matter of using a wrench to remove that (supposed) bolt by feel and then a simple matter to pull the old one out and then insert the new one and replace the bolt and then reconnect?

Are there any tricks to doing this? Is it likely that the bolt may be hard to break loose?

Any particular torque I should use to re-tighten?

I guess I should use anti-seize lubricant on the bolt if that is how the VSS is fastened.

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But I should note that now another person says I have two VSS's. (I guess that's not counting the likely ABS speed sensor associated with the rear brakes.) So that makes more sense vis-a-vis the available parts found. So now I should ask: Which one of the two VSS's could cause a trouble code 29? Or can either one of them cause code 29?

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I have a hunch that the VSS I should pay attention to is on the transmission, especially since the one shown here ( http://www.oldfuelinjection.com/?p=33 ) is also more easily available at parts stores.

It also happens to be like the one at the link above for the Motorcraft VSS.

Purplehazeir

Richard;

I would concentrate on the VSS in the transmission. As is stated above, I think it would be best to head to the parts counter at your local Ford dealer and have them help you in determining the correct part/part number for your truck.

RockAuto has good info and shows 6 different sensors and for each one there are four or more alternate OEM part numbers. I can see the confusion you are faced with. But, I have found the parts counter at my local Ford dealer to be very helpful and the prices were not bad...especially when you tell them that you can get the part cheaper elsewhere.

rfhaney

I went to the local Ford dealer. The service dept has a strict policy of not letting their technicians talk to do-it-yourself-ers. So no help there. They say they want their technicians earning money every minute they're on the job. (Is suspect they also want to spur do-it-yourself-ers to spend the minimum $98 to get a diagnosis.) Seemed very inflexible. But I guess Ford is a huge bureaucracy. Can't have any exceptions to the rules.

But I did get a look in the Ford parts dept at the RABS speed sensor that mounts on the differential. And after talking with people at the local Car Quest parts store, it became clear that the RABS speed sensor is likely not involved in engine management.

In fact, the RABS system has its own RABS control module and its own self-test procedure and its own trouble codes, but it apparently does not have its own keep-alive memory (KAM) because the Haynes manual says it looses its trouble codes when the engine it turned off. So I guess maybe the RABS control module stores the code 29 in the EEC-IV module's KAM. Seems odd. But that the only thing that makes sense if the EEC-IV module's trouble code 29 refers to a problem with the RABS speed sensor as several people have been saying.

Anyway, after clearing the EEC-IV module's trouble codes, neither the "check engine" light nor the anti-lock brakes light has been coming on. So, apparently, problem solved. See details at https://www.f150-forums.com/1987-1996-ford-f150-16/1988-5-0l-2wd-thermostat-lower-temperature-consequences-egr-pcm-functioning-1233/.