Vehicle Speed Issue

[ltemup4650]

Hello all. A family member has a 1937 Chevy with a engine and transmission from a 2005 GMC Envoy 5.3 swapped in (E40 ECM). The car is 2 wheel drive. We want the ECM to see the vehicle?s speed. When I open VCM Scanner and drive the car, no vehicle speed is displayed. I?ve also hooked up other scanners and they won?t read speed as well. I know that the previous owner of the car edited the tune file (at least deleted VATS). I downloaded a couple Envoy stock tune files (one 4x4 and one labelled as 2WD) to compare to mine. I can?t find any obvious differences under the ?General? tab of the transmission. I?ve attached the current tune file. Could someone help me? What am I missing? Thanks!

[blindsquirrel]

First, according to the VIN in your file the donor vehicle used to be a 4WD. (1GKET = 4WD, 1GKES = 2WD)

Here are some diagrams for you. Does the VSS work (meaning, does it output a valid waveform if you backprobe the connector and turn the output shaft)? Is it wired correctly, to TCM pins 16/41? Do you have working comms between ECM & TCM?

screenshot.23-04-2021 15.03.50.png screenshot.23-04-2021 15.05.39.png screenshot.23-04-2021 15.03.54.png

The description in the service manual is kinda cloudy on this, but I believe that the VSS shown at ECM C1 21 in images 1 & 2 is an output, used as the primary source for the radio & ECAS but only a backup speed signal to the IPC if class2 data fails - class2 is the primary source for the stock vehicle's speedo. If I'm correct on that I am guessing that you can ignore ECM C1 21 since you have none of those components anymore that used that as an input. It should work if the VSS is generating a valid signal to the TCM and the ECM & TCM have working comms.

[ltemup4650]

blindsquirrel,

Thank you for your reply.
1) Yes, the vehicle speed sensor works. I say that because it has a dakota digital VHX control box that is taking a single wire input (I assume pulse) and displaying the speed on the cluster. I assume this wire is coming from the TCM but I will have to do some wire tracing.
2) The pinout table shows the E40 ECM as taking in a speed signal via both pins C1 21 and C1 49.
3) I assume the TCM and ECM are communicating. I assume this because when I read the vehicle via VCM Editor, it showed both the TCM and ECM. Since the OBE2 is only connected to the E40 ECM, it must see it?

The more I think about it, I don't remember seeing a green wire (pin 16) to the OBD port. Perhaps the ECM is reading vehicle speed, BUT, the OBD can't see it because the class 2 serial data wire isn't there (I think it just has the C1 pins 1 and 2). I'll have to look at this.

First, according to the VIN in your file the donor vehicle used to be a 4WD. (1GKET = 4WD, 1GKES = 2WD)

Here are some diagrams for you. Does the VSS work (meaning, does it output a valid waveform if you backprobe the connector and turn the output shaft)? Is it wired correctly, to TCM pins 16/41? Do you have working comms between ECM & TCM?

screenshot.23-04-2021 15.03.50.png screenshot.23-04-2021 15.05.39.png screenshot.23-04-2021 15.03.54.png

The description in the service manual is kinda cloudy on this, but I believe that the VSS shown at ECM C1 21 in images 1 & 2 is an output, used as the primary source for the radio & ECAS but only a backup speed signal to the IPC if class2 data fails - class2 is the primary source for the stock vehicle's speedo. If I'm correct on that I am guessing that you can ignore ECM C1 21 since you have none of those components anymore that used that as an input. It should work if the VSS is generating a valid signal to the TCM and the ECM & TCM have working comms.

[blindsquirrel]

The speed signal is generated by the sensor in the tailshaft housing, and sends the signal to the TCM. The TCM puts that speed data onto the high-speed GMLAN bus, where the ECM has access to it. The ECM then puts the speed data into a different format and sends it out over the low-speed class2 bus and also via C1 pin 21 for use by the modules that are not GMLAN-equipped, like the radio, ECAS, IPC, ABS, etc.

You are assuming a lot of things that will get you into trouble, diagnostics is best approached by assuming nothing. First thing I would do is make sure the aftermarket speedo isn't using an aftermarket add-on speed sensor on the driveshaft, like a lot of them use. If this were an unmodified 2005 Envoy brought into the dealership I do think it would probably be safe to assume that the ECM and TCM have working communication because of the way the GMLAN wiring is arranged, but in a swap vehicle with a modified wiring harness? No way, Jose.

The E40 and T42 use high-speed GMLAN (CANBUS) for all data to the diagnostic port; the low-speed class2 line is ONLY for the non-GMLAN modules in the original vehicle; which, being a swap with all those modules removed, you do not use and do not need, because there are no non-GMLAN modules present with only a E40 and a T42.

[ltemup4650]

Thanks again. You know your stuff. I'll take what you have said and do some digging in the system. One last question (assumption? haha): The transmission shifts perfectly. Wouldn't the TCM (T42) need a speed signal for proper shifting?

The speed signal is generated by the sensor in the tailshaft housing, and sends the signal to the TCM. The TCM puts that speed data onto the high-speed GMLAN bus, where the ECM has access to it. The ECM then puts the speed data into a different format and sends it out over the low-speed class2 bus and also via C1 pin 21 for use by the modules that are not GMLAN-equipped, like the radio, ECAS, IPC, ABS, etc.

You are assuming a lot of things that will get you into trouble, diagnostics is best approached by assuming nothing. First thing I would do is make sure the aftermarket speedo isn't using an aftermarket add-on speed sensor on the driveshaft, like a lot of them use. If this were an unmodified 2005 Envoy brought into the dealership I do think it would probably be safe to assume that the ECM and TCM have working communication because of the way the GMLAN wiring is arranged, but in a swap vehicle with a modified wiring harness? No way, Jose.

