I have a vss out of a 91 Blazer S10. It is in the transfer case and has the same two wire hook up and plug that my donor 730 had. However, my logs show that the mph isn't right. It stays at 0 mph until I get going around... 35 to 40 and then it will only get up to the high teens as I keep going faster.

Since the T-case is the only place I can have the vss, I can't decide if I have a bad vss or if I need to find a different one that will fit. Any thoughts?

Thanks in advance.

 

How did you do the wiring (is the pair of wires twisted together)? Do you have an electronic speedo?

 

Well, as my projects usually go... I'm always doing a weird combination...

I have a Buick 272 cu in race motor and a 727 (730) computer. I just followed the wiring diagram and when I got to the vss I noticed that the sensor in the back of the T-case (from a 91 Blazer S10) had exactly the same connector, and it is also a GM part. So I hooked it up with the HOPE that it would work with the 730.

Unfortunately, hoping didn't work. I know that the Blazer was a 4.3 TBI setup but I didn't look to see which computer it had in it. IIRC it was a pair of wires, yellow and purple. (I cut the wires on the donor when removing the T-case rather than unplugging it.)

I guess a more specific question is on the wiring diagram for the Corvette it shows the vss going to C2 and C8 (727) with a note saying it is transmission mounted. Yet, on Ludi's site I noticed three different variations for vss.

My current plan of action is to remove the sender from T-case and take it to the parts store to see if a tranny mounted sensor will fit in the same spot, (Hoping again) and purchase a Corvette sensor.

I was hoping that someone in here might have some info about these sensors to help me with other options.

Thanks again

BTW No speedo. Racing application with a Tach. However, I know that the vss needs to be working for the ecm to work properly (fuel).

 

is the speed sensor a gear driven type or does it use a reluctor?

 

I'll pull it off tomorrow. Hmmm good question. I ASSumed it was gear driven. Shame on me. We'll know tomorrow.

I thought the reluctor models have three wires?

 

The late model TPI VSS is gear driven, just like the old school cable drive.

If you are able to, try this somewhere safe while logging ALDL: bring the truck up to a reasonable speed and then kill the engine and shift to neutral while maintaining power to the ECM. If your indicated MPH suddenly starts working, I'd suspect interference on the wiring. The fact that the ecm starts to register some mph as you get going faster could be a clue, because as the VSS signal generator spins faster, the amplitude of the signal gets higher.

Mind you, all the above could be in vane. I don't know how many pulse-per-mile the truck VSS uses, or if GM even bothered to change the part between truck/car platform. GM isn't known to change things too much... LOL

 

That's good info. I'll give the wiring a look-over. Do those wires need to be sheilded?

Unfortunately, It is "race" wired. My on/off switch controls the whole works (through a few relays). This way I can shut down almost all the power. ... got me thinking though. I can hot wire the ecm pretty easy. I'll give that a try as soon as the weather gets above 30 deg.


Well, as seems to be the case for the last month, I didn't get a chance to go out and work on my project. Thanks

 

i have a 700 trans from a full sized truck, it has the standard truck VSS, 40 pulses per revolution reluctor type sensor. the speed sensor connector is the same as what they used in the 92 camaros.
im not sure which speed sensor they used in the 4 wheel drive trucks, but my books are showing the same 40 PPR sensor.

i was using a DRAC from a truck for the speed signal to the ECM, speedometer & cruse control.
i wanted to do away with the DRAC & just use the ECM instead, so about a year ago i split the VSS signal to both the DRAC & ECM.
for my 7730 ECM to see the signal & show the right speed, i don't remember for certain, but i think i had to change the setting for magnetic speed sensor. i did have to change the VSS Gear Ratio in constants. the factory AUJP is set at 230.00, with 215/65x15 tires & a 2.42 rear gear i set it to 12.00. i used TunerPro with 1227730_8d.xdf because its easier to change the setting with that XDF than the others.
with the stock setting my ECM speed was reading 255 MPH by the time i got to about 15~20 MPH.
i do have a problem with the ECM speed reading above about 65 MPH, the speed starts to drop out, the faster i go the worse it gets. it reads a fluctuating lower speed than what im going, at around 80 the ECM shows about 35~55 MPH. i have the same problem with the DRAC removed & the VSS signal going to just the ECM. it is causing problems with my TCC unlocking & relocking above 80.
i think the 730 ECMs have problems reading the hi resolution speed sensors at higher speeds. if you do have the 40 PPR sensor you may need to use a DRAC from a pickup for the ECM to get a good reading.

 

This seems like useful information:

http://www.tbichips.com/drac/

 

Got it out today and it is the reluctor type (magnetic sensor in the end). So, from Monte's link it appears that the way I have it wired now I was sending an AC signal to the ecm. That can't be good. I think I have a DRAC in one of my misc parts boxes... Now I just have to track down a schematic.

Denn_Shah, do you have a schematic for the trucks using the DRAC? how did you wire it to your 730?

Thanks,
Charles

 

How is the DRAC different from the "buffer box" that RBob is refering to in this post? https://www.thirdgen.org/forums/diy-...ease-help.html

Is the buffer box for a specific ecm or is it just a signal converter... can I use it with a 727 (730) ecm?

Just when I thought I had most of this EFI stuff figured out (for my application)... the smallest thing can get complicated in a hurry.

