Vehicle Speed Sensor (VSS) - Electric or No??

OK, folks...here's the question. I'm looking at trying to rig up a MPG monitor, much the same as the Scangauge II for the 96+ vehicles. The major problem that I'm running into is that my XJ is an '89....before even ODB I. This limits my options pretty severly...means that I've basically got to start from scratch. BTW.....Any ideas/suggestions would be helpful here :looney:

The first question is the same as the title: On my 1989 XJ Limited, 4L with AW4 tranny....is the VSS electric or not??

I don't think your '89 has a VSS. Look at the transfer case, where the speedometer gear is located. If you have a VSS, you'll see an electrical cable attached there, instead of the standard speedometer cable. There are aftermarket VSS devices that can be inserted in line with your existing cable.
If you have cruise control, however, I believe there is a VSS in line with your speedometer cabe, located close to the speedometer; but it has nothing to do with engine management.

xjbubba said:

I don't think your '89 has a VSS. Look at the transfer case, where the speedometer gear is located. If you have a VSS, you'll see an electrical cable attached there, instead of the standard speedometer cable. There are aftermarket VSS devices that can be inserted in line with your existing cable.
If you have cruise control, however, I believe there is a VSS in line with your speedometer cabe, located close to the speedometer; but it has nothing to do with engine management.

Click to expand...

Mine is an 87 XJ (Wagoneer) 4X4, with 4.0, and cruise control. I noticed a VSS about 24" from the Speedometer gauge near the steering coulmn, that is inline on the speedometer cable on mine recently. I though it might be connected to the Renix, but had not considered the Cruise control. The Haynes manual, pg 12-21 shows an engine speed sensor, but not a vehicle speed sensor attached to the Renix ECU, so I guess your right! It also shows a sync sensor, so I guess the CPS is the engine speed sensor and the sync sensor is in the distributor, leaving the VSS to talk to the cruise control (or pehaps the TCU?). 1991 version shows a CPS, Sync and "Distance sensor" all talking to the PCM (Old name ECU).

I would imagine the VSS is electric?

The CPS (crankshaft position sensor) tell the ECM where the crankshaft is . The ignition module tell the ECM engine speed.
Your VSS talks to the cruise control electronics--I don't think it has anything to do with anything else.
If the VSS has 3 wires, it's probably electronic, but could be just like the CPS, which senses magnetic flux.

xjbubba said:

If the VSS has 3 wires, it's probably electronic, but could be just like the CPS, which senses magnetic flux.

Click to expand...

So you are distinguishing between electronic and electrical, as opposed to electric versus mechanical output?

It's not likely to be mechanical, IE gear box inside the cruise control module. More likely it will be magnet spinning past a coil (electrical) or a magnet spinning past a field-effect transistor (electronic). The electronic unit requires a voltage to the transistor, hence, the third wire referred to in my last post. The electronic unit would output a square wave that could be read directly by the control module, while the electrical out put would be a sign wave requireing additional circuitry in the module to modify it to a digital signal.

This is a simple setup on OBDII vehicles since the ECU/PCM already has all the info (injector pulse width and vehicle speed) and the scanguage is just querying it from the ECU.

For an 89, you'd be starting from scratch, having to measure those values somehow and then figuring out the math to calculate mpg. You'd need to measure injector on-times and get a vehicle speed signal, and then custom make something to convert that to a mpg display. Oh, if you have an automatic, you could tap the tranny output speed sensor and just assume the xfer case is in high.

xjbubba said:

I don't think your '89 has a VSS. Look at the transfer case, where the speedometer gear is located. If you have a VSS, you'll see an electrical cable attached there, instead of the standard speedometer cable. There are aftermarket VSS devices that can be inserted in line with your existing cable.
If you have cruise control, however, I believe there is a VSS in line with your speedometer cabe, located close to the speedometer; but it has nothing to do with engine management.

Click to expand...

I had a little bit of time to play with the XJ this weekend while dodging raindrops/hailstones....and saw that I've got a 2 pin electrical connector in the T-case, right next to the (assumed mechanical) speedo cable is. What could this guy be??

Is it connected to anything? My NV231 did not have any electrical connectors, however it did have a vacuum switch to activate the front disconnect axle. Depending on what transfer case you have, maybe what you see has something to do with 4wheel drive--like turn on the 4x light??

I believe I've got the 242 t-case....and that may just be to turn on the 4WD light, IDK. Guess I thought that was somewhere in the 4WD shifter.....

The light sensor is atop the transfer case on the driver's side.

Probably doesn't work anyway.

If you have the 242 and the non-disco axle, then the switch replaces the vacumn switch that plugs in near the speedo gear.