The E40 and T42 use high-speed GMLAN (CANBUS) for all data to the diagnostic port; the low-speed class2 line is ONLY for the non-GMLAN modules in the original vehicle; which, being a swap with all those modules removed, you do not use and do not need, because there are no non-GMLAN modules present with only a E40 and a T42.

[blindsquirrel]

ECM C1 49 is another one of those replicated VSS outputs, this time going to the ABS, which I assume was not used in the swap, which means there is no module for the wire to go to, which means you do not need it.

screenshot.23-04-2021 23.03.28.png

[blindsquirrel]

Being that this is 1) a swap vehicle with who knows what modified and 2) that used to be 4WD and now isn't, I'd step back, trace out every wire, and draw up diagrams of how everything is connected. Knowing which parts of the factory diagrams are still true and which aren't is going to be the biggest source of trouble here.

[blindsquirrel]

Another thing to keep in mind for after the wiring situation is at least advanced to the known-unknowns stage - you might consider having the ECM GM-flashed (not HP Tuners) with a stock calibration from a 2WD Envoy, just because.

[ltemup4650]

Thanks again for your time replying. I'm going to try and do more digging on Thursday.

[ltemup4650]

OK, I've done a lot of digging and have one issue left. But first, to clear some previous questions up:
The VSS is a stock sensor. It has a yellow wire and a purple wire leading to pins 16 and 41 of the T42.
The E40 has a purple wire leaving C1 pin 21 and ending at the Dakota Digital VHX box. This is what is giving the instrument cluster its speed signal. (I've been able to see speed on the instrument cluster this whole time). Thus, the E40 is seeing vehicle speed.

This is how I've wired the cruise control:
E40 C1 pin 17 is cruise on/off
E40 C1 pin 27 is cruise set/coast
E40 C1 pin 33 is cruise resume/accel
E40 C1 pin 32. This is wired for +12v pedal applied, 0v pedal rest
E40 C1 pin 46. This is wired for 0v pedal applied, +12v pedal rest

PROBLEM: For some reason, E40 C1 pin 32 is getting +10v pedal rest, +12v pedal applied
I disconnected E40 C1 pin 32 at the brake switch and metered it. It is showing +10v, thus, the E40 ECM is supplying the +10v.
I don't understand how this can be.

[blindsquirrel]

In the factory diagrams the wire to ECM C1 32 has a splice in it, wire also goes to TCM 42.

screenshot.30-04-2021 01.01.56.png

Isolate both (all?) ends of the circuit - unplug ECM and TCM and brake switch. Get 0v then? OK, reconnect each component one at a time. If not OK with everything unplugged, find what else is tied into that wire somewhere.

[ltemup4650]

I isolated it. It was literally down to a wire only coming out of C1 pin 32 and was still seeing voltage anytime the ignition was on (+10v pedal rest, +12v from pedal switch when pedal depressed). No connected to anything, straight up only connected to the ECM. Weird.
Interesting that your wiring diagram also has connection ID (CONN ID) with C1 pin 56 BU listed. I wonder what that is for?
One of the pin-out tables I have labels this wire as "CHMSL Supply Voltage" (Center High Mount Stop Light). This would lead one to believe that this is a +12v output.
But, the wiring diagram I have from eautorepair for the 2005 Envoy 5.3 clearly labels it as an input (attached).
Maybe the cruise is turned off in the tune somehow and this is the way it inhibits itself. I don't know.
My wrenching skills are solid, but my tuning skills are lacking and end pretty much with knowing how to turn VATS and DTCs off.
You will see in my tune I posted with the original post that most of the DTCs are set to no error reported.


C132.jpg

[blindsquirrel]

That's the connector ID at the ECM - C1, C2, C3, and the color, and number of positions total. What gets confusing is in some years they list the total number of cavities, other years they count only the ones that are populated.

[blindsquirrel]

Did you verify that the wire running to the brake switch has 0v when disconnected from the ECM? You might have to de-pin that one wire from the connector as unplugging C1 will shut off a bunch of other stuff.

[blindsquirrel]

Wait a minute... so the brake lights are on all the time? At 10 volts or whatever?

[blindsquirrel]

I think I figured out what you're seeing. There may not be a pulldown resistor inside the ECM on that pin, since the factory wiring would essentially be using the brake light bulbs as the pulldown. Are you by chance not using the wire going to C1 32 to power the vehicle's brake lights, and only using the switch as a signal for the cruise??

[ltemup4650]

My friend, I think you're right. And we came to that conclusion at roughly the same time. Two parts to this post. One, I shot a video of my issue:
https://youtu.be/XKEhSfUCMlY

Two, while doing research (including using aftermarket cruise control), I found the Dakota Digital CRC-1000 does exactly as you describe. With a relay, it grounds the wire going to its control box when LEDs are used. Thus, it grounds the wire at pedal rest, and releases the ground when the pedal is pressed. I'll wire it slightly different for the E40:
Relay pin 30 TO C1 pin 32
Relay pin 85 TO ground
Relay pin 86 TO cold side of brake switch (0v pedal rest; +12v when pedal pressed)
Relay pin 87 TO cold side of brake switch (0v pedal rest; +12v when pedal pressed)
Relay pin 87A TO ground
^this will ensure +12v is fed to the ECM C1 pin 32 when the pedal is pressed

See attached picture.
CRC-1000.jpg

[ltemup4650]

I got it working!

blindsquirrel, thank you so much for your time and responses!

[blindsquirrel]

Or you could have just used a resistor between C1 32 and ground, something like 10K to start with. C1 32 isn't really a power source that can supply any real current to anything, it's just floating, and the resistor would never be passing any real measurable current.

[blindsquirrel]

What's the status of the ECM VSS issue? Was that only a concern as a possible cause of cruise control ops?