The 1947 - Present Chevrolet & GMC Truck Message Board Network - View Single Post - Using an electronic speedo with TBI and 700R4?

ChevyTech · 08-07-2010, 02:50 PM

Do a search on DRAC module and another on vehicle speed buffer.

The instrument panel in the photo does not look like the earlier one on C/K trucks that had the DRAC/ buffer built into them.

I agree with raycow. The ECM does not have output on the VSS terminal. It is an input to the ECM

The DRAC takes the alternating currant wave from the VSS and converts it to a square wave and has a ratio adaptor to condition the signal.
Here are typical DRAC outputs for C/K trucks of that age. The signal it is sent at the rate of:
4000 Pulses per mile to the cruise control, speedometer, and odometer.
2000 pulses per mile ECM
128,000 PPM to the RWAL module

08-07-2010, 02:50 PM	#10
ChevyTech Registered User Join Date: May 2004 Location: Minnesota Posts: 4,620	Re: Using an electronic speedo with TBI and 700R4? Do a search on DRAC module and another on vehicle speed buffer. The instrument panel in the photo does not look like the earlier one on C/K trucks that had the DRAC/ buffer built into them. I agree with raycow. The ECM does not have output on the VSS terminal. It is an input to the ECM The DRAC takes the alternating currant wave from the VSS and converts it to a square wave and has a ratio adaptor to condition the signal. Here are typical DRAC outputs for C/K trucks of that age. The signal it is sent at the rate of: 4000 Pulses per mile to the cruise control, speedometer, and odometer. 2000 pulses per mile ECM 128,000 PPM to the RWAL module __________________ For those of you that are wondering why you are not getting replies to your thread: Did you give the model, year, engine, fuel system type, and transmission information? If it is modified from what came stock from the factory, let us know that too. Last edited by ChevyTech; 08-07-2010 at 02:55 PM.