80Sierra

Hey guys!

My truck is currently running a Speed Density TPI setup (7730 ECM), and currently has an old school SM465 / 205 transmission / transfercase setup. As such, I still have the old stock speedometer cable that runs from my transfercase up to my speedometer, which I retrofitted a 2000 pulse per mile optical VSS onto.

However, I am swapping from the 4 speed to a 5 speed, with a modern transfercase. This newer transfercase has a 40 tooth reluctor ring VSS in it, and I'd like to simply run it to the ECM, and have it act as the DRAC, putting out the required signal wires to my cruise control, and to my electronic speedometer.

Reading online, I've found that it is no problem wiring the reluctor VSS straight to the ECM, and that it is able to convert the signal no problems, but I don't know how a guy goes about changing the prom to reflect my truck's rear diff ratio, and tire size so that the speedometer is accurate.

Is there a specific formula to use for calculating a new constant somewhere? What address is the constant located at, and what is the formula if it exists?

I'm already aware of how to change my prom to tell the ECM that it will be recieving the magnetic VSS signal.

Thanks for your help!

Russ

DENN_SHAH

iTrader: (1)

to change what you want, with TunerPro use the 1227730_8d.ecu file, then in constants find VSS Gear Ratio. there are several other ecus/XDFs that you can use to set it, but i find the 1227730_8d.ecu the easiest to use for this as it uses numbers from 0 up to 255.
as far as a chart figuring out what setting you need, i can't help you, i got mine close by trial & error to start with, later i got it set as close as i could with the dyno at work.
with 215/65x15 tires & a 2.42 gear, mine is set at 12.

now for a bit of bad news, it may just be something with my system, so this may not affect you.
i have a 700 from a truck which has the 40 tooth reluctor.
i am having trouble with the ECM doing the work of the DRAC, so i still have a DRAC for my speedometer & cruise.
the problem im having is my speed reading in the ECM starts freaking out around 62 MPH in overdrive. the speed reading will start to dropdown & bounce around, the higher i go above 62 the worse it gets. at around 80 MPH the ECM reads about 30~50 MPH. in third gear it starts in at a little lower speed.
so far i can't find anything wrong, the wiring is all good, i have tried 3 different ECMs & 2 different speed sensors, so i think the 730 ECMs may a problem with the high resolution speed sensors at high speeds.

not to hijack your thread, i would really like to do away with the DRAC too, so if you or anyone else has an ideal as to what may be the problem, i am open to suggestions.

JP86SS

iTrader: (1)

Found this in my notes, Ludis's info from the DIY archives 4-2-1999:
These are the three bits at $8014, bit 5, 6 ,& 7 (in $8D)
There's more to his post (very detailed on pinouts etc)

Another note found while digging on this:

Formula for calculating Road Speed Constant:
7372800/PPM=RSC
For '92 - '94 5 speeds.
RSC = 24015.64, Divisor = 6

RSC is at $8015 (in $8D)

Not sure where that last formula came from but I believe it to be true.
HTH

87TPIYJ

Im digging up an old thread out of the grave, because I am having an issue with a smilar set up to the one listed above. I have a 91 5.7 z28 TPI and I am trying to use a 40 tooth reluctor ring on a newer NP241 transfercase. When i hook it up it starts and runs fine, then i try to move it and it dies. If I unplug the VSS i can drive all day long but I throw code 24. Any ideas????

morgsie

iTrader: (1)

If the ECU doesn't like the 40 tooth relutor wheel siginal then just install a DRAC to handle the signal and feed your ECU the correct 2000 PPM. Will also work for cruise and ABS if needed.

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

elesone

I have a 36 Chevy with a 91 F-body 305TPI and 700R4. Also, the dash is all VDO gauges with a programable speedo. I had the same issues, especially duplicating what DENN-SHAH experienced. Connected the stock purple/yellow wire connector to the ecm and fine tuned the speedometer. BUT, the moment I hit over 60mph the speedo was erratic. VDO suggested I change the speedo drive to gear and use their pulse generator. Well I knew that because that's the setup I had with the TH350.
I had plans to change the setup to Dakata Digital in the future, which I already had and I was waiting to redo the entire dash. I did not want to change the speedo drive, so I called DD and they suggested their product SGI-5 will work with any trans with pulse generators or gear drive with a generator adapter. They were very confident it will work without any drac. I got one, hooked it directly to my 700R4, fine tuned the speedo and it worked perfect. I then placed the digital gauge pod on the seat to test it, flipped some dip switches and it worked instantly, talk about a cool setup. You can read about it and you'll find it will also support cruise.
It was easy to install and setup. Yeah, I had issues at first, that's because I was in a hurry and missed one line in the instructions. Go figure.
NOTE: I have no ties with DD other than being a customer. Here's the link for the unit.
http://www.dakotadigital.com/index.c...rod/prd126.htm
Good luck.

