I recently swapped my automatic for a t5 (excellent project and I Recommend it to ANYBODY), however my speedo cable did not fit the new speedo gear, so I did not connect it. Not surprisngly, I got a VSS error. However that got me thinking and I am now VERY curious as to how that actually works? How does the car know it has no VSS attached? ITs a cable speedometer so when the car is not moving the speedo cable should not mvoe at all anyway, so how can it differentiate between nto moving and not being connected? At first I assumed it used vacuume to know when the engine was under load, but that also makes no sense because right before I trashed my auto, I stalled the torque converte and held it for a little while and that woudl have made the VSS error come on as well b/c the wheels were not moving but the engine was under load... So how does it know?

-- Henry

 

Hmmmmmm I'm confused. So you had an electric speedo right? That's the only reason for a Vss. The other reason is the computer has to know what gear the tranny is in to lock up the TC, but that's on another connector. Here's my guess about the code. Since you didn't connect any of the tranny sensors it's reading zero voltage which will trow an error. Another reason is the speed limiter that also came with TBI equiped cars.

 

No, no, no! Every computer controlled vehicle (at least GM) has a VSS.

For a cable-drive speedo, it is located on the back side of the speedometer, a little yellow box with a 2-wire connector (I think it's 2-wire, anyway, been awhile) going to it. The speedometer has a magnet in it, and the VSS simply senses the pulses as the speedo turns.

If it isn't turning, the ECM figures out through other data that the car is moving, but it isn't receiving any pulse data. So, sets a trouble code. The data is used for engine control as well as TCC lockup. On later vehicles ('84-on, I think), the ECM even uses the VSS for the cruise control.

 

But does anybody know what other data that is? Because this morning my car threw the code without moving. I don't have ABS or anything with speed sensors on the wheels. So how can it know it is moving?

-- Henry

 

By "speedo cable", do you mean a wire connector? Or, a sheathed, rotating, mechanical piece that has a screw-on retention device?

A mechanical speedometer cable should have connected to either the auto or manual tranny. If you're talking about a wiring connector, then not having it would connected to the sensor would set the code immediately.

Do you have a mechanical or digital speedometer? What year/model car are we talking about?

 

its a cable that spins in its sheathe, very much not electric. its an '89 firebird. When I swaped, the speedometer cable did not reach the new speedometoer gear location on the manual transmission (Which is further away than that of the auto), and if I could make it reach it is obvious that the cable would not fit the fitting. Its nto a wire, and sometimes I can get pretty far befoer the error comes on. I totally understand why it is coming on, as mys peedomster is in fact not connected. However, I was just curious about how it knows.

-- Henry

 

The ECM will presume an engine load based on TPS input, MAP/MAF input, and distrtibutor pulses (RPM). If the ECM notices a given throttle position, along with a high MAP (low vacuum) or high MAF (high flow) reading, and an RPM that is relativelty low for that TPS, it will presume that the vehicle is moving, applying load to the engine. The same TPS input with a low MAP (high vacuum) or low MAF (low intake flow volume) and relatively high engine RPM indicates the engine has little or no load and is being "revved" in neutral. This presumptive logic that decidees whether the engine is loaded or not will also start a timer. The presumed loaded condition will be allowed to continue until the time limit expires. If no VSS pulses are generated within that time, an error code is set. That's probably why you can drive fairly "normally" for quite a distance before the VSS error is set. Drive a little more aggressively and the error may set sooner since the ECM will presume a load earlier.
Incidentally, the ECM also knows what gear you are in by comparing the DIST pulses to the VSS. If engine speed = driveshaft speed, it knows you are in DRIVE. If engine speed is less than driveshaft speed under load, it knows you are in OVERDRIVE. That's the basis for TCC operation and the SHIFT light in manual trans cars.

At least that's my understanding.

 

Not that your car is setup this way, but all the Firebirds I have seen have the VSS on the tranny. Is there a 2 wire connector at the back of the trans you left disconnected?

