Does VSS matter? (you should already know my answer)

[dyeager535]

What isn't clear about "if in doubt"?

Pretty simple in my mind: I am installing my first used EFI setup. Off the bat I have no idea for SURE if it will run correctly when I'm done installing it. There is plenty of history to back that statement up. Could be miswired power feeds, broken sensors, you name it. I decide not to use VSS. I start it up, thing runs terrible. So is lack of VSS a problem, or is it something else? MAP, vacuum leak, injectors, fuel pump, wiring?? Do I go back and install VSS now, and do I know if it's going to fix the problem? That's what I'm getting at. If the truck runs fine, there is no doubt in someones mind. If it DOES run bad and we are starting with no baseline, we can't just attribute it to the VSS can we? And if we're new to this, that is just one more headache or potential problem area.

Like I said earlier, I don't think it's "experiences" that are governing why people do or do not have problems. The trend seems to be largely that the '747 and early (160 baud) '165 ECM's run ok without it, while my later '165 for instance, for some reason will not. Another variable is the transmission. You said it helped driveability a bit with a manual, same as mine, except mine was quite a bit worse. I don't see any other mention except ours about manual trans/lack of VSS functioning, but that is a trend worth noting as well. I'm certainly picky about how my vehicle runs, and I suspect most of us are, so I don't discount anyones experiences, I'm sure they are being honest. No one would say their vehicle was running fine if it ran like mine did without VSS.

We have a pretty large sample of '747 users here that say they have no problem without VSS, so it stands to reason that if there were more of us running say, a later '165 that did have a problem, we could narrow it down to the programming. At that point a statement like

I say skip the VSS unless you have a cruise control

would be perfectly valid when applied to someone contemplating a '747 TBI swap, and it likely is, I'm just not certain it's solely the programming and not a combination of factors, like donor vehicle, engine modifications, and so on.

 

[dogdaysunrise]

Well I have to take mine out on my 4.3tbi and 700r in my 92 s10. This has been a good read and I am anxious to see how my truck runs without it hooked up.

I will definitely post up with my results.

 

[dyeager535]

I was adjusting my new cruise setup tonight, decided I'd take my digital camera and shoot a bit of video. It's not the best obviously, but it's pretty big, sorry dialup folks.

http://www.youtube.com/watch?v=QbnMgrNj64k with VSS

http://www.youtube.com/watch?v=g-Bd1VXongU without

First off, without VSS you do have to pay close attention to whats going on. You can tell by sound the engine isn't running as well as it is with VSS. Yes, the exhaust isn't solid right below the drivers seat, and it's reverberating off of the garage door, so it sound bad anyway. You will notice however, that the RPM's without VSS are about 200 less than they were with, the angle of the shot of the tach is different before/after, so that throws the reading off a bit. I kicked the throttle just to prove idle wasn't "hung up" or anything.

I shot the "with VSS" first, after having driven around the block, about 3 minutes of driving, after letting it get up to temp. For the without, I turned the truck off, disconnected the VSS (DRAC in my case) then drove it around the block, same distance/route, without VSS. Immediately upon restart the engine idled as it did with VSS. As I drove around the block (couple of stops) idle started to noticeably drop, and towards the end I was losing power steering as I was on the clutch, turning a corner, the engine wasn't compensating for the increased engine draw. I didn't leave the tach hooked up while I was driving, as you can see I was just doing this temporarily. With any sort of throttle, it seems to run fine. When I kicked the throttle in the video it sounds/felt just as smooth as it does with VSS.

Based on my previous experience (what drove this thread) had I driven this thing around the block maybe two more times, it would have been as bad as I first posted about. I just didn't have the inclination to drive around a few more times. Nonetheless, idle RPM drop of 200RPM and loss of power steering at idle RPM's within 2 miles of driving is certainly a driveability problem, especially with the manual transmission.

Again, to make sure we all stay one big happy family, I'm not saying this will happen in all cases, this is what happens when I don't run VSS on MY setup.

 

[4X4HIGH]

Bottom line is, if it wasn't needed GM wouldn't have installed VSS. There is literally hundreds of thousands of dollars saved by not installing VSS or anything else people "think" isn't needed and GM and any other maker would love to save that kind of money if they could.

 

[dyeager535]

While for the most part I tend to agree with GM doing nothing more than they have to, to save a buck, in the case of VSS I think it's a bit more difficult to say. I think if you search for past discussions on this I have said different, but now I don't think quite the same way.

First, off, starting in 1983 (earliest I've seen it) cruise control required VSS in trucks, and that was it's sole justification to exist up until 1987 and TBI. So making the leap from cruise control to using it to control emissions devices and EFI wouldn't have been hard to justify cost-wise. The device was already there in many trucks, add a few wires, some functionality to the ECM, and voila, easy way to control anything that speed was a factor in.

So in the case of VSS, even if it is/was unnecessary for driveability (and at this point I'm going to go with "depending on application" it may be) AIR, EGR, and TCC all use it, so thats the justification cost-wise in a stock application, since those items are essentially mandated by law, at least if GM wanted to sell smog-legal vehicles.

 

[noahrob]

I have the VSS module, but need to hook it up. After reading this, that became a little higher priority...of course my truck broke down and got towed over 180 miles yesterday, so getting it to run is the first priority, but right after that, and fixing the leak in the 39." Irok with only 500 miles on it, the VSS is top of the list..!

 

[dyeager535]

After you get it up and running, don't forget to clear the ECM (pull the ECM fuse for 10 seconds) after hooking VSS up, as in my mind, this is a "learned" function. Since it doesn't start as soon as you remove VSS, any problems lack of VSS is causing (if any) may not go away unless the computer is forced to re-learn the IAC settings that work best.