6.2D to 350 TBI conv. VSS/DRAC needed?

[MartinAro]

Hi,I'm at the final stages of converting my '90 K2500 Silverado from a 6.2 diesel to a 350 TBI from a '94 burb. One thing I didn't realize before sending the donor to the scrapyard is the VSSB/DRAC module that's supposed to be in there some where.
Is that really a signal that's needed by the PCM (which is a 7427 btw)? The tranny is a 700R4, and the lockup is controlled by vacuum (kit by TCI). During the first startups I experienced some rough idling, just as if a cylinder wasn't firing. And on the test run it hit the rev limiter at 4000RPM, would at least expect it to rev closer to 5. Could this be the PCM stopping it prematurely because it thinks the car is stationary?

If a VSSB/DRAC is needed, what parts am I looking for? Where on the car is it located? Is it just a case of inputing the signals from the VSS from the back of the transfer, and then it splits the signal into one going to the PCM and two to the speedo?
The fuel vapor thingy is also missing (the black box connected to the intake with a line going back to the fuel tank). Will that have any side effects?

Kind regards,
Martin

[Six_Shooter]

For best results with any Delco EFI system, a VSS is needed, which means a VSSB (DRAC) may or may not be needed depending on the actual VSS output you are using.

Have you tuned out all of the transmission control stuff of the PCM? If not that's the bigger issue. Without the PCM seeing the transmission inputs (and outputs) the PCM will be in a limp mode.

The PCM can control your lock up, and would recommend doing it that way.

[MartinAro]

I don't know what kind of VSS I have. I have the instrument cluster that I think is known as an "all electric"? I've attached some diagrams of the original wiring. The OE part number for the sensor is 15547452.
Looking at the one for the instrument cluster, I do see some speed signal outputs from the cluster at 437A and B. Can these be plugged straight to the ECM, or do they still need to be converted in some way?

No, the transmission controls are not programmed out of it. I actually didn't think it was that smart to be honest, nor have I got any clues on how to do so.

[ony]

I think on that model the vss is built in the speedo cluster.

[MartinAro]

That would be great if that's the case. Then I'm sure that wire to the PCM is part of the old harness behind the glove box. That I'll figure out.

Then the next issue is programming out the transmission control. Guess I'll need to get my hands on an old Windows laptop and an ALDL cable, and get my hands dirty. Unless there's an easier/cheaper option.

[Six_Shooter]

ALDL cable isn't exactly what is need to do that, you need a programmer and an EEPROM (or two) along with a MEMCAL adapter (or modify your MEMCAL).

The trans controls need to be PROGRAMMED out via a modified BIN programmed to the EEPROM.

There are details in threads on this site how to do it.

You don't need an old laptop, just one that runs windows. Tuner Pro RT will run on everything up to Win10 I believe (I only have 7 on my systems, and only tested 10 briefly when it was in public beta). I know I've read discussion of using Win10 with Tuner Pro RT.

Using an ALDL cable will help to see what the PCM is doing and seeing, but won't on it's own help you get rid of the trans controls.

Look at moates.net for the hardware that you need.

[MartinAro]

Ok, thought it would be possible to change the parameters on the chip that is currently in place. If I've done my tiny bit of research right I would need to remove the current chip (MEMCAL), mount the G1 adapter to this, burn/flash a new program on to the blank EPROM which is included in the adapter kit, then install it all. Found the support article at Moates.net while writing this post. Very informative!

Then I'd need a tool like BURN2 to write the new program to the EPROM. Is this the part where I either read the old program from the MEMCAL and change the parameters, or choose to use a community made BIN with the changes I need already in place?

I mentioned "old Windows laptop" as both cumputer I have available are running MacOS. Getting an old used one is surly better than either trying to emulate TunerPro in WINE or purchase and dual-boot the MacBook.

[brian617]

The 88-89-90 has the drac in the cluster. The two wire twisted pair goes from speed sensor to the cluster, then the cluster outputs a single wire to the ECM. In your situation you want to get the twisted pair from the drac connector to the cluster. Then put the single wire from cluster to pin F13 at the PCM. The missing transmission inputs wont cause "limp mode" Drove a 94 model auto truck with a manual trans conversion for years with no limp, or SES light long before I got into tuning.

[MartinAro]

Oh really, Brian?! Gonna have to give that a go this weekend. Were about to give up on this for now, you know with the winter coming closer every day. Getting cold working in a garage with no door.

Will also change the IAC and temp sensor, then we'll see how it performs.


Thanks for all the help, guys!