 

Madmax is correct. The firebird uses a VSS in the transmission, and the one from the 700R4 is too big to fit into the T5 hole. Buy a VSS for a T5 and you're back in business.

 

My speedo doesn't work either and i get a vss. but in the haynes manual it says that if a code 24 is set without the wheels turning just disreguard it. just my FYI

 

What kinda crap is that? "If there's something wrong with your car, just ignore it."

Oh well. Not flaming you, flaming Haynes.

 

The two main times you're going to get a VSS error code set is under hard driving, or coasting (deceleration, once you get under probably about 15 mph).

And a couple small corrections:



The yellow box on the left is the VSS buffer. Follow the three wires to the right and the little black unit there is the actual VSS pickup. It's actually optical, not magnetic. It has an infrared emitter and reciever which is interrupted by a reflective spinning bar (the bar has a magnet on it which moves the speedometer needle). The "chopped" up infrared signal is sent to the yellow buffer module which converts it to something readable by the ECM.

 

On my 86TA, my 85TA I had, and my GTA the VSS is on the back of the trans in the extension housing. The yellow buffer box is attached to the back of the plastic panel the ECM is mounted on. I'm guessing his Firebird is the same setup, since all the Firebirds I have seen are identical to mine.

 

Max,

Maybe I have the odd one (I know the '86 is a bastard), but my '86 TA has the VSS on the back face of the speedo head in the cluster, and a regular old Bowden cable to drive it. At least that's the way I recall it from removing the cluster before, and I certainly don't see anything down here:



I'm not going to pull the cluster or dash pad just to check it, though...

 

I knew that...

"My mind's okay, it's my memory that's bad."

 

Vader,
I guess GM has been sourcing 2 different parts then. My 86 cluster as you know has a different tach than yours and theres no buffer or VSS attached to it. My 85 was the same way. My 89 is also like that. The VSS is a huge round thing at the back of the trans, almost where the driveshaft is. 2 wire connector, I think yellow and brown wires or something like that. Its to the right of where your pic ends. So... your speedo isnt electric eh? I wonder if that is the deciding factor? That means this guys 89 has the VSS on the back of the cluster. Thats how my dads G-body is, its on the back of the cluster. Its a puke green box for the buffer tho

Thinking out loud... I thought my 85 speedo was cable driven... hmm. I know the VSS was in the trans though, I had to replace it.

 

Max,

When I pull the cover off today I think I'll run it up on the ramps and take a quick look. I'm working from memory, and 5-7 has already pled my case.

There were two assembly plant for the '86s, and mine was slapped together in Van Nuys instead of Ohio. Maybe there is a little difference there.

 

Sorry this isnt Thirdgen related. but on my beater car 89 caprice estate wagon. (yes it has wood paneling) If I am driving around in drive the speedometer doesnt work. But If I put it in neutral (while rolling) about three seconds later the speedometer jumps up and reads the correct speed. But if I shift back to drive the speedometer dies again. (this is a 307 w/ a 200-4R)
Does anyone know what this could be (Im going to try to fix it so I can sell it. (and get some money to finish my motor.)

 

Max,

<h1>You were correct, I was mistaken.</h1>

I took a dive under the old 'Bird, and sure enough, the VSS is on the OD extension housing. I must have been thinking of one of those POS Fieros I had, I guess...



I learned something again today...

 

The Birds changed over to the trans-mounted VSS several years before the Camaros did... I think it was in about 85 for the Birds, and when the dash changed in the Camaro (90?) but I could be mistaken about the details. I'm sure it was after 87 in any case, as I have a 87 dash in my 83 car.

Jza'a description of how the older one worked is correct. The newer design generates the pulses directly; and the speedo in the dash is electric, not mechanical.

So I think everybody is at least partially correct here.

 

Yea, but I got the wire colors wrong.