electronic speedo help

[KSommer]

I've been fighting with this for almost a year now and am right down to the end of my rope. my truck is a 2wd with only 2 wires at the VSS. VSS replaced about 8 months ago, no change. I checked all the gears and stuff in the transmission and it all is working order. i put in a new cluster today and ran a wire (white with an orange tracer) right from the VSS to the speedo.

can anyone tell me where the other wire on the VSS comes from? it's black or brown, i'm not sure. does it carry 12 volts or something less?

thanks, kent

 

[Paychk]

Link to some previous speedo trouble shooting (with diagrams, quality on poor side)



https://www.turbodieselregister.com/forums/showthread.php?t=111006



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[Paychk]

The other wire is a black /light blue strip this is a ground and runs to the K4 splice (somewhere on the engine harness) then runs into a black 10 pin connector (on the left rear of engine) which then goes to pin 4 of the PCM connector (computer).



The two wires from the VSS or VDS (same thing) should be twisted around each other, this is to prevent electrical interference from mucking the signal up.



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[KSommer]

thanks for the drawings, you say the other wire is a ground? how does the VSS make a signal with no input power? the top drawing shows three wires going to the sender, am i missing one? I'm real close to trashing it all and gettting one with a cable. but it only goes to 85mph. my dad days you can only go 55 anyhow.

kent

 

[dan brooks]

I don't have the wiring diagram in front of me, but IIRC, the orange/white wire goes from the VSS back to the PCM, then to the speedo. The other wire ties into the "sensor ground," which is a common ground for the various sending units/sensors and also goes back to the PCM.

 

[Paychk]

Go here for the complete electrical diagrams; read the notes on both pages.



https://www.turbodieselregister.com/forums/attachment.php?attachmentid=48353&stc=1



https://www.turbodieselregister.com/forums/attachment.php?attachmentid=48357&stc=1

 

[KSommer]

so if i'm reading this right i should just have to ground the other wire and things should work? shouldn't matter if it's common ground with the other sensors, right? ground is ground, isn't it? still not sure how a sensor makes pulses without some input...

kent

 

[dan brooks]

Actually, it looks like the LB/BK wire is called the "sensor return" - not sure if that is the same as a ground or not. All this computer stuff is a real pain... .



Looking at the wiring diagram (page AD29), it appears that the WT/OR wire has a splice, with it going to both terminal #47 of the PCM plug and to #6 on the printed circuit board connector for the instrument cluster. Wish I had a scanner so I could post a pic.



I think I'd start out by inspecting the harness from the sensor back to the PCM and see if it has been damaged in any way. You could always piggyback a new wire and see if that corrects the problem - if not it may be an issue with the PCM, or maybe even a bad speedo head unit.

 

[Paychk]

The PCM has nothing to do with the function of the speedometer; it just reads the output from the VSS so it can control the throttle actuator for the cruise function. The VSS signal goes up to the speedo head and signals it to dispay speed & milage.

 

[Paychk]

Have you pulled the trouble codes? the code for no VSS signal is 15; you may want to check that first instead of chasing wires. Also the Black/Light blue wire should be grounded already, to pin 4 at the PCM connector. The orange white should connect to pin47 at the PCM and pin 6 of the printed circut board (PCB) connector behind the gage cluster.



BASICS



1. check for trouble codes

2. check wire continuity from the VSS connector to the PCM & gage cluster PCB.

 

[Crash_AF]

Have you checked the speedometer gear to make sure that it is engaging the main shaft properly? I had a Jeep Cherokee who's speedo quit after I rebuilt the transfer case. Turns out the speedometer gear is 'clocked' depending on the tooth count of the gear and if it's not turned to the right position it doesn't engage the shaft.



Later,

Joe

 

[KSommer]

i will double check the sender if it is clocked right, i will run the dark wire to a ground, i will scrap the whole damn thing if i can't get it working by the end of the week.

kent

 

[Moab4x4]

My Speedometer worked

The PCM diag manual says to test the voltage to the VSS and it must be over 4. 9 volts to operate the VSS.

This voltage supply is provided by the PCM, so it is an important link in the electronic speedo.

If there is no 4. 9 volts, the book says to check wiring continuity to PCM harness and if that is good with no opens or shorts, then replace the PCM.



My solution as I have no PCM in my truck, was to creatively provide voltage to the VSS.

I jumpered the signal ground line to the VSS to chassis ground.

I then jumpered the Ignition voltage to the VSS signal line with a 10k ohm resistor. Another 10 k ohm resistor is jumpered from the VSS signal line to the VSS ground line.

This provides a 6 volt supply to the VSS. As the vehicle moves and triggers the VSS, I believe the signal is triggered to ground. The 10k resistors keep the VSS from frying and allow the signal to drop.

The speedometer now works! Oo.



EDIT Speedo quit after ~ 1000 miles. My 6V method may not protect the VSS as much as I thought. I will rework this and post better solution later. My voltage regulator went a little off and my voltage divider went from ~ 6V to ~8V. I don't know if this was enough to fry the VSS or not, but it is dead now. I will keep working on this after I get the truck back on the road.



Good Luck,

Ken



BTW Thanks Paychk for the diagram, that is what led me to my solution.