DRAC Module clicks?

[AzDon]

I recently completed my 69 van project with the complete TBI and harness from a 91 Wagon....<br>I also installed a new Intellitronix instrument cluster that fits 62-65 Nova and 67-70 G-Vans...<br>All of the gauges work awesome, except the speedometer, which I hooked to the GM DRAC module speedometer output.....<br>When it didn't work, I did a web search for DRAC module troubleshooting (and to better understand how it works) and found a pretty good article here:<br><a rel="nofollow noopener noreferrer" target="_blank" href="https://atracom.blob.core.windows.net/gears/2005/2005-04/2005_04_04.pdf" style="color: rgb(25, 106, 212); font-family: &quot;Helvetica Neue&quot;, Helvetica, Arial, sans-serif;">VanDyke-DRAC GM.qxd (windows.net)</a><br><br>I think I learned that the DRAC module produces negative clicks to ground for a speedometer and ECM that are providing positive five and ten volts....<br>The Intellitronix speedometer only appears to show two-tenths of one volt on the speedo wire and the instructions suggest that this speedometer wants to read (receive) positive clicks if I'm understanding things correctly....<br>If this is the case, I'm wondering if there are relays that can handle 4000 clicks per mile and produce crisp enough positive clicks to make the intellitronix speedo happy.....<br>They suggest possibly tapping into the VSS, but I don't think it produces clicks......<br>I'm really not looking forward to stringing more wires and using a JTR VSS with cable hookup so I can use the Intellitronix VSS in tandem with the factory one (for the ECM)...<br>It might be cheaper to just buy and install the GPS sender that Intellitronix offers...

[tayto]

The VSSB outputs 4000 pulses per mile. Looks like your speedometer is originally calibrated for 8000 pulses per mile. if you have it wired correctly it sounds like you just need to calibrate the speedometer.

EDIT: i assume you're using the yellow VSS buffer that came in the 91-93 Caprice?

[AzDon]

The VSSB outputs 4000 pulses per mile. Looks like your speedometer is originally calibrated for 8000 pulses per mile. if you have it wired correctly it sounds like you just need to calibrate the speedometer.

EDIT: i assume you're using the yellow VSS buffer that came in the 91-93 Caprice?

Yes, the FSM calls it a VSS Buffer.... not really sure if DRAC module is an appropriate description....it's a white box with a small PC board inside....
FSM shows it producing 4000 click-per-mile to the speedometer and 2000 clicks-per-mile to the ECM....
The Intellitronix panel has a calibration button, but it won't calibrate unless it is already showing some MPH (it isn't)
The article referenced suggests that the speedometer is supposed to provide 10 volts back to the VSSB, which disconnects and connects to a ground creating clicks 4000 times per mile....
I'm seeing only two-tenths of a volt from the speedo wire when it's powered up, so either it's defective or incompatible....
Intellitronix provides a VSS with the kit that has power and ground wires in addition to the signal wire, which suggests that their speedometers expect positive clicks....
I'm not really sure what to think since I'm basically lost beyond simple electrical circuits....
I'm waiting for a reply from intellitronix to help me make some sense of the entire setup...
I am considering just buying their GPS sender and not concerning myself with the VSS hookup...

[1project2many]

DRAC is a specific speed sensor buffer used in pickup trucks. It can be configured with a little effort for a number of different axle / tire / transmission combinations.

[tayto]

so here's the difference. if you are using the speed sensor from the 91-93 b-body (yellow box) it needs a 4000 PPM AC signal. this setup uses speedometer gears still with a speed sensor. it will not work with the truck DRAC (white box). The truck DRAC requires a 40 pulse per revolution AC signal. this setup uses a 40 tooth reluctor wheel like the 4L60E. You can use either system in your 700R4. I like the later myself. Either way you need to use the correct buffer for your speed sensor.

