VSS Related...

[Supertex]

So... Where does my 93 1500 gauge cluster get its vehicle speed signal?

May sound like a stupid question, but considering this was a TBI/4L60E, later swapped to an L98/700R4, and currently an L98/4L80E, and only thing receiving the VSS or TOSS, is the standalone transmission controller... I would think that would leave the speedo SOL.

Buuuut... Cruise works, speedo works... What gives?

 

[Schurkey]

Download the service manual set for your vehicle from the links in the Sticky section of the Engine web-page.

On my '88, the VSS signal is routed to a heap of circuitry within the instrument cluster (the DRAC) which processes the signal, reduces the frequency, and distributes the processed signal to the speedo, odometer, cruise control, ABS, and ECM.

At some point, the DRAC was pulled out of the instrument cluster, and put in it's own lil' box near the ECM. Later still, the DRAC was integrated into the ECM.

 

[Scooterwrench]

88-91 Drac in speedo,92-95 stand alone DRAC,96 up Drac in PCM and programmable.
On yours DRAC is a little white box usually over near the PCM with a three letter broadcast code on it.

 

[Supertex]

See, that much I know. Thing is...there is NO INPUT from the tranny. So is it making some calculation or estimation of speed? I have the drac. It's behind the glove box. But the ONLY speed signal leaving the trans is direct to the aftermarket trans controller. There is no output from that controller to the ecm, or drac or anything else. The controller connections to vehicle are TPS, 12VDC, and 2x GND. There are no other wires to the truck...Just control circuits to the tranny itself.

Schematics dont help here. This is an L98...TPI, and was rewired for the 7730 ecm + 700R4. Then later I had a tranny shop swap in the 4L80E and it's controller.

Reason I started into this, is that while the speedo works... the ECM has no VSS signal. At least Tuner Pro can't see any VSS. So...how is it that the speedo works, but the ECM doesn't see speed? Again...the TOSS or trans output speed sensor is clearly only feeding the US Shift controller, and that controller is NOT outputting speed to the vehicle.

 

[Schurkey]

C1500 or K1500?

 

[Scooterwrench]

It's got to have a VSS on there somewhere or the speedo wouldn't work. Can't run off a tach signal or your speedo would be going up and down with gear changes. Are the gauges stock? I wonder if your stand alone trans controller is using GPS for VSS.

 

[Supertex]

It's got to have a VSS on there somewhere or the speedo wouldn't work. Can't run off a tach signal or your speedo would be going up and down with gear changes. Are the gauges stock? I wonder if your stand alone trans controller is using GPS for VSS.

Indeed. You're right. After mulling it over...there would just be no way to NOT have some connection. Crawled under it this eve to figure it out. Thing is, when the harness folks rewired for the 7730 ECM, they left the part of the original harness that ran the 4L60E, in case I wanted to switch back from the 700, I could piggyback my original ECM. I suppose they spliced things like the tach, tps, etc...no idea, really. So at the tranny, there is the original 4L60E control and VSS connectors, in addition to the TCC and VSS connectors for the 700. I should have told them not to, because now I have a wad of unused wiring right at the tranny's speed sensor, but it made sense at the time.

Anyway...later, when the tranny shop swapped in the 80, they DID tap the VSS harness, However it looks like they tapped the wrong one. It's a purple and green pair in a shielded sleeve. The 700's VSS is a yellow and purple pair. Really makes me wish I'd done all this myself because this is one hell of a mess. They used wire taps FFS. I'm going to have to get it in the air and figure it out. I guess the original VSS fed the DRAC, and subsequently the old ECM. But then I have no idea how the 700's VSS ever fed the gauge cluster. Looks like I'll be using that schematic after all :/

 

[Supertex]

C1500 or K1500?

C1500

 

[Scooterwrench]

Cool! glad you figured it out.
The 700R4,4L60E and 4L80E all use the same magnetic pickup type VSS. The 60 and 80 have two,one in the front for input shaft speed and one in the rear for tail shaft speed. 4WD have three,another one in the tail housing of the transfer case. On those that is the one that runs the speedo. Yeah, you want a good,sound electrical connection on those VSS wires. The signal is not really strong to begin with and a bad connection will cause erratic behavior in the speedo,shifts and TCC in that 4L80 you have in there now. It relies on that sensor for tail shaft speed data to tell it when it can shift and lock up the convertor.

 

[Supertex]

Yeah, there is just so much wiring there, I missed it. It's a pretty decent-sized wad of wire, and it was really hard to find the splice. I didn't expect it to be in the original VSS wiring either, but rather in the 700's that they replaced with the 4L80.

Now it kind of begs the question: How did my speedo ever work with the 700 in the first place? Since the truck uses a DRAC, and it expects the 4L60's 40 pulse per revolution, and the 700's VSS provides 4 PPR, the harness shop couldn't have tied those two together...surely...

Buuut....Looking further, one of the two VSS input wires into the ECM (BB10) is cut. I didn't do that, so it had to be the tranny shop. I put in a jumper just for kicks, and before my dash even showed 5 mph, TunerPro was showing 60 mph...

I think Im either going to have to get another DRAC to use solely for the ECU, or perhaps splice off of one of the existing DRAC outputs, although I'm not real sure what signal is leaving the DRAC. I see is has a 4000 pulse per mile output for the speedo, but it's just one wire. Not sure if I would use that and the ground or...

We'll see.