Need help with DBW cruise control

[93blackbeast]

Yea, that makes sense. I called my buddy who has a GM dealership and he sent me the wiring schematic for the TAC setup and I see now exactly what you're talking about...all except the lt blue wire being T'd into...hmmm....

So, what i need to do is replace the inputs to the TAC module with the outputs of the 93 cruise switch and that should fix me up. I'm going to have to find a normally open relay though and hook the brake signal up through that since my brake switch gives a constant 12V signal UNTIL I hit the brakes. Just gotta flip flop it.

Well, I'll give this a go now...

Thanks again for the help!

Cody B.

[GasGuzzler]

Let us know.

[93blackbeast]

Ok...I did all that is listed last night and....drumroll....didn't work....I don't know what the deal is...

I did do a lot of cutting on my harness before I put it in my truck to delete unneeded wires (or what I thought was unneeded at the time) and may have some continuity problems. But here's what I did exactly:

I used a single pole, double throw 12v relay.

I hooked the positive 12V of the coil to the existing brake switch from the truck (12v when brake is not pressed, 0v when pressed).

The other side of the coil is to ground.

The top terminal of the normally open relay has 12V on it from the ign key, so its +12V when the key is on.

The middle terminal is to the Lt. Blue wire, pin 6 of my TAC module (So it gets 12V activated when the brake is pressed)

The bottom terminal has my TCC/Brake Signal wire on it (I unhooked it from the existing brown brake signal wire from the 93 and ran it through the relay so I could use both signals) and it gets 12V when the brake is not pressed and 0V when the brake is pressed, releasing the Torque Converter Lockup.

I know the relay is wired right, because I can get the voltage out of each terminal that I need with the brake in the right position.

My TCC works again, so I know that's right, but I can't get anything to work on the cruise.

At first, I vampire clipped my into the wires going into the plug on the TAC module to save from cutting anything and nothing happened. I took the vampires back off to find that they didn't go through the wire coating, therefore, no electrical connection. Then I cut the wires going into the plug and ran my 93 wires (set, resume, and on/off) to the corresponding wires in the plug.

Still nothing worked. And that's where I'm at now. Stuck...I don't know what to check next to make sure things are working...

Any more hlep is appreciated.

Thanks,

Cody B.

[GasGuzzler]

Any codes in the PCM?

All the wires at the TAC now do what they're supposed to under each condition?

They're all installed in the proper place?

The TACM is from the same vehicle as the PCM (I assume so since it runs)?

Do you have a scanner to check inputs?

Have you checked the tune?

Where are you located again?

[93blackbeast]

I'm in Muldrow, OK....its on the far east of the state, midway up. I'm about 10 minutes from the Arkansas line.

I don't have any codes...and I believe everything is hooked up fine...i think

I am a little curious about the connections from my 93 though to the TAC module. I got an explanation of the system from my buddy at the GM dealership and it said the following: The TAC module gets inputs from the switch on the turn signal lever and sends them to the PCM per the UART serial wires in the TAC module. I need to make sure that I have the UART wires hooked up still to the PCM. I'll check that next, the rest all still looks fine...oh well...having fun doing it still...but its getting to a point

Thanks for the help agian man!

Headed off now to the LSX shootout in Memphis!

[GasGuzzler]

He's right except UART is the grandfather for Class2 and hasn't been used for years and years. The TACM and the ECM have a distinct comm circuit not shared by any other module. It's kinda like the older DMax...the FICM on those is a slave module that carries out functions but makes no decisions....it does what the ECM tells is to....just like the TACM on your harness.

Then again, if the comm circuit was gayed out, the throttle wouldn't work and you'd have reduced engine power.

You're only 5 hours away...tow it down.

[93blackbeast]

Lol...I drove another vehicle over to Memphis...so no need to tow

Hopefully, I'll be running my car in it next year...LSX true street

But anywho, I've yet to revisit the truck with what you said earlier...I'm a little confused...are you saying that the PCM doesn't use the UART signal anymore and uses some comm data?

Here's what was said in the manual I have from GM about the cruise setup...I don't know how this is going to copy and paste over...so bear with me...

