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Np205 Vss

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pseudomike

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Ok, guys need a little help. I know this has been talked about on here but I was unable to find specifics. I am trying to locate a VSS for my NP205 (1985 slip yoke) that will run the 4L80E transmission controller on a '94 TBI harness. I originally purchased a mechanical to VSS adapter from jagsthatrun.com but I think they sold me the wrong # pulse unit. I have the original NP241 case w/VSS still in it that came out of the donor vehicle. I read somewhere that I can swap that VSS and reluctor ring in place of my mechanical speedo drive in the NP205. Can anyone confirm this? I plan to have the 205 overhauled here very shortly and it would be very relieving if I could solve all the issues in 1 shot.

Also, I am looking for that 5/8" adapter plate that goes from the long input NP205 adapter to the 4L80E. I thought somebody was making these but I have only found the 2wd 4L80E adapters on the web. I had a machinist buddy make my original one but I think it is machined incorrectly because it is leaking, Any help greatly appreciated.
 
If you search for 4L80, there alot of posts, but there is one that goes into detail. IIRC you can take the 241 tone ring, turn out down to fit the 205. Can't remember the particulars....
 
I had found the write-up on here for the 205 with fixed yoke. I guess I can assume it is the same as my slip yoke? Anyone tried this on the slip yoke 205? Thanks for the link guys.
 
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As long as the output housing (the removable cast iron piece the speedo gear is in) is the same as the fixed yoke 205's, I can't imagine it's any different, short of the length of the output shaft. It wouldn't make sense from a production standpoint.

Then again, the slip yoke doesn't make sense either. Ditch it. :) lol
 
I agree, I'll give it a whirl and see what happens. On those slip yokes, they were a nuisance in my lifted K5, but with a 130 some odd inch wheelbase and no lift I've never had a problem....knocking on wood. For this application it really equates to less maintenance and parts to wear on the drive line. I'm really not a fan of those slip drive shafts, you have to grease the pi$$ out of them constantly and they vibrate like a bit(* when they get even the slightest slop.
 
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I can try and get some pics of the one I did on mine, 93 blazer with a 4L60 and a klune and a 205. basicly I tried 2 different 200 and 400 hundred dollar speedo parts that were suppose to work , but didnt , so I built my own,,, it cost about 2 bucks and 2 hours worth of time.
 
Same problem here, tried a couple different units but no avail. It wouldn't be a big deal but i need it to play nice with the controller for the transmission. I should have purchased the compushift unit from the get go and been done with it...and probably less money (at least in time) then I got into it now. I did find a used NP 241 tone ring at a local drive train shop so I won't have to tear apart a good t-case. Out of curiosity what is the best way to turn this thing down to correct thickness? I do have a lathe, a friend with a milling machine, and I'm pretty sure he has a surface grinder. I'm guessing that it's probably fairly hard material so the surface grinder may be the best bet? Maybe I let him decide.

Any case if you got some pics that would be great. I'm going to start digging into the case this week to see what else it needs.
 
Not sure why the aftermarket has been so slow to understand the 40 pulse per revolution setup. Even jagsthatrun makes that mistake from what I've seen, unless they've corrected their site lately. I guess it doesn't help that GM used three different input measurements for speed signal.
 
To my current understanding, the 91-93 models used the 2000 (not sure on the correct lingo here, may well be 20pulse/rev) pulse system. 93-96? used the 4000, and the newer stuff including LS1 is yet different again? I was trying to use a '94 K2500 Suburban harness and ECM but I had the 2k sensor from jagsthatrun..not gonna work. I tried swapping to another they recommended, the 4K, and either they sent the wrong unit or? I thought I had a good understanding on the different systems but I have heard too many conflicting statements to discern now (willing to bend my ear if someone knows differently). So, from there I went with a harness and ECM out of a '91K3500 which should? use the 2k sensor. I think this would have worked ok but I had a '95 (actually unsure other than it is '96 or older) 4L80E and I am told now that it was incompatible with the 91-93 application and I likely cooked it up. From what I understand now the 91-93 4L80E are compatible and use the 2K system. The '94-95 transmissions use the 4K system. In '96 GM went to a PWM torque converter so these again are not compatible with the prior years. This is also when GM moved the return oil line on the 4L80E (a better system). Along with all of this are differences in the DRAC systems that run the electronic speedometer and TBI motor meaning you can't really mix and match. Unfortunately I don't have access to diagrams to sort this stuff out (a little beyond the haynes manual). I've also found a number of tranny guys, injections guys, and everyone in between claiming to know which systems are which, most of whom disagree with the other and none that had a viable solution to my problem.

So, I pulled the 80E out for now, fluid smelling like burnt toast (after a $2400 rebuild and 3.5k mi) and swapped the trusty TH400 back in until I find a good solution. For the time being I'm trying to get my NP205 rebuilt w/vss so i can run the TBI correctly without swapping the harness in the truck again to the mechanical speedo with DRAC behind the cluster. Once that is done, I am going to attempt to rebuild the 80E myself and run it with a compushift box to simplify things. I also picked up a 2005 model 80E but it is 2wd (does anyone know if the LS1 bellhousing is the same as my L05?). I think I will swap the output shaft out of my '95 into this transmission and simply freshen the clutches with some red eagles and kolene plates. So a long sigh to wasted wages and hopes that I get this damn VSS running in the 205 for the winter.
 
