I am taking your advice and installing a new speed sensor to begin to address the NON-TC lock-up in my '94 4.0L/A4LD. My current problem is I can't find the *&^% thing on the vehicle. My Chilton's manual is nearly worthless and doesn't list this important sensor anywhere. I know, the tranny modulator is next even though it shifts fine. Where is this sensor located??

Also, would a bad speed control sensor effect the speedometer...the needle bounces around and isn't steady. This bouncing needle causes the cruise control to act weirdly and surge in response to this needle bounce. Or do I have something else going on?? Help!!

Michael

 

aka Differential Speed sensor in Ford Svc cd.
Top of rear axle (F9)

DP4 CHECK VSS RESISTANCE

l Key off.

l VSS disconnected.

l Measure the resistance of the VSS.

l Is resistance between 190 and 250 ohms?


yes, DSS can affect cruise control, TC LU, speedo readout stability.
also can cause the PCM to go crazy, jacking air/fuel mix around due to erratic input.

can also be damaged corroded electrical connector or wiring, damaged tone ring teeth on ring gear or metal cuttings in diff. lube

 

Hmmm, the sensor in my diff looked much simpler than that. This one looks like one of the sensors that goes into the transmission, and costs a bundle. The one that goes into the diff was only like about $20 from Ford.

 

that's the part that Rockauto shows for VSS, speed sensor. they've got it wrong. OD'd on pies and whipped cream yesterday, brain was soaked in sugar and Jack Daniels high.

correct part. Rockauto calls it an ABS wheel speed sensor.
correct picture. $10 bucks from RA

 

hah! mercedes truck for example use only 2 exangable speed sensors made by Wabco.... they have different a bit connectors as wheels speed sensors, cam and crank sensors... this is 1.2 KOhm coil sensor....

 

The wobble may or may not be fixable. If the speed sensor is bad you won't have any speedo..I have 2 eratic speedo's and never have been able to correct the problem.. The only thing that makes it stop is disconnecting the abs computer under the ash tray.. Then the needle is steady... On one van I replaced the tone ring the cluster and sensor to no avail.


Dick

 

OK, I just exchanged the VSS. This part does look like the first pic 96_4wdr posted. The speedo cable connects to the right side of the abov pic AND the sensor wires connect to the blue connection at the top right of the pic to gather vehicle speed data for the PCM. Yeah, the wobble continues with the speedo and the TC still doesn't LU. What next before I start looking at more complicated solutions? Should I confirm I have two good wires going from this VSS back to the computer? If so, how do I do this?

Michael

 

later Aeros went to an all electronic speedo doing away with the spinning mech. cable shaft and the VSS in the rear end.
how did the electrical connector on the sensor look, any corrosion?

unplug the ABS controller under the dash ashtray. they often fail loading down the VSS circuit which feeds the ABS, the PCM and the electronic speedo if there is one.

Ford must have had a bunch of older A4LDs with the mech speedo output and rear ends without the VSS hole, so they used them up in the Aero line which they decided to end when they started design on the WindlessStar in the early 90s
Ranger/Explorer got the creme of the crop.

 

I disagree with the asessment that a bad VSS will cause a no speedo situation. Our E-350 will jump around at speeds about 65 in an erratic way. The problem was the VSS, but the speedometer still had a readout. When these sensors are on the fringe of failure,t hey will act erratically, and can be affected by temperature, vibration, subtle voltage variations, etc.

Since you replaced the sensor and it still acts up, I feel I should suggest that battery cable and grounding problems are known to cause erratic behavior in the instrument cluster.

Someone correct me if I am wrong, but I'm fairly certain that the computer does not get its speed signal directly from the sensor. Rather, it gets its signal from the instrument cluster, which contains the circuitry for speedometer adjustments, so that actions are based upon the adjusted speed. So instrument cluster issues can cause transmission lockup problems. On my moms van, intermittent instrument panel problems were resolved by replacing the ground cable.

 

I was talking about a totally bad VSS....NO SPEEDO! 96 I've swapped abs modules to no avail..I have 4 vans so lots of goodies to play around..I can't figure it out I just unplugged the darn abs until I have time to fiddle which I don't seem to anymore..can't get the rears to lock so I'm wondering why its there in the first place..


Dick

 

from Ford Svc cd
new term DSS Differential Speed Sensor, same component
---
How The Circuit Works


The Programmable Speedometer/Odometer Module (PSOM) receives a speed signal input from the Differential Speed Sensor (DSS) and uses a programmed conversion constant to convert the signal to the standard 8000 pulses-per-mile speed signal output. In the vehicle, the PSOM and Rear Anti-lock Brake (RABS II) Module are parallel users of the DSS signal. The speed signal output is proportional to the road speed of the vehicle. The Programmable Speedometer/Odometer Module supplies this signal to the Speed Control Amplifier, the Instrument Cluster (Digital), and the Powertrain Control Module (PCM), all of which require vehicle speed information.

----------------

the pulse signal from the DSS feeds to the RABS controller and then to the input of the PSOM on the instrument cluster. the PSOM outputs a digi pulse signal to the digital instrument cluster ( if equip), the Speed Control amplifier (cruise control) and the PCM.

any loading change, short or wiring defect from the DSS aka VSS to any of it's fed devices can cause problems.

the PSOM has a design problem in that it makes contact to the dash circuit board by pressure contacts only. vibration and moisture corrosion can cause problems. i've hauled mine out twice and sprayed the contacts with Caig Deoxit Gold.

 

Well, something strange has happened that confuses me even more...

I finally got around to cleaning the MAF sensor as you guys have suggested to address my pinging problem. I also removed the battery to clean the poles whilst the MAF soaked and then dried.

When I replace the MAF and took the old girl out on the road to check for pinging, the TC locked up just as it is supposed to do. Now, how did that happen. I have done nothing to address TC LU except replace the VSS. Does TC LU get data from the MAF...or could disconnecting the battery clear out nasty codes?

Michael

 

Is your speedo driven by cable or do you have the electronic cluster ?
If drive cable rotation is erratic it will make the speedo needle bounce. Dont know if it can be lubed , but maybe it should be replaced?

 

Speedo driven by cable. Also, where does the cruise control get its data? If the cable causes a bouncing needle, would that explain why the CC responds with surges to try to maintain the SET speed?

 

My 94 doesn't have a cable. Electronic cluster. I thought they were all like that..Hmmm