What's the expected output from the Speedometer Sensor wires?

[phil_xjs_conv (Philip Brown)]

What type of signal or outputs should I expect to see
on wires that come from the speedometer sensor
(part DBC2632) mounted on the rear differential ,
when it’s working correctly?

I need this info to help troubleshoot and diagnose the speedometer
sensor on my '93 XJ-S convertible with the 4.0L six,

As a diagnostic for a dead speedo, I want to try reading a signal directly from the transducer, or simulating an input signal from the transducer into the speed sensor unit mounted in the boot, but to conduct either of these diagnostics, I need to know more about the output that is expected from the speed sensor transducer.

My speedometer has no readings, (I know… that’s the FIRST time you’ve heard this complaint!) and I’ve verified that my odometer, cruise control and the speed functions of my trip computer aren’t working either.

This means that problem is probably:

1. In the speed transducer that goes into the differential, or the wiring from it

2. In the speed unit mounted in the left side of the boot.

3. (less likely) in the wiring going from the speed unit, to the speedometer, ECU and trip computer.

I’ve read every thread I can find on Speedo’s, but I didn’t find the answer to this question.

The closest I found was this very interesting thread:

---

[xj-s] Re: speedo sender

Kirbert wrote: I just realized that the two wires to the “pulse generator” in the diff that provide a signal to the speedo are a signal wire and 12V power. I was a little surprised that this thing needs 12V power. I expected it to be a coil-and-magnet type of affair, generating its own signal with no need for a power source. Can someone elaborate on such a scheme? I pulled the sender out when I dropped my rear-end, and briefly eyeballed it to try to determine how it worked. The sen…

---

But it’s speculative and not conclusive, so I thought I’d ask the forum for help.

My hope is that collectively we can figure this out so that rather than partially diagnosing speedo problems, and then taking a leap of faith by guessing which part or parts are malfunctioning, – and risking being wrong – and I can actually test each component, and KNOW which one is bad.

I have ordered a used speed unit from the UK, so I can swap that out when it comes, but I’d really like to do a better diagnostic.

Here’s what was proposed in the earlier thread along with a drawing of a possible schematic or logical diagram.

@Ned

I will take a stab at this,  someone
    can shoot me if I am wrong.....

    It sounds like the device might be a VSO (vane switched oscillator).
    This device are triggered by ferrous metal.  Basically a
    proximity switch.  They are available in two and three wire
    configurations.  If the two wire style,  the 12v that
    Kirby is talking about must be pulled up using a
    resistor between the 12v and the device.  This allows
    the 12v line to also act as the switched line.  My
    guess says that the 12v is coming from the trip computer and it
    is pulled up inside (I dont have the schematics so this is a guess).

    Here is the basic Circuit Idea......

    12v
    |
    |
  -----
  |   |
  | R |     Pull Up resistor
  |   |
  -----
    |
    +------- Switched Line (This point would measure 12v with no metal present)
    |       This output would be in the form of a pulse train fro 0 to 12v that
    |       varies in frequency as a function of speed.
  -----
  | V |
  | S |     Detector
  | O |
  -----
    |
    |
    |
  -----
   ---
    -

    My Two Cents..

(9/13 -updated my post for better formatting and better readability.)

[Andy_Broadbent]

Interesting. I thought that all 89 convertibles were 5.3 V12s? I also thought that the 4.0ltr six was 1st fitted to 1st stage Facelift cars & I have not seen any 4.0ltr convertibles that were older than 92MY Facelift cars. Do you think your car is a factory original OR some sort of conversion?

Rgds.

Andy.

[phil_xjs_conv (Philip Brown)]

Hi bigpigpants,

OOPS… my mistake. I have a '93 XJ-S convertible 4.0L with
the 4 speed ZF automatic trans, not an '89!,

I updated my original post.