I've been reading about this stuff all night and I'm pretty confused at this point (AI summaries regurgitating false info certainly hasn't helped). I've been under the impression that TPI/TBI ECUs expect an AC/sine wave output directly from the VSS, but now I'm reading that the VSS instead sends a signal to the DRAC which then converts that AC signal to DC/square wave signal which the ECM can read? Does anyone know what the ECM actually expects to see? Specifically in regards to a 7730 ECM. Thanks!

 

Heya! If this is regarding the ECU in your 3.1 swap, then the VSS signal should be going straight into your ECU. 3.1/ TPI 7730 ECUs act as the buffer box internally, instead of having a separate processing box like earlier/ TBI cars. The raw input from the VSS is taken by the ECU, and if you're looking at the value in Tunerpro, the "Road Speed Constant" will be the setting that tells the ECU how many pulses per mile to expect from the VSS. Stock setting should be 4004 in the ECU. If you have the means to burn a PROM, and you need to correct the VSS for your gauge, it'll be pretty straightforward. Take your tire height in inches, and convert it to rotations per mile. Multiply that by your rear gear ratio, and finally multiply that by the number of pulses your VSS sends out per rotation of the driveshaft. The 7730 cars will have a magnetic VSS, same signal type that gets sent out by the T56 VSS (can't recall what signal it is specifically, wanna say sine wave).

Anyways, if you have a factory service manual on hand (or the PDF for a '91 Camaro from AustinThirdgen), head to page 6E3-A-3; On the 24 pin AB connecter for the ECU, pins BB9 (purple wire, low signal) and BB10 (yellow wire, high signal) are connected straight to the VSS. If you have the PDF, it'll be page 816 Pin BB11 (grey) will be the ECU output to the speedometer (it'll match whatever your Road Speed Constant is in the ECU, GM gauge expects 4004 pulses per mile). Pin BC1 (red) will be the ECU output to the cruise control module, if so equipped; this outputs at 2000ppm