david auster

iTrader: (10)

I swapped my drive train from my 92RS into my 87 which uses the mechanical cable speedo. Since my 92 used the electronic guages I don't have it hooked up. The car is a 5 speed and doesn't have cruise so I figured it wouldn't make a difference but I am having a high idle problem and I was thinking maybe my lack of a VSS signal could be the cause. Can somebody tell me what the VSS actually does as far as the running of the engine? Is it possible to reprogram the chip so the computer doesn't look for that signal? Any help here would be appreciated.

JPrevost

No, but it helps. Especially if you've still got a lockup torque converter. The fuel and spark calculations have nothing to do with vehicle speed.

89 RS W/ T-56

Does it effect your cruise control if it is not hooked up? I also have a TBI. My speedo cable will be unhooked soon.

JPrevost

Yup, big time. The cruise requires the vss input, how else do you expect it to know what speed to keep the vehicle?

david auster

iTrader: (10)

So bottom line is that since my car is a 5 speed without cruise, my high idle condition has nothing to do with a lack of a VSS signal and I should start looking elsewhere for the cause? Also can you see any reason why I even need to consider hooking it all up?

Thanks

RBob

iTrader: (1)

Your high idle is being caused by the lack of VSS. . .

The '87 was TBI also? It should have a VSS pickup on the back of the speedo. A small green box w/3wires to the speedo and a connector into the harness?

RBob.

david auster

iTrader: (10)

Ok now I'm confused

First off the 87 was a LG4 carbed engine. I do know that the speedos that use the cable setup like the 87 have the VSS signal coming from the back of the cluster. I am using a 92 TBI wiring harness so I don't know or think the connections would be correct going to the computer, since the 92 used the electric VSS sensor in the tail shaft of my tranny which is part of the 92 engine harness, while the 87 VSS is part of the dash harness.

So thats my dilema, but I would really like to know what the VSS signal to my computer is telling it to do as far as the engines running is concerned? I was under the impression that it was mainly for TC lockup and cruise control, which I don't have either one. If this is causing my high idle problem, is it possible to program the chip where the computer doesn't look for the VSS signal? I already deleted VATS out of the program and that doesn't seem to have caused any ill effects.

Please help, thanks

RBob

iTrader: (1)

The '92 had the VSS in the tranny tailshaft. The signal then went to a 4-out buffer box under the dash (yellow, sometimes white). From there it went to the ECM ('8746).

Find the signal coming from the speedo and hook it into the ECM VSS input. I am not positive on this but look for a small lime-green box with a three-wire cable going to the back of the speedo. If you have one the other end of the box will have a connector. The brown wire goes to the VSS input of the ECM.

The ECM requires the VSS input for a proper idle.

RBob.

Kat

iTrader: (4)

The VSS in the 87 is on the back of the speedo... Find the wire that comes off of that and run it to the VSS buffer ( I think thats how it works still on 87)

I'm willing to bet if ya looked at a wiring diagram for a 88 TBI car you'll find what I mean and how to wire it up..

Doesnt seem that hard.

Kat

JPrevost

I remember reading that and seeing it somewhere in the hack but it never changed any of my idle characteristics.
I guess I should be more clear on just saying it doesn't effect anything because obviously it does.
In stock calibration, EGR has an on/off speed threshold, Highway mode has it's threshold, TCC manditory lockup has it's threshold along with the unlock tables, knock detection has a 10mph threshold, the TCC lock/unlock with compression breaking, and those idle thresholds. All in all it's important to keep the VSS but from my experience with it unhooked and when it thought I was going 110mph when only 70 I noticed no difference. Idle was fine, a little high but it was still well within the 0-50 counts norm.

RBob

iTrader: (1)

I think what happens is that the ECM has not been reset. It still has the minimum learned idle IAC step count and then the VSS goes away. So in general the idle is still decent.

If the ECM gets reset it can't learn a new idle step count w/o an operational VSS. Now the idle is high. This also depends upon the IAC steps cal data. The idle IAC control logic uses the VSS in several areas. There is even a check for a VSS malfunction with the code defaulting on the IAC.

I do know one thing for sure: the idle logic is complex. As much as I've dealt with code the IAC stuff is very difficult to follow.

RBob.

david auster

iTrader: (10)

Thats exactly what I am trying to figure out. When my drivetrain was in the RS everything was great, it idled at about 650-700. Now when I did the swap into the 87 I'm ideling at 1,500. I am pretty sure its not a vacuum leak since the guage shows 23. I do know my VSS isn't hooked up and I was hoping I would be able to get away without it, but that certainly sounds like it could possibly be the cause. I will have to see if I can get it wired up this weekend.