Hi, I have an aussie car with an engine transplant from a 91 Z28 (L98 engine & ECU). It has a cable speedo which has an inline 4 pulses per turn tranducer to provide the Vss to the ECU. Using an ALDL monitor, vehicle speed is always zero to the ECU. Ive checked all the wiring and and all seems fine and the cable speedo works fine. Ive taken the car out and driven it with an AC volt meter across B9 & B10 on the ECU and when stopped i get nothing and as speed builds it raises until at about 15MPH its at 0.202 VAC and thats where it stays as the car gets faster and I guess from that point on only the frequency of oscillation of the voltage changes and the voltage remains stable. I would have thought this means that the sensor is working ok but I have not been able to find out anything on the net about what to sample and what ranges to expect. Regardless the ECU shows hard zero the whole time. The VSS ratio setting in tuner pro for the chip is at 230. Any one got any ideas where I should look next to get it working ? Thanks in advance.

 

I don't know too much about all cars and ECUs, but I know that some ECU's expects a full 5V digital signal from the VSS (which often is optical), and it sounds like you have an inductive pickup that would need an amplifier to satisfy the ECUs signal level requirements. Just a thought to check into..
Btw., my ECM expects 2000 pulses per mile, and that is alot more than just 4 pulses per revolution. It also has an amplifier between the optical pickup and the ECU.

 

Thnx for that fullsizewagon, i'll see if I can find anything that indicated the correct voltage for my ECU and look for an AC amplifier to suit. It may be that the tranducter is faulty too and that a good one puts out 5v, it has a 12v input wire do its possible. According to the books i have it says that the voltage rises with speed which would suggest that the voltage is analogue rather than a fixed value.

The ticks per mile could still well be right. Its based on the speed of the speedo cable spinning in this case x 4. My tailshaft spins 2515 times per mile but i dont know what speed the cable is geared at relative to that but in any case there is digital compensation in the ECU to sort that part out, any speed registered would do at the moment.

Thanks again for your help.

 

Some ECM's are well documented, even with schematics if you can read those.
I don't know which one you got but you could try to look through Ludis's pages and see if you can find it there: Ludis's Home Page

 

Hi and thanks again. I checked out that page and yes its there. The ECU is the 1227730 but i dont understand what im looking at enough to tell whats what unfortunately. My VSS has 12V when ignition on and 2 other wires which I have connected to B9 (vehicle speed sensor ground) & B10 (vehicle speed sensor input). The VSS seems to be covered in the top of the page you referred me to (1227749 Schematic) but i dont have a clue what im looking at to be honest.

 

Ok, the VSS input is configured as a digital 12V open collector. It has a 1.2k pullup resistor internal to the ECM. One way to test the VSS is to use an LED in series with a 1k resistor. Connect one end to Vign (12V) and the other to VSS (sensor). If the LED starts to blink as you start to drive then the sensor is ok, otherwise it's a gonner. Alternatively you can just use a Voltmeter (put to DC) connected to the VSS input on the ECM. Then you turn the sensor on the speedometer wire slowly around and see if the voltage jumps up and down between 12V and 0V.
At least this is the way I'm able to figure out it must be..