I like the DRAC setup because you can make changes in the DRAC for gear ratio, tire size, etc. With the yellow box VSSB you have to change speedo gears in the tail shaft. The ECM needs VSS input for stuff like lockup (VERY important for the 700R4)

[AzDon]

so here's the difference. if you are using the speed sensor from the 91-93 b-body (yellow box) it needs a 4000 PPM AC signal. this setup uses speedometer gears still with a speed sensor. it will not work with the truck DRAC (white box). The truck DRAC requires a 40 pulse per revolution AC signal. this setup uses a 40 tooth reluctor wheel like the 4L60E. You can use either system in your 700R4. I like the later myself. Either way you need to use the correct buffer for your speed sensor.

I like the DRAC setup because you can make changes in the DRAC for gear ratio, tire size, etc. With the yellow box VSSB you have to change speedo gears in the tail shaft. The ECM needs VSS input for stuff like lockup (VERY important for the 700R4)

It is definitely a white box and it came out of a 91 Olds Custom Cruiser, which is essentially identical to a Caprice wagon...... The FSM calls the box a VSS Buffer and identifies the click outputs as 2000 (for ECM) and 4000 (for speedometer)....
I'm just stymied by the negative clicks versus positive clicks thing.....If the VSS expects power from the speedo and the speedo expects powered clicks from the VSS... and neither is providing the power........Problem?

[tayto]

yes i am aware how the buffer works. NO signal or voltage is sent from the speedometer to buffer. The buffer sends 4000 PPM to the speedometer. The VSS actually generates it's own AC voltage by design and then the buffer "translates" it for the ecm, speedo, cruise. I don't know what you are referring to negative and positive clicks.

EDIT: Are you seeing speed signal in the data stream of the ECM?

[AzDon]

yes i am aware how the buffer works. NO signal or voltage is sent from the speedometer to buffer. The buffer sends 4000 PPM. The VSS actually generates it's own AC voltage by design and then the buffer "translates" it for the ecm, speedo, cruise. I don't know what you are referring to negative and positive clicks.

I'm not equipped to look at the data stream, but I do have the "pull-up power" referred to in the article below on the ECM's VSSB wire (10 volts) so I believe that the VSSB is communicating properly with the ECM (even though I can't prove it)
It's the communications with the speedo I'm concerned with, since the speedo isn't working...

I'm referring to info in this article:
https://atracom.blob.core.windows.ne...2005_04_04.pdf

Specifically:

Once you’re sure the DRAC has the basic signals it needs to do its job —good power, good ground, and a good signal from the VSS/OSS — you’re ready to look at what the DRAC needs to transmit its speed signals to the PCM and instrument cluster.1. 5-volt pull-up sources from the PCM for Output Speed and VSS Output signals (DRAC terminals C11 and C13)2. 10- to 12-volt pull-up source from the instrument cluster(DRAC terminal C15)Output and VSS Signals to PCM and Speedometer You might ask: “What the heck is a5- or 10-volt pull-up source?” To answer this question, let’s briefly go over the function of the Output and VSS circuits, and how the signals are transmitted from the DRAC to the Speedometer and PCM. Figure 3 shows a wiring diagram of a typical DRAC and its related circuits. PCM terminal F12 is connected to 5 volts inside the PCM through a1.2k-ohm (1200 ohm) pull-up resistor. PCM terminal F12 is also connected to the DRAC terminal C13.Figure 6 shows the Output Speed signal at DRAC terminal C13, with the vehicle at about 12 MPH. When the DRAC sends the transmission output speed signal to the PCM, it grounds and ungrounds terminal C13 at regular intervals. The result is a square wave that toggles up and down continuously from 5 volts to 0 volts and back to 5volts. If the DRAC doesn’t have 5 volts supplied to it through a pull-up resistor, the PCM won’t receive the speed signal from the DRAC. The DRAC can’t send out a speed signal without a pull-up voltage supplied by the PCM. Failure to understand this is one of the main reasons for misdiagnosis! The DRAC is grounding terminalC13 to pull the voltage to zero, then ungrounding terminal C13 to let it return to 5 volts. The key word here is ground; if the DRAC itself doesn’t have a good ground at terminal C8, it won’t be able to pull the Vehicle Speed, Output Speed, or Speedometer signal lines all the way to ground. The DRAC doesn’t have another magical ground circuit to toggle the Vehicle Speed,
Output Speed, or Speedometer signal lines with; DRAC terminal C8 is thebe-all, end-all ground. Make sure DRAC terminal C8 has a good connection to engine ground! Quick Test: With the DRAC harness connector unplugged, you should have 5 volts on terminals C11 and C13(figure 7). According to Ohm’s Law, grounding C11 or C13 should develop about 4.2 mA of current (figure 8) in the circuit, because the 5 volts is supplied through a 1.2k resistor inside the PCM. You should have 10 to 12 volts on harness connector terminal C15(Speedometer Signal; figure 9).With the DRAC connector still unplugged:• Probe harness connector terminal C15 with a jumper wire.• Turn the ignition switch on.• Rapidly tap the other end of the jumper wire against a good ground You .should be able to get a 10 to 20MPH reading on the speedometer. This verifies the speedometer can create a reading if the DRAC is doing its job. You can perform the same test on DRAC harness connector terminalsC11 and C13 while watching the Vehicle Speed and Output Speed readings on the scan tool, but this isn’t a conclusive test. Even if everything is okay, you may not get a steady reading for Vehicle Speed. The Output Speed may only read a low value and will come and go. This is because the frequency of the Output Speed signal is pretty fast and difficult to duplicate by hand.