Thanks

 

im not sure what the difference is between the buffer box RBob is talking about & a DRAC from a pickup. i do know the DRAC can be easily changed like the site MonteCarSlow posted shows, i have no ideal about the buffer box, im thinking the buffer box isn't changeable because the thirdgens used a gear driven speed sensor.


right now i have the speed sensor split off going to both my ECM & DRAC, as i said in my other post, it works fine at speeds below 65. i just thought of this, im thinking maybe its not the hi resolution sensor but maybe the sensor output signal gets gets too high for the ECM to handle, i wonder if a filter to clip the maximum peak to peak voltage output would fix the problem?

i did have it set up so the DRAC was sending the speed signal to the ECM, i plan to change it back to this some time soon.
when i had the DRAC sending the signal to the ECM, i had one of the outputs from the DRAC wired into what i think is C6 on the ECM, also i don't remember if i had to make any changes to the bin as far as getting it to read the right speed or if i changed the magnetic setting, again im not sure on either of these.

here is a diagram for a 92 C1500 pickup DRAC, open them in windows picture & fax viewer so you can zoom in to read them, you may need to save them to your computer to do it

*EDIT* to correct terminal for optical speed sensor input on a 730 ECM

 

Thanks!

 

The main difference between the 4-out buffer and the DRAC: the DRAC can be adjusted to the proper pulses per mile (PPM).

The 4-out buffer receives the VSS signal, converts it to digital, and outputs it. Two of the outputs are 4000 PPM, the other two are 2000 PPM. However, the output PPM is dependent upon having the correct input PPM. So if the incorrect VSS to tranny gears are used (such as after changing the rear end ratio), then the outputs will also be incorrect.


The DRAC (digital ratio adaptor controller, or close to that) also buffers the VSS signal. And has various outputs including a high PPM for anti-lock systems. The real difference is that via jumpers on the controller board the actual output PPM can be adjusted.

This makes it a versatile little box that can be real handy. Change tire diameter, change the jumpers to set the correct MPH. Change the rear ratio (gears), change the jumpers.

RBob.

 

this post turned up out of the blue, nice timing i think,

https://www.thirdgen.org/forums/tpi/...v-truck-w.html

if you read 80Sierra's first post, it has some useful info about using a DRAC, on a 7730 ECM, use C6 for the VSS input to the ECM instead of B10 like i said, & also about the changes that need to be done to the bin.



i thought that was the differance between the two, i just wasn't sure, thanks RBob

 

Thanks RBob,


I've been doing some searches and reading. Do I have this correct?

I can run my reluctor type vss directly to the ecm (727) via pins C2 and C8 as long as I have the magnetic speed sensor flag turned on, has checkmark, using Tuner Pro?

However, my xdf file only has a constant called "vehicle speed sensor" (discounting the instrument divisor). It is set for 4006 right now and that is what the item definition states. I'm thinking that this is the gear ration constant that I can try to change. It is at address 15 hex (as shown in TP). I downloaded a couple fresh xdf's and bins from Moates and get the same thing.

It seems to work as an inverted number . With the slide bar all the way to the left (lower setting) I get a huge number in the data box.

 

i forgot your using a 727. the terminals i listed are for a 730 ECM, i'll edit my other posts so everyone knows those are the terminals for a 730.

which XDF are you using?

 

I'm using S_AUJP.xdf but I have made a few changes to some of the names. I also tried the stock 8D, same thing.

Thanks for the help

 

im pretty sure that like the 730, the 727 uses different inputs for magnetic & optical speed sensor input. so i think once you have the things set right in the bin, its just a matter of finding the optical speed sensor input on the 727 ECM & hooking up the DRAC output to it.

 

I have been trying to read Ludi's schematics... I thought I was pretty good at figuring out how things work, but I can't read these to the point of telling me which pins are for the mag sensor.

On his "connectors" link from the 727 ecm I find 5 plugs J1 - J5. My ecm has 4 plugs and I can sort of see how he is listing the 730 and 727 together but it looks like the diagram is only a 730 with the three big connector blocks, J1, J2, J3, J4 looks like the memcal, and J5 - I can't really figure out.

Can anyone help me figure out how to read this diagram for a 727?

Thanks in advance.

 

it looks like on J2, its C6 just like a 730

 

 

don't thank me yet,....
i was doing some more looking around on ludis's site, i think i am wrong on the terminal number.
i think the pages are crossed or something, going to 1227727 & then schematic & then connectors, it shows to be for 1227730 & 1227749 in the lower right hand corner.
going to 1227727 & then more info brings up what i think is the right page. there C6 shows to be a ground for the 1227727 & it shows that B21 is the VSS Optical input for the 1227727, so go with B21.

 

AtomicTruck: If you get stuck trying to find that optical input, let me know. I have a 727 running with the optical input. Hopefully it's in my notes somewhere what pin it is, otherwise I can go out in (really) cold garage and have a look.

 

Thanks Monte,

I'm real busy at work right now but I hope to poke around on Ludi's site some more this weekend.

If you can dig that up it will really help. It is always better to use one that is known to work.

So, it sounds like I need to use a DRAC and connect the single 2000 output to the ecm (B21 on my 727). Do all DRAC's use a 2K output? I couldn't find the one I thought I had... I'm going to have to go to the bone-yard this weekend.

Thanks again to all.

 

as far as i know, all truck DRACs have a 2000, 4000, & 128000 PPM output.
the antilock brakes use the 128000. i may have this backwards, but i think the ECM uses the 2000 with the speedometer & the cruise using 4000.

you may need to do some calibration work on the DRAC to get it close, MonteCarSlow posted a link on how to do this in his third post.
i believe the site is wrong as far as what years GM used a DRAC that was separate from the instrument cluster, my books say 92 was the first year the DRAC wasn't part of the cluster.

 

Thanks to all. I am armed with enough information to melt some wires. The problem is that it has been below zero here and I don't want to freeze my tuccas off wondering around the local yard. We still have the old fashion junk yards where nothing is on computer (inventoried). You have to go find it and pull it off. I like it this way. I find all sorts of treasures.

I'll post results when I get a chance to play in my shop.

Thanks again