1227747 to 16197427 Conversion PCM Swap with Wiring Pinout Directions!

[1project2many]

VSSB is a type of DRAC. Original reprogramming docs didn't even say "VSSB". All my data shows the "white box" versions are plug compatible but if you have something different I've got an open mind. Earlier trucks had DRAC built into the back of the dash and was programmed with a special fuses instead of resistors. Reprogramming meant popping all the fuses then installing a specal programming key which was nothing more than a series of jumpers. I've seen some vehicles where dash programming clip is set for "divide by" ratio of 1 and white box DRAC is installed.

[EagleMark]

Well they are both white boxs and they both have the same plug. But inside DRAC is one set of jumpers, inside VSSB is 2 sets of jumpers. If you plug a DRAC into a VSSB system it melts the new DIP switch. I haven't done much testing after that...

[Six_Shooter]

I think you had other issues, since I haven't heard of anyone else having that issue.

[1project2many]

If you plug a DRAC into a VSSB system it melts the new DIP switch. I haven't done much testing after that...

No?? Can't imagine why not.


I've never had problems swapping them around before. Maybe a blob of solder in the wrong place?

[CDeeZ]

The VSS information from JTR, http://www.jagsthatrun.com/SpeedSensors_Order.html shows early Pickup TBI ('7747 / ' 8747) is 2K PPM.

I've never tried to use the 4K PPM on a '427 PCM. The 4K PPM might work on the '427, but I don't have any personal experience doing so.

dave w

Wow, and here I thought all this time I have a 4K system in my '90 C-1500. BTW, PPM means pulses per mile correct?

Yes, 7747 applications expect 2k ppm. They were developed when the speedometer cable drove the speed sensor. I can cross check a 7427 wiring diagram against the DRAC outputs to determine speed signal frequency on F13. The "output speed not from DRAC" selection might be shown in the manual trans comparison thread currently running?

Does this mean that my C1500 that was originally a 7747 system is 2K PPM, the reason I ask is because on my application it never was a cable driven speedo. I need to boot into the windows side of my HDD when I get back to have another look at all that info as far as the comparo, wondering if maybe I should have that flag checked since I'm manual trans.

FYI DRAC on 1227747 and VSSB on 16197427 and they do not interchange.

Well the 7 jumper and 14 jumper versions are interchangeable, but on my 7747 the original gauges had the DRAC built into the cluster itself, not a stand alone unit so that is obviously not interchangeable with anything, not easily anyways. The white boxes all interchange though. I think the VSSB/DRAC is basically a nomenclature difference, nothing more. Technically the DRAC is the 7 jumper variant and it was produced from '90-'91 as a stand alone white box unit. Earlier applications using "DRACs" may have incorporated the DRAC into the cluster as my original "moonie" gauges did in my truck. The 14 jumper version is the VSSB and it was produced from '92-'95

VSSB is a type of DRAC. Original reprogramming docs didn't even say "VSSB". All my data shows the "white box" versions are plug compatible but if you have something different I've got an open mind. Earlier trucks had DRAC built into the back of the dash and was programmed with a special fuses instead of resistors. Reprogramming meant popping all the fuses then installing a specal programming key which was nothing more than a series of jumpers. I've seen some vehicles where dash programming clip is set for "divide by" ratio of 1 and white box DRAC is installed.

I like how descriptive "white box" is Yes I went through that very thing, my truck had what are referred to as the "moonies" gauges and the DRAC is built into the cluster itself... I just desoldered those "fuses" and recalibrated the DRAC for my 4.88 gears by using some high tech jumpers fashioned out of a butchered paperclip. I recently swapped to the "needles" gauges from like a 94/95 C1500 and those did use the stand alone DRAC/VSSB/white box lol, whatever ya wanna call it. While we are on the topic of this I wanted to mention that during my research for how to wire the DRAC, and new gauges and everything, I stumbled across all kinds of conflicting and/or vague information. What I could gather is that the DRACs/VSSBs are basically all pretty similar, they all perform the same function of converting the incoming VSS signal from AC to calibrated DC signal. Some DRACs have slightly different wire locations: for example, occasionally the cruise signal wire from the DRAC is off on a 2nd plug by itself instead of pinned in with the rest of the wires in the main plug. Some have TWO outputs for VSS to the PCM (VSS and TRANS OUTPUT SPEED) and some others I've seen only have one output to PCM/ECM for VSS.

Well they are both white boxs and they both have the same plug. But inside DRAC is one set of jumpers, inside VSSB is 2 sets of jumpers. If you plug a DRAC into a VSSB system it melts the new DIP switch. I haven't done much testing after that...

