So I have some interesting recent experience with this vehicle speed sensor (VSS). Short version: I think you want it but may not need it.
Long version:
When I researched my swap, I found the Hesco VSS adapter unit (the only option out there to my knowledge) to stick on there to give you both the mechanical speedo output and the VSS feed for the computer. I could not find anything definitive to tell me what the VSS was used for by the computer, so I ended up deciding that if it was there, it was for a reason and I bought the unit and installed it.
When you get it, you have to buy their plug or make something else work. I made something work out of a weatherpak connector (just had to trim back the plastic cover over the metal connectors). HOWEVER, I could not figure out how to wire it, since it wasn't a stock connector/sensor and all of the years of these have different wiring colors. I never found anyone who had directions posted for a 97. So I figured I had a 50/50 shot at getting it right OR it would work either way. So I put it together and ran it for years like that.
I think it was after I rebuilt the tcase, the large (and cheap) metal nut that screws onto the tcase split up the side. It leaked like a seive and I tried to repair it with JB Weld, but it wouldn't stop leaking. Rather than spend on a new unit, I decided to pull it off and just wire the connector out of the way, to see if anything would change with how it ran/shifted.
I didn't notice a difference in anything at all. So I just ran it without a VSS for a number more years.
Well, recently, I've had some questions about my IAT readings and a couple misfire codes. I've also been battling the IAC motor going bad/acting up on me and some hot weather stalling when getting off the highway after prolonged high-speed driving. So I invested in a bluetooth OBD2 scanner and app for my phone to try to dig into things more.
When I did, I found that there was a readout for the speed that the VSS was sending to the computer. That got me thinking about my IAC/stalling issues specifically, thinking that since the VSS was seeing 0 MPH, maybe not having the VSS feed was overworking the IAC and causing it to go bad prematurely. Basically, that the IAC kept up for a long while, but eventually was overworked and couldn't react fast enough without the proper VSS signal when getting off the highway with high engine temps and IAT readings. That's all total conjecture on my part, but I'd finally gotten sick enough of the issue that I decided to buy another Hesco unit now that I could definitively see the VSS feed on my phone.
Well, I got it, installed it, reconnected my homemade plug and went for a drive, eager to see if I needed to play with the speedo gear to get a proper reading.... 0 MPH
I checked that everything was good and connected solidly and it was... then I remembered about the wiring conundrum. So I swapped the wires, reconnected, drove and voila! I had VSS speed!
So (remember, I said it was a long story!) all this time, even when I had the first unit installed and connected, I functionally was running WITHOUT the VSS. IIRC, that's 10 years. In that time, you know how far the Jeep has been driven and wheeled, so do you NEED it: I can definitively say no. Does it potentially cause issues without it: Yes.
I've only had the new one in and working for about a month. I have made a couple trips to and from the dunes in that time and run it around up there a lot on the dunes and back roads.
Now, to make this story longer... I DO have a new issue that I did not have prior to the VSS being hooked up that just came up on my last two trips to/from the dunes.
I have always been able to run my Jeep as fast as I feel comfortable. It's been over 90 a couple times (and there's a fun story about getting pulled doing 85 in Minnesota on the way to the Black Hills National... and not getting a ticket
) Anyway, in these last couple drives, I have gotten a HARD misfire, almost like the fuel pump is cutting off, when I hit right around 80. It's repeatable in that it seems to happen every time I get in that area, but not exactly at the same GPS speed on the Speedut or my phone. What's REALLY interesting to me is that it's not RPM-related. I can rev the engine in 3rd WAY past the RPM where I get the cut-out in 4th when I hit that speed. I ran up to 80 in 3rd, just to see what happened at a different RPM and it did the same thing. It
definitely seems to be specifically speed-related and I can't imagine it's not related to the VSS.
What I have seen in the VSS readout is that it's reading much higher at times than my actual speed. Maybe 100-110 when I'm actually doing 70-75, but it's not consistent and bounces around a lot. I meant to look to see if there's some sort of governor that is built-in to the 4.0 or something to cut off fuel/spark when going too fast. That's exactly what it feels like to me. It's so similar to the rev limiter, which I am VERY familiar with from running the dunes
So I've been meaning to try to figure out the right speedo gear to swap in and see if that improves things.
I can't really yet say that the VSS has cured my IAC valve issues because it's not been hot enough to induce that issue. It only ever happened on really hot days when driving in the heat of the day.
Aren't you glad you asked??
