upgrade to '7427 from '7060 - ready to pull the trigger

[Daveo91]

Yes, the diagram shows A10 being used while the more generic pcm pinout shows it unpopulated. In some cases when this happens both pins are connected internally so it probably won't hurt to use an ohmmeter to check the connections. And yes, definitely confirm every wire location on the vehicle as you are making changes.

As the model years went by the TOSS was removed and a calculated TOSS value was created using VSS and gear ratios. For those calibrations it was imperative to know if the transfer case was in low range in order to scale TOSS correctly. You should use a calibration that matches the vehicle wiring. I am thinking that it would be best to stay with the '94 wiring.

OK, I've done a bunch more digging today - answered some questions but also created additional.

1. I think you're right on the 7060 A10 pin. Looking at the existing connector for the 7060: A10 is unpopulated but there is a brown wire at B11. So I think this should be mapped to F12 on the 7427. But is this correct since I have a 4wd and the service manual says it's for 2wd?

2. 7060, Pin B5. There is a light blue wire on existing B5 connector that I'm fairly certain goes to the rear sensor on the trans, along with the dark green/yellow wire at B6. (I will need to check with multimeter for continuity on esp. light blue wire - the color is faded and I'm not 100% sure it is the same wire that is on B5. If so does it seem correct that I have this mapped to F13 on the 7427?

3. 7060, Pin B6. There is a dark green/yellow wire at B6 that again I'm pretty sure goes to the rear sensor on the trans. So I agree this needs to be mapped to something on the 7427, but where. I have looked at all the available connections for the 7427 and it's not obvious where it should go? FSM lists B4 for 7427 as "Sensor Ground MAP, EGR, Transmission" Would it go here? (sorry if this is a dumb question)

4. A more general question. How does the 7427 know if the t-case is in 4-low? On the 7060, I presume it knows 4-low if it sees a difference between the output trans sensor and the t-case sensor. But on the later PCM, there was no rear trans sensor so I presume the 4-low input became a binary value, on or off? Where does that signal come from? Will I have to mimic it with my installation?

5. My t-case sensor has a purple/white wire and a green/black wire. I see that the purple/white goes directly to the DRAC, but is that where the green/black wire goes as well?

[Daveo91]

Well I have the answer to at least question 4 now. This thread helped a lot, especially 3rd page. Pin F8 on '7427 is the 4-low signal - if it get's grounded out, then PCM knows it's in 4wd. The reason I missed this when I was doing my mapping is I was looking at FSM and wiring diagrams for a G30, i.e. no 4wd options.

So it seems like I just need to figure out a way to ground out F8 when t-case is in 4-low. Might have to get creative as I don't think my t-case or lever has any electrical trigger for when it's in 4-low. No additional lights come on and PCM knows by looking at differential between rear trans sensor and t-case sensor. maybe I can devise a momentary on switch that goes to ground when the lever is moved to 4-low? Go to junkyard and look for the switch on a '94 or '95 with a t-case lever? I'm not sure I'll have any luck finding a ready-made solution since I'm sure my case of upgrading a squarebody to a 7427 is pretty rare.

Still need to figure out answers to the other questions. I'm pretty sure I'm OK on #1 and #2, number 3 is the one I'm most concerned about. Not sure where to map 7060's B6 on the 7427.

[Daveo91]

I've redone my Excel for the '7060 to '7427 conversion. I added a sheet that starts with pin A1 from the '7060 with description, wire color and mapped pin on the 7427. Should be easier to use during the actual re-pinning procedure as @1Project2Many recommends.

The questions I still have regarding this conversion have been consolidated down to those associated with the 4wd speed sensor differences between the '91-'93 7060 and the '94-'95 '7427.

7060 has five pins associated with speed sensors: B5 and B6 from rear (output) trans sensor, B9 and B10 from front (input) trans sensor, and B11 from DRAC which comes from sensor on output of T-case.

7427 also has five pins for speed sensors, but some differences: F2 and F3 from input trans sensor (map to 7060 B9 and B10), F8 is a 4-low signal from the t-case lever (grounds when in 4-low), F12 is trans output speed that comes from DRAC, and F13 is called vehicle speed signal but also appears to come from DRAC.

