Hey Crash, VSS question

TreeClimber · Mar 19, 2009

So I transplanted an 87 Suburban into a TJ. The check engine light comes on, with a Code 24; No input to the Vehicle Speed Sensor.

Looks like I forgot to wire that in during the swap, which shouldn't be too hard to add back into the harness.

My question is: Does the VSS need to be accurate? In other words, since I've got an independant Speedo head coming off the speedo input from the transfer case AND when I rebuilt the tranny, I added a speedo input for the TH400 (just in case:awesomework:, since 4x4 trannys don't have one). So, if I hook up the VSS to the TH400 and it's not feeding a speedo head, does it need to be accurate?

And if you want to type extra; what EXACTLY does the VSS do inside the ECM computer (other than turn on the check engine light)?

And I'm aware that my rig should use a buffer to change the signal from A/C to HZ signals.

It's an 87 Burb, 350 V8 TBI, K-motor. Wiring harness is fully intact.

Also... do you remember where on the Burb harness the VSS is generated? Does it come from the cruise control module or from the speedo signal at the tranny/tcase?

Thanks,
Tony

Binder · Mar 19, 2009

Your VSS will effect your EGR, closed-open loop operation, DFCO (deceleration fuel cutoff), at speed to idle threshold transition, stall saver, idle air control operation, and a bunch of transmission-torque converter functions but if I understand you correctly the trans settings won't effect you....
So yes for these functions to work right your VSS needs to be accurate or change a bunch of settings in your chip. The VSS being wrong will effect some things more than others, some you won't notice the difference.

Binder · Mar 19, 2009

Oh sorry my names not Crash...Never mind.

crash2 · Mar 19, 2009

For TBI, the only thing that ECm uses for fuel management is EGR and knock sensor diagnostics. I have never ever seen open/closed loop effected by the VSS input--book world to real life..

As for an input, whatever you can get. I did this once on a 4.3 swap in a toy (TBI) and grabbed the speed signal off the toy cluster which was like 2:1 where I wanna say the GM is 4:1 (pulses per revolution). Even though the speed showed half of what it was it was enough to run diagnostics but had zero effect on fuel trim.

Binder · Mar 19, 2009

crash said:
For TBI, the only thing that ECm uses for fuel management is EGR and knock sensor diagnostics. ..

I'm fairly sure you wrote this wrong because it makes no sence.??

crash said:
I have never ever seen open/closed loop effected by the VSS input--book world to real life..

Real life? This is real life chip adjustments. Something not normally done on the maintenace level....
Here's a picture..

crash2 · Mar 19, 2009

Just stating what I have seen in the field and in my shop on dozens of dozens of rigs over the course of years.

Binder · Mar 19, 2009

Tony here is the best place on the net for info like this.

TreeClimber · Mar 20, 2009

thankyou for keeping this on topic.

Thankyou both. I'll do some snooping over at the Binder site.

If you have any other stuff to add, please do so.

Blueleader · Mar 21, 2009

Binder said:
I'm fairly sure you wrote this wrong because it makes no sence.??

Real life? This is real life chip adjustments. Something not normally done on the maintenace level....
Here's a picture..

That "Item List Summary" feature in TunerPro looks kinda handy. I never noticed it before until I saw your screen capture. I've always just dug through the lists on the left side.

I ran my TBI setup for a couple years before I finally got around to installing a VSS. Only change I really noticed was with the idle control during braking. It used to get a little "stumbly" during hard braking. Adding the VSS seemed to help. I didn't have EGR enabled and ran a forced open-loop idle so I wouldn't have seen any changes there.

Binder · Mar 21, 2009

Blueleader said:
That "Item List Summary" feature in TunerPro looks kinda handy. I never noticed it before until I saw your screen capture. I've always just dug through the lists on the left side.

The item summaries in tuner pro are blank. You have to take the time to write them all in. Most of them are explained in tuner cats you just have to take the time to move them all over to TP.

TreeClimber · Mar 25, 2009

A little more help

Okay.

What I've determined is I need to send a VSS signal to the A10 plug on the ...747 computer wiring harness. That's the easy part.

However, I cannot seem to find in my literature if I need to send a square wave signal (I think it's this) or a sine wave signal (don't think so) or if it doesn't matter either way.

The donor TJ uses a VSS, and I could plumb that into the Chevy harness quite easily. However, it doesn't specify in the TJ manual if it's a sine / square sender either.

Any ideas?

Thanks,
Tony

Binder · Mar 25, 2009

JTR site (jags that run) has a bunch of VSS info, you could try there for more info....

TreeClimber · Mar 25, 2009

Thanks Binder for the IH site. Been very useful.
I just figgered out my Auto Meter speedo head has a buffered, VSS output to the ECM. So I'm good. Just got to run the wire now.

Binder · Mar 25, 2009

TreeClimber said:
Thanks Binder for the IH site. Been very useful.

You're welcome. Bill is sharp at this stuff and very helpfull.

Binder · Mar 29, 2009

If you're still working on this I could turn off the VSS error light in the chip for you if you like? The symptoms Blueleader described are the most common. If you're OK with that we can just turn off the light?

TreeClimber · Mar 29, 2009

Binder said:
If you're still working on this I could turn off the VSS error light in the chip for you if you like? The symptoms Blueleader described are the most common. If you're OK with that we can just turn off the light?

Nah,
I'm pretty sure now that it's re-wired for the VSS, that the light won't come back on. My sender sends 16 pulses per rev, whereas the factory sender sent 4 pulses. So the computer will think I'm going 4x too fast, but it still should recognize the sender and realize it's moving. The only downside is the ECM may overcompensate on the deceleration program (as it thinks I'm going 240mph down to 0, instead of 60mph down to 0). If the overcompensation becomes an issue, I figured out a way to get the signal reduced to the correct ratio by adding one more component. Since it's already wired, adding one item should also be pretty easy (and not too much $) The IH site is pretty good for EFI info. I've read almost the entire forum's content a couple of times to understand the issue.

I think it's done fixed.
But thank you for the offer.

Tony