[tayto]

get a scantool and/or a datalogging cable especially if you are planning on keeping the vehicle. reading the data stream is mandatory IMO. I think you need to read the manual for your aftermarket speedometer and figure out how to calibrate it. Have you tried contacting them for tech support?

EDIT: chances are if the torque converter is locking up, the ECM is getting a speed signal. this would be a quick and dirty way to test it.

[tayto]

disregard

[damanx]

@AzDon,

Have you figured this issue out yet?

[AzDon]

I came to the conclusion that nobody else has ever had to deal with this issue, so maybe I'm the guy that doesn't understand..... My mind is stuck in analog, so what I'm going to do is get a GM police car VSS unit from JTR Stealth that has provisions for a speedometer cable where I can add the intellitronix VSS that came with the cluster....Meanwhile, I'm using my eyephone as a speedometer to keep me outta trouble...

[dave w]

http://www.gearhead-efi.com/Fuel-Inj...GM-DRAC-_-VSSB

[damanx]

I came to the conclusion that nobody else has ever had to deal with this issue, so maybe I'm the guy that doesn't understand..... My mind is stuck in analog, so what I'm going to do is get a GM police car VSS unit from JTR Stealth that has provisions for a speedometer cable where I can add the intellitronix VSS that came with the cluster....Meanwhile, I'm using my eyephone as a speedometer to keep me outta trouble...

Ok, let me see if I can help you out here.

1. Are you using the intellitronix speed sender that goes into the transmission?
2. What model/year transmission are you using?

I converted my analog cluster to a digital cluster that required 2000ppm, (not clicks), signal to work. In addition to that, ANY DRAC can be used, but must be modified slightly for gearing and wheel size.

[AzDon]

I had intended to use the GM factory VSS and DRAC module, not realizing that (according to the posted article) the drac converts the signal it receives to negative clicks to ground and depends on the speedometer to provide the electricity......The intellitronix panel expects the vss/drac to provide a positive signal......For about two minutes I considered using a relay to flip the signal to positive before realizing that relays aren't designed to pass thousands of clicks....
It's possible that the Drac simply doesn't work, although the factory speedo in the donor wagon worked fine...
Assuming that it's okay and the posted article explains why my aftermarket won't work, I think it's probably best to leave the factory Vss and DRAC in place and simply add the intellitronix gear-driven VSS in tandem with the stock one using a JTR Stealth sourced unit that was original equipment for Caprice cop cars that needed the cable drive for their timing equipment....
I'm guessing that the reason this issue never comes up is because either 1) I'm crazy or 2) Maybe it's unusual for for folks restoring/building old nova or vans to try to use the GM VSS/DRAC to run an aftermarket speedometer....
Right now, I'm elbows-deep in adding more caster to my axle flip and my cell phone GPS can handle being a speedometer for now.....