- Trans input speed sensor connections are easy. Direct match between old and new PCM. Wire colors are even the same.
- 4-low signal at F8 is also easy. To figure out at least. I have to add a switch at the T-case lever that will ground that signal when lever is moved to 4-low. I have a plan that I think will work.
- It's the connections at F12 and F13 where I get fuzzy. At F12 I currently have it mapped to 7060 B11, but I'm not sure that's correct? I did it that way since they are both brown wires that come from C13 terminal on the DRAC.
- My main question though is F13. on 7427 this is a dark blue wire that also comes from the DRAC (Pin C11), but on the 7060, there's only one wire that comes from DRAC. I could map 7060 B5 (trans output) here, but not sure?

Why does the 7427 need two signals from the DRAC? In reading, it seems like one is 40 pulses per rev, the other is 2000 pulses per mile? Can I get both signals from my '91 DRAC? Or do I even need the second signal? (I did go to pick-n-pull and bought DRAC from a 1/2 ton '94 4x4 Burb. But then I'd have to reprogram that to get my speedo correct?)

One more question: does anyone know where the DRAC is on a '91 squarebody suburban? I'm not seeing it mounted by the PCM and I recently had the gauge cluster out for something else and I don't remember seeing it in there, but I wasn't looking for it. Maybe behind the radio?

[brian617]

Some years of 4l80 4x4 had three speed sensors, input, output, and vss on t-case and some only two, input and vss on t-case. The 4x4 ground input is used on the two speed sensor trucks for couple of reasons one being that it wont set gear ratio error codes for output shaft speed in 4x4 low.

91 suburban has DRAC integrated into cluster if i remember correctly.

[Daveo91]

Some years of 4l80 4x4 had three speed sensors, input, output, and vss on t-case and some only two, input and vss on t-case. The 4x4 ground input is used on the two speed sensor trucks for couple of reasons one being that it wont set gear ratio error codes for output shaft speed in 4x4 low.

91 suburban has DRAC integrated into cluster if i remember correctly.

Thank you - that makes sense that the DRAC is integrated into the cluster. I remember a bumped out section on the back which is probably where it is.

I understand the 3 sensors (7060, 91-93) vs the 2 (7427, 94-95). My question is how to make the 7427 play with the sensors, wiring and DRAC from 91-93 setup. Essentially, I have ONE brown wire coming from DRAC with a speed signal on it, I'm now reasonably confident that goes to F13 on the 7427. (my wiring diagrams in previous posts had it going to F12) But now I need to know if F12 needs to have a signal on it or if I can leave it blank.

Have since read and reread the 7747 to 7427 conversion thread. tons of good info in there. I know that:

F12 is looking for transmission output speed, needing 40 PPR.
F13 is looking for VSS, needing 2000 PPM.

Do I need both or will supplying the VSS that comes from the DRAC to F13 be sufficient for 4L80e? If I do need the 2K PPM on F12, how do I get it? would my '91 vintage DRAC that's built in to the cluster be able to supply that? Or can I take it directly from the rear trans OSS?

Thanks for the help

[Daveo91]

After a lot more research I think I have a plan going forward. I have a VSSB/DRAC from a '94 suburban that I will wire in parallel to the original DRAC. New DRAC will provide both inputs to the PCM (pins F12 and F13) and will probably eventually provide the output for cruise when I get that working. Original DRAC will just provide output for speedo. Using the info and calculator on the DRAC page on TBIchips.com, it looks like I will need to move two jumpers around based on my tire size and gearing.

I will have to splice in a new twisted pair for the two wires from the T-case VSS to feed the new VSSB. Any suggestions where/how I should do the splice? Wiring will be as follows for pins C7-C15 on the VSSB:

C7: VSS Input, lt grn/blk (twisted pair with C12, from t-case VSS)
C8: engine ground, blk/wht
C9: 12v power, hot in RUN, brn
C10: ABS (won't use), wht
C11: VSS Output to F13 on 7427, dk blu
C12: VSS Input, ppl wht (twisted pair with C7, from t-case VSS)
C13: VSS/Trans Output to F12 on 7427, brn
C14: Cruise control (won't use initially), red/wht
C15: Speedo input (won't use), lt blu/wht

I revised the original post by deleting the excel attachment and replacing it with my latest version that has all edits and the added sheet for new VSSB wiring.

[brian617]

I recently did a 0411 standalone swap in a 95 c2500 6.5 diesel with 4l80e. I stole my output shaft speed (VSS) from the DRAC plug by depinning the connectors and soldering my new twisted pair on top of the pin crimps, sort of piggy backing them and re-pinning. I left the DRAC functional for the speedo and ABS.