Gr8Dain's 4.0/aw4 swap from a 97 XJ

ag4ever

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I’m fairly certain the hesco unit is just an 88-93 Dodge Dakota VSS with an adapter to allow the stock speedo cable to screw on it. If so, you could use a post 93 VSS if you don’t have a mechanical speedo as they went digital on the 94 and later versions.
 

gr8dain

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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 :banghead:

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 :rotfl: ) 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. :shrug:

Aren't you glad you asked?? :rotfl:

I am very glad I asked. Thanks Eric. I appreciate it. But it makes me think That I still don’t know the answer.
 

gr8dain

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I’m fairly certain the hesco unit is just an 88-93 Dodge Dakota VSS with an adapter to allow the stock speedo cable to screw on it. If so, you could use a post 93 VSS if you don’t have a mechanical speedo as they went digital on the 94 and later versions.
That is correct. It is for a Dakota with the adapter on it. At least that is the one I bought.
 

gr8dain

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My issue is that I need to either add the gear in the t case or see if there is a gps unit that I can connect the vss to in order to tell the computer what speed I am going.
 

gr8dain

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Oh and I had my intake hot tanked today to clean it. After cleaning it, the guy noticed a crack. Damn my luck. I have another intake, but this one was the “better” horseshoe shaped one. My spare is not. I guess I can use it and swap over the better injectors to the older intake.
 

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gr8dain

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I already have a line on a replacement intake. Pick it up on Monday. So this weekend I will be working on mating the trans to the motor. And test fitting the dana 300 to the trans. Hopefully it lines up without issue. I need to make progress getting it back under the hood.
 

jpnmaine

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East Wilton
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ME
97 XJ 4.0l/AW4, OBD2 is what I am running.
Are you usng the instrument cluster for MIL or are you using a 96 ECU. Not trying to rob this from gr8dain....Just preliminary game plan in thought to my OBD2 adventure. Also it was my understanding that without VSS the transmission would not shift up into overdrive.
 

bigwalton

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No MIL, just check the codes occasionally.

Clearly, VSS does nothing for shifting and can’t be that involved in much else. Everything worked aside from, potentially, the stalling/IAC

I have a detailed swap thread on here from my install.
 

spankrjs

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I would say that you do need the VSS, the factory installed them/tied them into the PCM for more than just the speedometer :twocents:

You can probably run/drive just fine without it, and I would say it is not as important as having an operable coolant temp sensor, as far as just getting the vehicle moving :twocents:

Like BigWalton said, I am not 100% sure it is absolutely critical, but for $100-$150 why leave it out :shrug:

On my red Scrambler, stock 4.0 with Mopar MPI, I use the Hesco supplied VSS.
On my green Scrambler, rebuilt 4.2 (bored 40 over/mild cam) with Mopar MPI, I use the Hesco supplied VSS.
On my "sold" tan Scrambler, 350 with GM TBI, I used a VSS from "jagsthatrun".

I have/had the correct speedometer drive gear in each one (matched to gear ratio/tire size), so that the speedometer reads correctly AND the PCM gets the correct speed. I can drive any of them with the scan tool plugged in, go to the VSS screen, and see the speed in real time.

I would think that PCM uses input from the VSS to vary timing/fuel/injector pulse, just like it uses input from other sensors :twocents:

One important NOTE - all of mine are OBD1. But, even just being OBD1, if it is not important, why did Mopar/Hesco include them in the MPI kits :shrug:

One other NOTE dealing with something BigWalton said - I know that on both my 2005 TJ and 2005 LJ, the PCM limited top speed to 93mph. If you went over 93, say to 95, the PCM did something to retard the engine/vehicle speed, it instantly slowed down, until you hit 95mph again, then it would retard the engine again.
 
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gr8dain

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I would say that you do need the VSS, the factory installed them/tied them into the PCM for more than just the speedometer :twocents:

You can probably run/drive just fine without it, and I would say it is not as important as having an operable coolant temp sensor, as far as just getting the vehicle moving :twocents:

Like BigWalton said, I am not 100% sure it is absolutely critical, but for $100-$150 why leave it out :shrug:

On my red Scrambler, stock 4.0 with Mopar MPI, I use the Hesco supplied VSS.
On my green Scrambler, rebuilt 4.2 (bored 40 over/mild cam) with Mopar MPI, I use the Hesco supplied VSS.
On my "sold" tan Scrambler, 350 with GM TBI, I used a VSS from "jagsthatrun".

I have/had the correct speedometer drive gear in each one (matched to gear ratio/tire size), so that the speedometer reads correctly AND the PCM gets the correct speed. I can drive any of them with the scan tool plugged in, go to the VSS screen, and see the speed in real time.

I would think that PCM uses input from the VSS to vary timing/fuel/injector pulse, just like it uses input from other sensors :twocents:

One important NOTE - all of mine are OBD1. But, even just being OBD1, if it is not important, why did Mopar/Hesco include them in the MPI kits :shrug:

One other NOTE dealing with something BigWalton said - I know that on both my 2005 TJ and 2005 LJ, the PCM limited top speed to 93mph. If you went over 93, say to 95, the PCM did something to retard the engine/vehicle speed, it instantly slowed down, until you hit 95mph again, then it would retard the engine again.
Thanks Spanks (sorry, I could not help myself). I plan to tear i to it this weekend and call JB about getting a gear for it. Do it right, Right?
 

bigwalton

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To be clear, I put it in initially (wired incorrectly) because I agree, if you can have it, use it.
I am not advocating for leaving it out, just relating real-world experience.

.I would think that PCM uses input from the VSS to vary timing/fuel/injector pulse, just like it uses input from other sensors :twocents:

...

One other NOTE dealing with something BigWalton said - I know that on both my 2005 TJ and 2005 LJ, the PCM limited top speed to 93mph. If you went over 93, say to 95, the PCM did something to retard the engine/vehicle speed, it instantly slowed down, until you hit 95mph again, then it would retard the engine again.
With A/F ratios feeding the PCM in basically real time, I question the utility of vehicle speed as an input. Think about a carb, there’s zero speed input. Certainly, it would have to be SO tiny a difference for my Jeep to run as well as it has for years (and with no noticeable difference now that it’s working again). :shrug:

And that speed limiter info is exactly what I needed, that fits perfectly with what I’m seeing now. Got to swap that gear.
 

gr8dain

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Played in the garage some today. Took a long time, but got some accessories on the motor, and then mounted the tranny to it. I want to shove it into the Scrambler, but I need to get another i take and then mount that and the exhaust manifold before putting it in. Baby steps.

I hate that I have to put everything back into a yighy space so I can put two vehicles back in the garage for the night. But oh well.

“Good night Gracie”
 

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gr8dain

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Went to see what was missing in my transfer case to see about the speedo functioning in the future. It has an advanced adapters output shaft and it is missing the sleeve gear that makes contact with the speedo gear. Part 300625 in this diagram. I will be calling them tomorrow to see if I can just order the parts I need.
 

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gr8dain

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Oh and I got new studs for the fan clutch to mount to the water pump and ordered a power steering reservoir since the one that came on the pump I bought new for this was busted. And I did not open it ip to look u til after the return window was passed. Oh well.
 
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