Some other thoughts -

1. The reed switch becomes pitted after a period of time. Perhaps because a debounce capacitor in the circuit causes "in rush" and errodes the contacts. Maybe a series resistor could be added to increase life.

2. The switch fails and begins to bounce more. This makes more pulses, and the time between pulses becomes very short. The ECU interprets this as high speed operation and starts shutting things down (limiter). So could adding a capacitor between the signal lead and ground "smooth" out these small pulses, and make a flaky sensor behave?

3. Or just unplug - verify truck drives without limit function - and then replace sensor or use GPS for speed readings and don't use cruise control.

 

Or adapt a hall effect sensor. :-)

 

The problem turns out to be mechanical and can be corrected. I've posted a writeup here:

http://www.key-men.com/downloads/Lan...rebulid_v1.pdf

Comments welcome.

 

Kudos.

 

 

The previous owner of my truck said that the fuel pump was intermittent, but she really had no idea. Since the VSS could produce similar symptoms, I figured it was worthwhile to investigate.

I'm ripping apart just about everything else anyway. :-)

 

Thanks George!

I did replace mine with the Allmakes version. Even with the replacement, unplugged my Disco runs fine. Plugged in it shows all the classic symptoms of a bad VSS. Any ideas?

 

I have no clue how the Allmakes version is put together. Where did you get it and what did it cost? Does it look the same as the original?

Then there's the possibility that you problems aren't caused by the VSS. The same symptoms could be caused by wiring issues, or ECU issues.

If you have or can get hold of an OBD II scanner, you should be able to look at the freeze frame data in the ECU. Check to see what's stored there. Any Freeze frame data with a speed of over 127 miles per hour indicates a VSS problem. If there is none, I really have nothing to go on. Lots of different sensors can cause similar problems if they're intermittent or out of range.

 

I found the Allmakes after market version of the VSS at RovahFarm for $70.

It is identical to the OEM.

No freeze frame data with high speed values. Faults usually occur at 50 mph. I guess what is happening is that without the VSS it is going into a default mode that compensates for something else. Truck runs great without VSS, terrible with it. Without the VSS the ECU never completes a cycle and is never in "ready" mode, so I have no codes or data to analyze running without it.

 

I assume you mean that it looks identical to the OEM.

Does it run the same way as before you changed it, worse or better?

Without the VSS, the ECU thinks the truck is not moving and probably assumes it should be idling and may be trying to adjust it. If it weren't for the speed limiting function, it really wouldn't matter how the VSS performed as long as it sent out signals.

It's possible that the new unit is worse than the old. I did see some comments on the quality of the Allmakes unit on some other web sites, but it's not clear what those who made the comments considered to be the problem with it.

If you still have your old one, I'd try the rebuild.