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VSS Confusion

9K views 26 replies 7 participants last post by  Steve83 
This is the only thing it will affect in your aplication.

In normal driving and when you let off the gas wile rolling normally the IAC will is open all the way. This is so that if you come to an abrupt stop the engine will not stall. The VSS is telling the computer you are rolling so it keeps the IAC open.

If you dont have a VSS the computer thinks the vehichle is stationary. So as soon as you let off the gas the IAC closes.

This can couse 2 problems.
1. Coming to an abrupt stop you will stall the engine. The IAC will not have time to open in order to prevent the stall.

2. Slow stops when cold. (before closed loop)
When you let off the gas with no VSS the computer expects the idle to drop. When it doesnt it gets a bit confused. The EEC has no way of knowing that you are rolling in gear, it thinks the truck is stationary. So when the TPS shows Idle, and the engine doesnt slow down, the EEC trys to slow it down. First thing it does is close the IAC, then it starts playing with the timing and fuel tables. It is doing this to try to bring the engine down to idle. Now wyle you slowly decelerate it belives what it is doing is working, so it continues.

The problem is that at the point where the engine finaly pases idle, the IAC is closed, plus the timing and fuel tables are tweeced, and bonus, the engine is cold. The EEC just doesn't undo every thing fast enough, engine almost stalls.

Heres where it gets funny.
The EEC, gets the IAC open just before stall, the engine reves up, the EEC freaks out, like it was before, slams the IAC shut, and that + drag from the torque converter and a cold engine stalls the engine.

I dont have the VSS in my T-Case hooked up to my A9-L yet, thats how I know.

There are two things you can do to defeat this.
1. Set the base idle to 600 rpm and reset the TPS to read the correct voltage. And learn to live with it. This will all but cure the slow deceleration stall (exept when cold) but will have little affect on a panick stop.

2. Get a VSS and hook it up somewhere.

My 86 had the VSS in the T-Case. I have no idea how many pulses it puts out but since it told the factory EEC-IV how fast it was going, and the donar Mustang uses the same style VSS, it should work with my A9-L.
 
Hey Gack, wouldn't the EEC not even care about the number of pulses then? All it cares is that there is some kind of pulse, telling it you are moving. Don't the number of pulses only matter for ABS and/or an electronic auto?
Thats my theory :thumbup
But there could be a point where 16 pulses on newer EEC's would read as 0mph before you where actualy stopped.

Did that make any sence?
 
AFAIK, EEC-IVs don't use that speed/idle strategy. Only OBD-II. My truck ran perfectly fine before I added the ABS rear axle. In fact, I'm not even sure that I EVER connected the the speed signal to the EEC. It's probably only running to the PSOM & cruise..
I belive it does. It's just not super important. But in some cases it makes itself known.

Ryans stratagies page hights slightly on EEC-IV keeping the IAC open during driveaway and cruse.
http://fordfuelinjection.com/?p=64


BTW
You'd need a MONSTER engine & a large throttle plate with a scrawny pivot shaft for the engine vacuum to break the throttle plate. I doubt anyone has designed a throttle that weak in the past 60 years.
You get the bunny.
 
*hijack*

I've got a 93 (w/ E4OD) that I am swapping a 10.25 into.

If I recal the PSOM using the following:

ConvConst = RevsperMile x ToneRingTeeth / 8000

Will I be OK on shift points, IAC, speedo etc. etc.?

*hijack*
93 E40D + 10.25 = :thumbup

Does the newer style 92-96 speedometer/odometer use a cable, or does it run off vss?
If it uses vss Couldn't i just swap it into my cluster and run the EFI and speedo/odom off of it?
What computer are you wanting to run?

Yes the 92 up spedos are PSOM
But youll need a rear axle with a tone ring to run one.
 
A9L

So they won't work if you put the vss in the t-case whre the speedo cable went?
It will work for that computer and your existing speedo, but I belive the number of pulses per tire rotation would be diferant from an axle mounted VSS. This would make the PSOM be way off.

The way the early EFI trucks where the VSS was in the T-case with the spedo cable pluged into it. 87 up got an aditional RABS speed sensor in the axle and then later this become the VSS for the PSOM

Atleast thats the way I understand it.

Steve will know better than I what was done when.
 
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