Hi, just getting into the tuning aspects on my 88 tbi chevy 4x4. I don't currently have a speedo hooked up because I have a doubler setup (2 transfer cases) and the port in the rear that originally housed the VSS had to be blocked off for a shifting mechanism. So, my question is, is the VSS absolutely necessary in getting a decent tune on this thing? it's not highway driven or daily driven, just want to get it in the ballpark.
Without VSS the tune will be somewhere close to the ballpark parking lot, not inside on the playing field.
With that said, the VSS is very useful for drive-ability and smoothness. There are fuel and spark transitions that VSS will smooth out.
Common statements I hear before VSS, "seems touchy", "seems like something is not quite right", "I suppose I can live with it that way". Common statement with VSS, "pleasure to daily drive", "nice, just like factory", "wow, night and day drive-ability difference".
If you never had a VSS, you will never know what's missing.
dave w
Did you ever hear the story about the guy who bought a '90 Chevy 1500 with an inop speedo? Crazy thing seemed everytime he cam to a stop the truck would stumble. After some investigating he found there was an electric speed sensor in the housing with wiring attached, yet a plastic "mechanical" speedo gear on the output shaft. Replaced the plastic gear with the proper metal tone ring and voila` the stumble went away.
Moral of the story, it can make a noticeable difference even if your just crawling rocks or slingin' mud.
Buddrow
If it don't fit force it, if it don't force fit f&%@ it!
Driveability right now without it is very good, but my fueling is off (lean/rich).i wish there was an easy way i could hook it up but i havent found one yet. So is there anything critical like open/closed loop, fuel maps or timing that is triggered by speed?
I've tuned several vehicles without VSS with acceptable results. I think you will be fine running without a VSS, after tuning the chip. With or without a VSS, I think the chip needs tuned to fix the rich / lean conditions. Adding a VSS will not fix lean / rich conditions.
If you had the newer TBI PCM (aka 7427), which has two fuel tables and two spark tables the VSS is needed to switch between the two tables. The older TBI computer used in 1988 did not use VSS for fuel or timing tables.
dave w
Thanks for clearing that up for me. I was a little worried about spending all this money on tuning equipment just to be stopped by the vss.
Adding a VSS has improved the drive-ability and engine performance in every case where it has been added after the fact, that I have dealt with.
The man who says something is impossible, is usually interrupted by the man doing it.
^^ X2 ! ^^Originally Posted by Six_Shooter
You are dealing with one of the easiest ECM to go without a VSS. Some are what I would call impossible, although possible the added work just to make it drive is no where near the amount of money saved to still have a vehicle not drive as well!
In your case the ECM knowing if vehicle is moving or not is all that is necessary! There is no E trans or speedometer or? for accurate speed, it does not matter what speed is as long as ECM knows it's moving or not! But the idle stability when rock crawling is a huge improvement. Over the years with my involvement with the off road clubs I'd say I have had the opportunity to drive at least a dozen rock crawlers before and after VSS, so this is first hand info from someone picky about idle, drive ability, cold/hot start etc... There is always an improvement of idle with VSS!
1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
-= =-
Does it matter if this is a manual transmission or not? are there different parameters that are affected more or less between auto/manual bins?
Nope...
1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
-= =-
Is there anyway I could use the tps to fool the ecm into knowing if I'm moving or not?
Nope, there's already idle parameters that are TPS based...
The man who says something is impossible, is usually interrupted by the man doing it.
I've going to agree with Dave. The '7747 computers are pretty simple. Last one I played with I added a VSS to make the TCC lockup work and I had it tuned fairly decently before and really couldn't tell any difference in the tuning or how it drove after. The rich and lean needs tuning to fix, not a VSS.
Sorry to jump in on this thread, but it'[s one of my upcoming issues. I am in the process of adding TBI to my '83 motor now using a '7747 computer. I run a 16 pulse Autmeter VSS and don't really want to add a second 2 pulse VSS just for the computer. Would hooking up the 16 pulse VSS be better than leaving it out? The VSS option is currently turned off in the computer.
Eagle is right about "least sensitive" to not having a VSS. But I can tell you from having looked at the code of several GM $masks that a large number of important factors (true idle, rolling idle, IAC control, effect on Spark moving or not, fuel calcs, etc. etc.) are not properly controlled without a VSS signal. It'll run, but not like it should -- manual or automatic does not matter.
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