I have a PSI harness that I am using for my PCM and my question is can I run the black wire from the harness directly to the speedometer the directions are saying that it is 4000 pulse per mile. Is it true that a it would require a inline resistor to pull up the signal?

 

Has anyone been successful at running the speed wire from the PCM directly to the wire going to the speedometer? I am being told by PSI that this is the same output 4,000 pulse that came out of the buffer box on my 89 Firebird

 

TL;DR: Most likely will be fine to wire the output straight to your speedo wire

Depends on what ECU you're using. Not overly familiar with what folks use for LS swaps, but if it's similar in function to how the '7730 works in the '92 3.1 (no buffer box), and the ECU is getting a 4000ppm signal, then you should be set. Again, not sure what ECU you're dealing with here, but the '7730 gets an input straight from the VSS on the transmission, then internally converts it to 4000ppm (parameters can be changed to account for different VSS signals, like what they did on late 80s Cavaliers; if you get 4000ppm in from the VSS, you won't need to worry about that), then sends it to the speedo/ cruise control box (if you have one; factory thirdgen cruise control box expects 2000ppm if that's something you're trying to get working). Basically, the ECU just does the same job that most other factory setups (TPI/ TBI) would have used the buffer box for. As long as your ECU is putting out a 4000ppm signal, and has a speedo output wire, it'll read close enough on your speedo (technically the factory speedos want a 4004ppm signal, but 4000 flat is going to be indiscernible).

I don't believe you'll need an inline resistor btw, as long as you're getting the 4000ppm out of your ECU. I'm just finishing up a TKX swap, which uses the same VSS as the T56, and the signal is getting through the factory ECU just fine. Now, your ECU is most likely different, but all the speedo cares about is getting a 4000ppm signal. The VSS is a simple magnet dealie, basically like a little reluctor wheel that you'd find for distributorless ignition setups. It puts out a simple sine wave, which tells your ECU/ speedo what's going on.... shouldn't need a resistor if the ECU is truly putting out the 4000ppm signal

 

Thank you so much for all your information !!! my ECU is a 896 Thanks Rich

 

No worries! Just did some quick searching, looks like the 896 is pretty much the same as the later 0411 ECUs, and they should indeed both be putting out 4000ppm signals. Might be able to find more confirmation for this on the LS swap board here on TGO, but from everything I'm seeing, should be doing the same thing for your speedo that the stock ECU in a factory non-buffer box thirdgen would be doing