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Questions on 87k5 VSS

BEEBOB

BEEBOB

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I have been reading some post about the VSS module. If I am correct this is the harness thats behind the speedometer and goes down to a green connector box. I have been fighting my transmission hunting overdrive while driving down relative flat roads. I can hit the brake and can hear a change in rpms like the torque conveter lockup is changing. Another problem is that the speedometer is not accurate, it is off consistently like is off linear.
I have the 31 10.50 now but am going to 33s.
Is the sensor thats mounted behind the speedo giving a signal to ecm?
Is there a way to test the VSS sensor?
If it was bad or not giving a good signal would the ecm throw a code?

Sorry for the barrage of questions, just been fighting this stuff. not real worried about the speedo being off but if it hurting the TCC lock up i will look into fixing it..

Thanks.
Josh
 
Yes that little box is what feeds the ECM VSS.

I'd assume you would get an error if the ECM "knew" there was an issue with the VSS. If the speedometer is malfunctioning, I *believe* based on the design, that the VSS would still work as it is driven/reads directly by/from the cable, not indirectly like the speedometer needle.

One easy check of VSS is cruise control. If cruise control works, the VSS is working...does not mean the ECM is getting that input, but it would need further diagnosis to determine the ECM is not getting a speed signal, or the proper one.

Tunerpro RT or various scan tools will tell you what the ECM thinks vehicle speed is.
 
Make sure everything in the cruise circuit works (easier if it worked prior then just stopped) but then I'd focus on the speedometer issue, or vice versa, since you know both of them are problems.

A conversion away from the mechanical speedometer is nice, if the opportunity ever presents itself for you.
 
My '89 doesn't have VSS. I'm pretty sure that wasn't available until '90. Are you sure it's not just a cable that is driven off a gear in your transfer case?
 
VSS did not come into play on our vehicles until 1990. Prior to 1990 our system used an optic eye which reads pulses from the speedo cable (inside the speedo actually) and then sends the info to the ECM and the cruise control. If you've changed gear ratios or tire size then the ECM is receiveing incorrect info and it will affect your TCC lock-up and possibly some driveability issues.
 
VSS was used starting in approximately 1983 for the trucks, when the cruise control went from the transducer to VSS.

The buffer is what converts the optic eye "pulse" to the information the ECM needs. The 1227747 (for instance) uses the same input from the buffer 1987-1989 as it gets from the DRAC from 1990-onwards, even though the 1990+ setups don't have a speedometer cable and is all digital.

TPI (MAF or MAP) and TBI use the same input to the ECM's for instance, and they are completely different ECM's and how the vehicle speed is calculated.

VSS was used on all fuel injected GM trucks. There is no exception to that rule, at least for US vehicles. It is how deceleration fueling is handled, for one.
 
dyeager535 said:
Make sure everything in the cruise circuit works (easier if it worked prior then just stopped) but then I'd focus on the speedometer issue, or vice versa, since you know both of them are problems.

A conversion away from the mechanical speedometer is nice, if the opportunity ever presents itself for you.
Click to expand...

What is involved in converting it from mechanical? Been looking for info but coming up empty.
 
It's somewhat complex, but at the same time not.

Hard part is the transfercase. Finding a 1990-1991 NP241 from a donor truck (our bodystyle) is step one. Then you get a DRAC from anything that used one, Astro's the same vintage used them, pretty common. Need the twisted pair of wires from it too.

Need a cluster out of the same 1990-91 that donated the transfercase.

Change the wiring at the cluster plug a bit (simple) swap out t-case (should be simple if NP208 or 241 already) run wiring from t-case to DRAC, wire DRAC up, done.

Another plus is that tire or gear changes are handled with a bit of easy soldering.
 
Other than this " The two-pulse (2000 pulses per mile) square wave (D.C. current or direct current) used on all TBI engines through 1992, all computer-controlled-carbureted engines, and on 1985–1989 TPI engines." We know of course that it started before that, since cruise transducers were gone before 1985, and CCC started in 1981.

and "A 40 pulse per driveshaft revolution speed sensor used on 1993 and newer trucks with automatic transmission, 1994 and newer rear drive cars (Camaro, Corvette, and Caprice) with the automatic transmission" Of course we know that is wrong, since 1990/91 R/V trucks had a 40 tooth reluctor/electronic speedometer.

And if you keep looking in their VSS chapter, you will find where they state that you can't have VSS on the transfercase because it would screw up the vehicle speed reading in 4wd lol.

I asked them to correct all of that years ago, but they apparently were too busy or uninterested. Don't get me wrong, they do a lot of great stuff, but it would be nice if they would correct what is known to be wrong so they would be a go-to for info for everyone.
 
Or "Raising the minimum idle speed with the adjusting screw can eliminate stalling, but the engine will still not run optimally without a VSS."

That screw is called a Minimum Air Screw and does not control Idle RPM, the ECM does!
 
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