Variable reluctance sensor--what am I missing?

Discussion in 'General Electronics Chat' started by tab a, Feb 22, 2017.

  1. tab a

    Thread Starter Member

    Jan 14, 2015
    34
    0
    This seemed like it should be so simple.

    Changed transmissions in a 97 Ford Ranger, so there is no longer a Vehicle Speed Sensor on the transmission, as in the diagram. NOW trying to use the ABS sensor on the differential (red, on the diagram), which Ford also did, in '98, as the VSS. Both are variable reluctance, 2 wire sensors, but because they (diff and transmission) rotate at different speeds relative to each other, I put a Dakota Digital Signal Interface Unit between them, to adjust that.

    Thought it would be as simple as attaching the red wires, as shown, but all I get from the speedometer, is a brief spike to 15mph or so when put truck in gear, then it stays at zero. After many hours and trying all kinds of ground permutations I finally cut the GY/B wire behind the speedo and attached the signal wire from the DD to the speedo. That works! BUT, I still need a corrected speed to the PCM and the Cruise Control, and maybe the GEM.

    I guess it's possible there is a problem with the GEM or CC or PCM now, but they worked fine before the swap. Is this a ground problem of some kind?

    Appreciate any thoughts you may have!

    Thanks.

    ranger vss copy.jpg
     
  2. drc_567

    AAC Fanatic!

    Dec 29, 2008
    639
    94
    ... looking at the Dakota Signal Interface module ... Have you performed the coarse and fine calibration? Are the diagnostic lights green or ok?
     
  3. tab a

    Thread Starter Member

    Jan 14, 2015
    34
    0
    Yes, I've tried every variation of the Dakota Digital I can. No green lights on this model, but the flashing red dot indicates I have a signal (measured at about 4v ac. That and the fact that it did work when hooked directly to the speedo make me think the DD is not the problem.

    Any chance it's significant that the GEM and speedo don't show the PK/O ground on the diagram, like the PCM, CC and VSS do--or they just left them off, for whatever reason?

    Thanks.
     
  4. drc_567

    AAC Fanatic!

    Dec 29, 2008
    639
    94
    ... am not clear on the purpose of the GEM module. Does it serve a necessary purpose? With the GEM hooked up, does that constitute two speed sensor inputs to the speedometer? What would happen if the GEM were to be disconnected?
     
  5. tab a

    Thread Starter Member

    Jan 14, 2015
    34
    0
    I've never been sure why they send a VSS signal to the '97 GEM as it seems to mostly deal with windows and lamps. The '98 GEM OTOH has VSS in and VSS out--to instrument cluster. I definitely don't have that wiring.

    I've already tried unhooking it. Didn't change the speedo.

    As I mentioned, when hooked up as I think SHOULD work, I get a speedo 'pulse' when first put into gear, but then nothing. Does that pulse suggest anything?

    Thanks again.
     
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