Dakota Digital in 91 suburban with 4L80E

[jeremy.farlow]

I’ve got a 91 Suburban 2500. 454 TBI engine with 4L80-E transmission.

I’m attempting to put a set of Dakota Digital gauges in. Dakota furnishes their own water temp and oil pressure sending units, so those are good. Tach hookup was easy, but I’m having trouble with the speedometer and fuel gauge. I’m likely going to take Dakota Digitals advice and run my own wires from gauge control back to the fuel sender to fix the fuel level issue. But the speedo is still stymieing me.

Problem is there were a lot of changes/revisions between 87 and 91 and the bulk of the information out there is for the 73-87 trucks. I’ve pulled from AllData the complete wiring schematics for a 91, but I’m realizing that my understanding of how exactly the speed sensing circuits work is causing me problems.

My understanding is the VSS at the transmission tail shaft goes to the DRAC: digital ratio something controller. From the DRAC the signal goes to the gauge cluster and then outputs to the TCM, PCM, ECU. In my case it also outputs to the RWAL (rear wheel antilock) and cruise module.

I have a speedo signal, but the speedo only reads to 20-25 mph. A call to Dakota Digital tells me this MAY be a just a calibration issue... I’ve done the extraordinarily unsafe thing of running the truck, in gear, on jack stands... I know it’s shifting into third, I can’t tell if I’m getting overdrive.

I have all the wiring schematics and can share them by PM, but unfortunately I’ve run out of pictures I can post this month.

Lastly, the entire interior is ripped out at the moment, so I’m not exactly able to drive the truck and do the speedo calibration.

Here’s my questions:

First off, is my understanding: VSS to DRAC to gauge cluster to computers correct?

Does anyone know, will the 4L80E transmission even shift without a speed signal?

I’m not certain whether or not the trans is actually making the final shift to OD. Any idea where to start chasing this issue?

I’ll gladly accept any advice, or even kind words as I work through this part of my project.


Jeremy

[ehjorten]

From what I understand...I have a 1991 V3500 and I successfully modified my DRAC to install dip switches and adjust the speedo for gearing and tire diameter changes...The VSS outputs a 0-10 Vac signal that is proportional to speed and pulses 40 times per revolution.

The Digital Ratio Adapter Controller (DRAC) Processes inputs from the Vehicle Speed Sensor (VSS), and outputs signals to the Speedometer, Electronic Control Module (ECM), Cruise Control Module and the Rear Wheel Anti-lock Brake (RWAL) Module.

The DRAC takes the voltage pulses from the VSS, converts them to clean DC square-wave pulses, and uses them to open and close four solid state output switches to ground at a rate proportional to vehicle speed.

The ECM and speedometer receive 2,000 pulses per mile.  Cruise Control receives 4,000 pulses per mile.  RWAL receives 128,000 pulses per mile.

This site has a lot of great information: http://www.msgpio.com/manuals/mshift/drac.html

[VileZambonie]

Search DRAC on here and there are plenty of threads. http://forum.73-87chevytrucks.com/smforum/index.php?topic=18371.0

So is your setup 2 or 4wd? Which Dakota Digital gauges did you buy?

For vehicles which have a vehicle speed signal from a transmission sensor or ECM, tap
into the VSS wire and connect it to the SPD SND. Consult a vehicle service manual or wiring
diagram to determine wire color and location.
This system can accept 4,000 ppm – 128,000 ppm speed signals with room for
adjustment. The speedometer is fully adjustable and calibration is discussed in a later section.

Calibration is explained in their instruction manual. https://www.dakotadigital.com/pdf/VHX_manual_main.pdf

[jeremy.farlow]

My truck is 2WD. But I do have the rear anti lock and cruise control. The cruise I can live without, frankly the anti lock brakes as well. I just want to hedge my bets and do my best to get the whole truck as together as I’m able before moving it out of the shop.

[VileZambonie]

Why do you need to omit cruise and RWAL? What am I missing?

[jeremy.farlow]

I don’t NEED to omit anything. I’m just saying if I’m not able to figure out a system I’d rather lose cruise or antilock brakes and have the transmission still shift. Right now I’ve got several massive vacuum leaks so it’s likely the cruise won’t work even if it gets a signal.



Jeremy