Some of them have two plugs. I have used both DRAC and VSSB in my 90 C1500 without issue. Both with DIP switches.

I think you had other issues, since I haven't heard of anyone else having that issue.

Yeah I haven't heard of anyone either.

No?? Can't imagine why not.


I've never had problems swapping them around before. Maybe a blob of solder in the wrong place?

Very possible. Possibly the unit itself just crapped out?






I use VSSB and DRAC interchangeably, the function is basically the same between them. The one problem I can see with the DIP switch install is that with the white case closed, it seems awful close to the rockers on the DIP switch. I don't know if the particular DIPs they had at my local radioshack were bigger than normal or what, but I think I'm going to take one of my extra DRACs and just set it up with jumpers, its not like it's that hard to move the jumpers around if you have too. And if you don't change tires and gears every weekend I don't know if there is really much point to a DIP after having gone to the trouble to install a few DIPs myself.


Alot of the DRACs I've seen have a DK BLUE wire that is used for VSS output. These DRACs also typically have a 2nd brown wire (main brown wire is +12v for DRAC). Here is where it gets interesting: I opted to use the 7 jumper version the "DRAC" that I had laying around. This DRAC didn't have the DK BLUE wire at all, only the 2nd brown wire. I took the connector off and pulled the plastic comb off and ADDED the DK BLUE wire to a DRAC that didn't originally have it. Both (DK BLUE and the 2nd BROWN) appear to be VSS output signal for PCM: VSS and Trans Output Speed... Initially after swapping to the 7427 I kept getting "LOW TRANS OUTPUT SPEED" error. After connecting the BK BLUE and BROWN wire from the DRAC to the PCM at the respective pins, the code never came back.

[1project2many]

If you look around the web, or on this site, you might find a PDF called drac2 with calibration info and pictures and even a pinout. There are scanned documents with fingerprint smudges that show jumper settings for the various "divide by" ratios and images of the two DRAC circuit types. Had I known I'd be uploading those papers to the web in '97, I wouldn't have smudged them back in '93. In '98 or '99 a fellow with some technical writing experience took the original drac.pdf, basically a collection of technician's notes, and re-wrote it as a more presentable paper (drac2). That document also introduced the DIP switch. The included pinout was put together later, maybe in late 99 or 2000 and has a disclaimer as to accuracy. When I researched the pinout I'd often find that a wire or signal had different names in different vehicles but when I'd dig into the service manual the signal itself was consistent. But without a statistically significant sample I wasn't willing to skip the disclaimer. I haven't spoken with too many people who have actually checked these signals. I had two units to test on a bench and both produced signals which matched the pinout. When someone complains to me that they have a unit which doesn't match the pinout it's usually because their service manual calls the signal by another name. It would be awesome if someone either found OE documentation or had enough time and energy to do some component testing so the pinout could be confirmed or removed.

"White box" is a simple identifier but with some dealership background and some years working on these, I know there are speedometer buffers, ratio adapters, signal buffers, digital converters, and plenty of other monikers that have been applied in service literature. Experience has shown that the green box is the green box, and the yellow box is the yellow box, and the orange box is the orange box, and the blue box is the blue box, no matter what make and model it originally came in. Skip all the service manual nomenclature, just go to the junkyard and get the right color box and you're done.

Speedometer cables were standardized at 1000 revolutions per mile. Original speed sensors were two shutter wheels which passed through an IR LED and photo detector with each cable revolution. Much of the programming historically descended from code designed to work with these sensors and you'll find many GM speed signals to be an even multiple of 2000 pulses per mile.

I have a DRAC with a DIP used for confirming correct jumper settings before modifying a customer's part. It's always nice to know you'll get it right before the soldering gun is applied.

Some have TWO outputs for VSS to the PCM (VSS and TRANS OUTPUT SPEED) and some others I've seen only have one output to PCM/ECM for VSS.

Yes, and that's part of the question I asked. There is no way to generate a correct "trans output speed" signal with only a 4k ppm generator. So that signal will not be present in an older manual trans swap, even though the 94-95 S/T/C/K applications with 7427 family pcm and manual trans appear to have used it along with a "vehicle speed signal." Can you think of any specific applications which did not have the "trans output speed" signal but may have used the later 7427 family pcm?

[CDeeZ]

Can you think of any specific applications which did not have the "trans output speed" signal but may have used the later 7427 family pcm?

I can't think of any off the top of my head, but there was bound to be some. I know that the DRAC I ended up using APPEARED to only support VSS, but when I added another wire for TRANS OUTPUT SPEED, it worked just great, so it appears that the capability for the two outputs is always there on the DRACs, whether it is used or not...