73-87chevytrucks.com

73-87 Chevy _ GMC Trucks => Electrical => Topic started by: burbankraut on August 01, 2016, 08:22:01 am

Title: 1987 VSS Buffer and 1991 DRAC ECM Input (same signal?)
Post by: burbankraut on August 01, 2016, 08:22:01 am

Hi,
This is my first post - I am working on a Frankenstein Suburban. 1991 Suburban body on a 1987 3/4 ton 4x4 frame. The engine is TBI running off of the 1991 ECM but the speedometer is a cable drive because the transmission and transfer case were out of the 1987 (TH400). I am trying to:

a) Get the check engine light to stay off. This is because the 1991 electronic VSS input from the transmision is not connected.

b) Get the cruise control working.

Since I have a cable driven speedometer, I installed the "Green Box" VSS Buffer behind the speedo that will hook up directly to the cruise control (which I have from a 1987 truck) I am working on hooking up the electrical connections which are
Pink (Power)
Black (Ground)
Brown (ECM)
Yellow (Cruise Control)

On the non-electronic speedo units, the brown wire from the VSS Buffer went in to terminal 437 of the ECM. The same terminal, 437, on the 1991 ECM goes to the DRAC.

So, here is my question: Is the brown wire from the VSS Buffer the same signal as the DRAC output? Can I connect the brown wire from the VSS Buffer to terminal 437 on a 1991 ECM?

Thanks for your help!

Title: Re: 1987 VSS Buffer and 1991 DRAC ECM Input (same signal?)
Post by: Irish_Alley on August 04, 2016, 05:40:36 pm

i replying to give this post a bump, theres not many of us that are running the electric speedos.

Title: Re: 1987 VSS Buffer and 1991 DRAC ECM Input (same signal?)
Post by: hatzie on August 04, 2016, 06:42:22 pm

DRAC output on the T400 chassis is 4,000 pulses per mile.  Not sure on the RV chassis.
Optical VSSB is 2,000 pulses per mile.
I'm not clear on what type the actual waveform is.  Sine or square...
The cruise shouldn't care.  The ECM may.

Sent from my SM-P605V using Tapatalk

Title: Re: 1987 VSS Buffer and 1991 DRAC ECM Input (same signal?)
Post by: hatzie on August 04, 2016, 07:15:48 pm

Found some info from the 3rd Gen F-body boards.
The DRAC output is a 4,000 pulse per mile dc squarewave...  meaning to doesn't pass below zero it's zero to high and back to zero.
http://www.thirdgen.org/forums/electronics/650900-vss-output-signal-type.html

If the ECM won't get angry about half the pulses you can use the optical VSSB signal directly.  A 555 could be used to double the frequency if needed.



Sent from my SM-P605V using Tapatalk

Title: Re: 1987 VSS Buffer and 1991 DRAC ECM Input (same signal?)
Post by: Irish_Alley on August 04, 2016, 07:24:10 pm

not sure if this will help or not
http://www.jagsthatrun.com/V8-chapters/V8-tpi-Speed-Sensors.pdf

Title: Re: 1987 VSS Buffer and 1991 DRAC ECM Input (same signal?)
Post by: hatzie on August 04, 2016, 08:42:35 pm

A frequency doubler using a comparitor and an XOR gate.  Drive a FET to switch the output from the TTL XOR gate between gnd and system volts high.
https://www.maximintegrated.com/en/app-notes/index.mvp/id/3327

Sent from my SM-P605V using Tapatalk