DENN_SHAH

iTrader: (1)

87TPIYJ, i believe what your seeing is MPH governor kicking in. above a certain speed, the ECM turns the fuel off to limit vehicle speed.
a good speed reading to the ECM is needed for best performance & fuel mileage.
also without it, the ECM will stay in cell 4 & the motor may die when coming to a stop.

there are a couple of ways for you to go.

if its easy to get to, you can change the reluctor with one with fewer teeth. i would guess 4~8 teeth would work ok.
i have one that i had ground most of the teeth off of, i think i left 8 on it but i never put it on so im not certain how well it would work.

you can use a DRAC as morgsie suggested and like i did, the link posted by morgsie has pretty much all the info you need.
also,
https://www.thirdgen.org/forums/diy-...-v-sensor.html

https://www.thirdgen.org/forums/diy-...ico-trans.html


but either way you'll need to make some changes to the chip.


what elesone posted is another way.
maybe he will let us know if you would need to make any changes to the chip or not, but it is certainly something to look into.

87TPIYJ

That sure sounds like you might have hit the nail on the head with why its dying. When i first drove it with the guage hooked up and not the computer the mph skyrocketed almost emmidiatly. I remember the instructions said that it was pre programmed for 4000 pulses per mile. Which it stated was a standard V8 signal. When i calibrated it i got 107,337 pulses in roughly 2 miles. If im understanding this correctly then my out put signal is 26.8 times more than the computer should be seeing (2000 pulses per mile) for that speed. So it theoretically it would die at a little under 5mph? Am i understading this right?

Tuned Performance

iTrader: (92)

It depends on the calibration some $8d are around 110mph fuel cutoff and some are 255 mph.

elesone

DENN-SHAH, I did not make any changes to the programming. The unit is so flexible, it works with any tire size, any gear ratio or transmission. I just did a quick calculation of how many revs my drive shaft did with the tire/ratio in my hot rod, flicked some switches, went for a ride, came back to the garage, did some more adjustments because I'm **** and back for a ride, each time using the GPS as my checker. I found out that the family car is NOT accurate. For *****-n-giggles I went for a ride on the hwy doing exactly one speed against a known distance with green markers, timed with a stop watch and was within 1/2 mph. Told you I was ****.
In short, this unit is flexible and I like it. I'm sure there's others that will argue the point

87TPIYJ

My problem is not the speedo its the computer. After reading the installation instructions it said most tpi set ups us a 4000 ppm oc. I thought that the computer read an ac and had a buffer in it to convert it to a 2k/4K oc????

Tuned Performance

iTrader: (92)

https://www.thirdgen.org/forums/elec...rs-speedo.html

Six_Shooter

iTrader: (2)

The SGI-5 is basically just an expensive aftermarket DRAC, that comes with programming instructions.

It has a couple other features but it's basically a glorified DRAC.

Tuned Performance

iTrader: (92)

I like to go the cheaper rout to a drac and a dip switch is a cheap easy and effective modification.

87TPIYJ

well I got the sgi-5 hooked up and everything is good to go as far as vss. It has rough idle and misses under light throttle when its cold and after a while throws code 32. Another problem to tackle.

robotic_junky

The 730 ECM (speed density) uses a VSS that does 4000 PPM that goes to the the ECM. Then the ECM sends 4000 PPM to to the speedometer. The ECM also sends 2000 PPM to the cruise control.

The 165 ECM (MAF) uses a VSS that puts out 2000 PPM (TBI also uses the same PPM) then goes to a buffer. The buffer sends 2000 PPM to the ECM and cruise control and sends (I think) 4000 PPM to the speedometer. There are some (Camaros) that use a cable driven speedometer and has the optical sensor and the buffer right behind the speedometer.

87TPIYJ

yeah my computer receives a 4000ppm AC signal and the buffer is inside the ECM. It outputs 4000ppm dc square wave to the speedo and 2000ppm dc square wave to the cruise (which I wish I had).