[Six_Shooter]

The VSS behind the cluster is not a DRAC, it's simply an optical VSS, that outputs a 2000 PPM signal. A DRAC (and VSSB) converts the magnetic (Variable Reluctor) signal of the output speed shaft sensor (VSS) to a digital signal of varying pulses to other devices, including the ECM, ABS, Speedo and cruise. It could be possible to use the VSS output, but may require changing some parameters in the BIN for it to recognize the signal. The TBI PCM should be able to use this signal directly but the indicated speed will likely be incorrect, by half usually since the PCM of the TBI trucks usually uses a 4000 PPM input for speed.

In my experience without the trans connections made there is a limp mode initiated, not full blown ruing off the RFD chip limp mode, but the way the engine runs is much reduced over what it could be.

[Fast355]

The VSS behind the cluster is not a DRAC, it's simply an optical VSS, that outputs a 2000 PPM signal. A DRAC (and VSSB) converts the magnetic (Variable Reluctor) signal of the output speed shaft sensor (VSS) to a digital signal of varying pulses to other devices, including the ECM, ABS, Speedo and cruise. It could be possible to use the VSS output, but may require changing some parameters in the BIN for it to recognize the signal. The TBI PCM should be able to use this signal directly but the indicated speed will likely be incorrect, by half usually since the PCM of the TBI trucks usually uses a 4000 PPM input for speed.In my experience without the trans connections made there is a limp mode initiated, not full blown ruing off the RFD chip limp mode, but the way the engine runs is much reduced over what it could be.

Give the PCM a Park/Neutral switch signal and a 2000 ppm signal. The engine code uses a 2000 ppm speed signal in a TBI PCM and it can come directly from the optical VSS. My old 83 G20 ran GREAT on a TBI PCM even before it was tuned to eliminate the 4L60E codes.

[brian617]

Oh really, Brian?! Gonna have to give that a go this weekend. Were about to give up on this for now, you know with the winter coming closer every day. Getting cold working in a garage with no door.Will also change the IAC and temp sensor, then we'll see how it performs.Thanks for all the help, guys!

After thinking about your set up more it occurred to me that most of the wires you will need to re-pin are going to be located in the harness connector just behind the glove box. You also need to make sure the communication wire is also correctly pinned. The communication wire will also have to be re-pinned at the DLC and the truck scanned as a 94-95 auto.

[lionelhutz]

Is the cluster 2000ppm VSS output available on diesel trucks? I'm thinking the cluster might have the ability but no wiring will be present since the signal wasn't used by the diesel.

[MartinAro]

The VSS behind the cluster is not a DRAC, it's simply an optical VSS, that outputs a 2000 PPM signal. A DRAC (and VSSB) converts the magnetic (Variable Reluctor) signal of the output speed shaft sensor (VSS) to a digital signal of varying pulses to other devices, including the ECM, ABS, Speedo and cruise. It could be possible to use the VSS output, but may require changing some parameters in the BIN for it to recognize the signal. The TBI PCM should be able to use this signal directly but the indicated speed will likely be incorrect, by half usually since the PCM of the TBI trucks usually uses a 4000 PPM input for speed.

In my experience without the trans connections made there is a limp mode initiated, not full blown ruing off the RFD chip limp mode, but the way the engine runs is much reduced over what it could be.

Even if the signal is wrong, it would at least give the PCM the impression that the vehicle is moving, if that's something that would put some restraints on it. I'll give that a try before purchasing a lot of hardware and equipment I might not need.

Unless of course the VSS output is AC, and not DC. Which I can hardly believe, as all the ECMs/PCMs require a DC square wave input signal, as far as I've read. And my VSS is already producing a DC signal, although half the pulses the PCM "needs".

After thinking about your set up more it occurred to me that most of the wires you will need to re-pin are going to be located in the harness connector just behind the glove box. You also need to make sure the communication wire is also correctly pinned. The communication wire will also have to be re-pinned at the DLC and the truck scanned as a 94-95 auto.

Yep, that is true. If only the 94 wiring schematic had been as good as the 90 ones most of these issues could have been solved already. At the time of the swap I found no info about those connectors when looking at the schematics, but as I get closer and closer to completion I find more and more wires that need to be connected. Just recently I figured out that the LO signal to the wipers go through that connector, while the HI goes through the firewall at the conv. center.

Is the cluster 2000ppm VSS output available on diesel trucks? I'm thinking the cluster might have the ability but no wiring will be present since the signal wasn't used by the diesel.

According to these two diagrams, yes. 437 A and B is VSS output. Knowing the VSS is also a vital part of the cruise control, which is present in the truck.

[lionelhutz]

Are you referring to the VSS in the bottom left of the diagram? That will be an AC sensor feeding the cluster. The diagrams are way too fuzzy to attempt finding the numbers you are referring to.