The powertrain control module (PCM) will engage and adjust vehicle speed based on the information received from
the TAC module via the UART serial data link. For further information on the TAC System, refer to

Throttle Actuator
Control (TAC) System Description in Engine Controls - 4.8L, 5.3L, and 6.0L or to Throttle Actuator Control (TAC)
System Description in Engine Controls - 8.1L.
The cruise control switches are located on the multifunction/turn signal lever.
Ignition positive voltage is supplied from the 10-ampere CRUISE fuse to the cruise control switch via the ignition 3
voltage circuit. When the normally open cruise control On/Off switch is turned ON, the switch closes and the TAC
module detects a high signal voltage on the cruise control On switch signal circuit. When the normally open
set/coast switch is pressed, the switch closes and the TAC module detects a high signal voltage on the cruise
control set/coast switch signal circuit. To engage the cruise control system, turn the On/Off switch ON and
momentarily press the Set/Coast switch. The TAC module sends a UART message to the PCM, in order to confirm
that the cruise control enable criteria has been met. The PCM will engage the cruise control system and record the
selected vehicle speed. The PCM sends a class 2 message to the instrument panel cluster (IPC) in order to
illuminate the cruise control indicator. Pressing the accelerator pedal, while the cruise control system is engaged,
will allow the driver to override the cruise control system in order to accelerate the vehicle beyond the current set
vehicle speed. When the accelerator pedal is released, the vehicle will decelerate and resume the current set
vehicle speed. The driver can also override the current set vehicle speed via the set/coast switch and the
resume/accel switch. When the cruise control system is engaged, pressing and holding the set/coast switch will
allow the vehicle to decelerate from the current set vehicle speed without deactivating the cruise control system.
When the set/coast switch is released, the PCM will record the vehicle speed and maintain the vehicle speed as the
new set vehicle speed. When the cruise control system is engaged, momentarily pressing the set/coast switch will
allow the vehicle to decelerate at 1.6 km/h (1 mph) increments for each time that the set/coast is momentarily
pressed, with a minimum vehicle speed of 37 km/h (23 mph). When the normally open resume/accel switch is
activated, the switch closes and the TAC module detects a high signal voltage on the cruise control resume/accel
switch signal circuit. Activating and holding the resume/accel switch, when the cruise control system is engaged,
will allow the vehicle to accelerate to a greater vehicle speed than the current set vehicle speed. When the
resume/accel switch is released, the PCM will record the vehicle speed and maintain the vehicle speed as the new

set vehicle speed.

So...I've done what I think needs to be done...I just need to triple check everything...it was like midnight when I was soldering the last few things on...hmmm...

Thanks again for looking at this stuff for me!

Cody B.

[GasGuzzler]

Good call I guess. I did not and still have not read the up to date GM Description and Operation because I speak schematic. They used the term UART but I'm not sure why since that, by definition, hasn't been used in years. Fear not though, as we really don't care with they call the protocol as long as our crap works.

The TACM is a slave module. Did I already say that? LOL

Look at this quote you provided:

"he TAC module sends a UART message to the PCM, in order to confirm
that the cruise control enable criteria has been met. The PCM will engage the cruise control system and record the
selected vehicle speed."

That's why I keep harping on the tune.

What's next?

[93blackbeast]

OK everyone..the fight is over....finally figured it out this weekend with the help of a friend. I'll post it up shortly how everything went down. Turns out, the stop lamp switch on the new trucks need to come from an electrical line that has a brake light bulb in it. When I ran just a relayed 12v signal to the TAC module for the stop lamp switch, it was never seeing the resistance of a brake light bulb in the circuit and hence wouldn't work. I found this tidbit out and hooked the stop lamp circuit into the line that turns my brake lights on (directly out of the brake pedal switch) and BAM! Worked like a charm.

I'll do a writeup on this whole venture now that it is done...DBW cruise in a 1993 ftw....also, since we figured out how to do this, we hooked up my buddies 1968 Chevy w/ 5.3L DBW with cruise as well. Wired the set and cruise on to one of his pull knobs in his dash...looks super clean and factory...even for a 68'!

Thanks for all the help guys.

Cody B.

[GasGuzzler]

Aha! Thanks for the update.