a question about DRACs.

my understanding is that their conversion range is designed around the idea of a 40-pulse-per-rev sensor.

a t56 outputs 17 pulses per rev.

is there any way to "double" the pulses per rev that a DRAC sees as input, (i.e. make the t56 vss act like a 34 pulse per rev vss) in order to get close enough to the DRAC's expected input that it is actually useful?

or more simply, has anyone successfully married a DRAC to a T56 17 pulse VSS?

 

i take it your talking about a DRAC from a pickup, right?

the output on them can be changed for different size tires & gears.
i don't think you can change what speed input they use.
does the T-56 use gears to adjust the PPR? or does it use a reluctor?
do you know what transmission the standard trans trucks came with?
if it uses a reluctor, maybe you could change it & use a speed sensor from a truck & then adjust the DRAC to give you the signal speed you need.

 

well, that's getting a bit ahead of where I'm at right now.

The T56 has a 17 tooth reluctor.

I've had the T56 in my '91 for a few years now, and with my 3.23 gears and stock size tires, I generate around 43,000 pulses per mile with this setup.

It just so happens that the highest divisor built into the stock speedo hardware is almost perfect for this combination (well, it reads about 3% too high but was "good enough" for the time). and naturally the $8D has a road speed constant which was easy to calibrate to 43,000 pulses per mile as well.

So, my present setup has no DRAC at all because it's not needed.

But now I'm changing gears (posi died, might as well go to 3.42 or 3.73 while I'm in there), which is going to increase my pulses per mile (higher trans revs for same MPH), but I have no further possible reduction in the stock speedo hardware, so I'm going to end up at least 8% off with 3.42's, worse with 3.73's.

The DRAC has three outputs, which I will call "low", "mid", and "high" (the intended 2000/4000/128,000 pulse per mile outputs). The DRAC generates the outputs through the following math:

low output:
VSS input / (input ratio * 64)

mid output:
VSS input / (input ratio * 32)

high output:
VSS input / input ratio

If I run 3.42, I'll generate ~ 46,000 pulses per mile, I would get the following possible drac outputs:

low:
46000 / (0.5 * 64) or approx 1430 pulses/mile
to
46000 / (0.988 * 64) or approx 720 pulses/mile

mid:
46000 / (0.5 * 32) or approx 2860 pulses/mile
to
46000 / (0.988 * 32) or approx 1450 pulses/mile

high:
46000 / (0.5) or approx 92,000 pulses/mile
to
46000 / (0.988) or approx 46,000 pulses/mile

low and mid are too slow/low to be useful, and high is too high to be useful.

my reason for asking about doubling the input (to somehow double the VSS signal) is that the mid range output of the drac would then have a range of 2900 to 5700, which I could adjust to 4000 with an "input ratio" of 0.718 and achieve a nice 4000 pulses per mile out of the DRAC mid output.

 

The output freq can be doubled easily. Not sure about the input as it is a sine wave. Although, could buffer the VSS signal, double it, then feed it into the drac. Once the signal is digital only need a capacitor, resister, and XOR gate to double the freq.

RBob.

 

Interesting, I never thought of doubling the freq on the output but it seems more doable than doubling the input.

I did some reading about an XOR gate doubler, and it seems that the downfall is that the duty cycle will not remain 50%. Will the ECM tolerate this?

I've also founda few other links that I'm trying to digest that might have some good info:

http://forum.allaboutcircuits.com/showthread.php?t=2250

http://www.du.edu/~etuttle/electron/elect28.htm

 

It looks like I could also use a frequency divider setup on the highest pulserate output, specifically a /16 divisor (divide in half four times) to achieve what I'm looking for:

http://hyperphysics.phy-astr.gsu.edu...ounter.html#c3

I think that there are J-K flipflops with four flip-flops per chip (14 pin package), so perhaps could implement the whole thing on one chip.

with my setup and a DRAC, the ABS output is between 92,000 and 46,000 pulses a mile. take that and divide by 16, and I get 5,750 to 2,875, which leaves me plenty of room to adjust for a standard 4000 pulse per mile output.