There very well could be more to it than what I know...R/V for 1990/91 with absolute certainty use DRAC, which means the 40 pulse per revolution VSS on the output shaft. C/K may have been different from '88+. 40PPR is based on the reluctor ring on the output shaft having 40 teeth...this obviously makes the PPR significantly higher than 2000 or 4000PPM. But it would stand to reason that if the output shaft uses a reluctor ring with the same number of teeth, the signal from the VSS would be the same, no matter what or how it's interpreted. (With a DRAC, in the ECM, or however)

2000/4000 is PPM, which is pulses per mile. Our trucks, up to 1989, used 2000PPM to the ECM. Cars were a conglomeration throughout the years, so this is where the aftermarket companies kind of lead truck people astray, as the trucks didn't follow the cars with VSS implementation.

It is a pretty confusing area...at least for the late 80's TBI/TPI, the ECM still wanted to see the 2000PPM signal, regardless of what ABS or cruise used, which may or was higher. And even though our trucks ECM's in 90-91 still took the 2000PPM signal, the DRAC had to convert the signal from the VSS before it got there.

DRAC is real simple though...as long as the input signal is the same, and it's output is what you need, it should work. It will output 2000, 4000, and 128000. I've not seen anything that says GM used other numbers than those three. All it does is convert that VSS signal from output shaft revolutions to what it would be in pulses per mile. As I understand the 4L80E though, it uses two of it's own "speed sensors", and since the 4L80E equipped trucks have PCM's and not ECM's, it would make sense that everything is handled in the PCM for the transmission. But I could see the early 4L80E applications using the DRAC to convert/transfer those signals to the ECM.
 
My props, I have to say that is more than most seem to know about the early VSS systems. I think a lot of confusion comes in with the car/truck bit and also the confusion that the earlier vehicles had a DRAC or VSSB. Bottom line... I think GM engineers were trying to figure out the best method for controlling the electronic transmissions, and they really didn't do that until the late '90's so there are many variations from the early to late '90s.

What I know for sure is that all 4l80E regardless of year require 3 speed sensor inputs to the controller whether its and ECM, PCM, or stand alone controller. There are two sensors on the side of the case (4wd version). I think these actually measure speed of internal components of the transmission like the drum. You still need a VSS of the final drive output to drive the electronic shifting and other functions which is where I ran into problems. As you presumed the 91-93 4L80E is driven by a DRAC (although I have also heard the cutoff was '92?). As an assumption, this is why the '91 K3500 ECM for 4l80E has 5 more pins than my 700R4 '91. The 94-95 transmissions have updated speed sensors on the case and are wired differently. They also have built in oil-temp sensors and various other enhancements. I had a '94 harness in my truck and it does not have a DRAC, it has what is called a VSSB. I am not sure what differs from the DRAC as I believe it just converts the signal. I have not seen a '95 harness but I assume it also uses a VSSB as the transmission is listed as a direct fit 94-95...although a few guys told me a '95 4L80E will only fit a '95 harness? And then there are the '96 up version, all controlled by PCM behind some derivation of a Vortec (or diesel I guess), all of which have the PWM torque converter and the rearward return line.

All that rubbish aside, the 4L80E to get is the '96 up model. Actually, the later the better. Aside from the much improved oiling system, the later models also had much improved electronics, better valve bodies, and probably most importantly reinforced cases around the reverse band which is notorious for breaking and requiring welding on the early 4L80E. Had I known this before, and had someone told me that the later transmission wouldn't play nice with the early C/K harness I probably would have just a late model tranny out of a scrapyard and a stand-alone controller as they can be programmed to play with just about any VSS, TPS, and so on. But I was trying to do it without giving TCI or Compushift a grand so I guess thats what I get. Bottom line, if you're really going to go through all the trouble of getting that 4L80E in that C/K get the later tranny because it truly is a much better unit but it will require a controller unless you are running a PCM controlled motor like a Vortec or LS block. I wish I knew more about this because I would do a nice write-up here for people looking to do this conversion.
 
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here you can see the senser in the side of the middle ( big cast iron ) tail housing, I ground it flat so the senser would be straght with the exciter ring. there is a metric nut on the inside of the tailhousing, the metal is very thin and I didnt think there was enough metal to have good threads. the nut is also bronzed to the cast iron tail housing.
the exciter ring is the same spline as the plastic speedo gear that is on the tail shaft, but the exciter ring is wider, take it to someone to have them mill it down to the same width as the plastic speedo gear . I used a grinder and took my time to keep it straight on both sides, then just slip the exciter ring and all the other pieces back together, One note , the exciter ring and the senser can not touch or be over ( I think ) 5 thousandths of an inch away from each other , or it will not work.
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I know this is a breif desription, but it was easy to do , and is cheap !
I have done this to my 93 s10 blazer , a 91 K5 , and a 90 c10
Hopefully this will help you and get you going.
 
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you will also have to plug the hole where the old speedo cable went into the housing
 
Thanks for the pics. It seem pretty straight forward but how do you determine where to drill the hole? I was thinking I could re-use the original speedo hole but I presume there is a reason not to do that? I guess the key would be making a good measurement from the front of the case to the center of the tone ring and then from the mounting flange measure that distance and drill. I will probably have my machinist buddy help me out so I'm sure he can figure it out. Done correctly I assume I can use a stock NP241 VSS? No depth issues or anything?
 
The problem with the stock speedo hole is that it's offset. The VSS needs to be "centered" on the output shaft, so thats the reason. It wouldn't read correctly (if at all) if it were angled in relation to the reluctor ring, as it would be being below the centerline of the output shaft, if that makes sense. :)

I've noticed my output shaft wobbles a bit, and that the VSS' I've looked at that were in use look to take a bit of abuse from the reluctor once in awhile. So far no problems with mine, but I do wonder about how long it will last, or how sensitive they are to wear...
 
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