L18 cruise control lockout

[Mullet5]

I have 1994 GMC Suburban with a 2001 DBW L18/4L80E. I recently ordered a cruise control wiring kit from Hot Rod Therapy because, well, I wasn't smart enough to do it myself. This kit basically just allows you to pin the original cruise control stalk wiring into the TAC module. The instructions were so bad they were unusable (the instructions weren't kit-specific, they were one-size-fits-none for all of his cruise products. Honestly NO instructions would have been quicker) and the tech support/business owner is a super nice guy, but impossible to get a hold of, so it took several days to get it all sorted out.

Now it is all installed, and I can look at the cruise-related channels in VCM Scanner and see that the on/off switch, the set switch, and the coast switch are all working properly, but the cruise won't set. I assume this has something to do with the parameter "Cruise Lockout" which has a value of "yes". Any ideas how I can kill the cruise lockout?

I have mostly been testing the cruise control on the lift, could it need to see some load to engage? I did try it on the street once, didn't work.

The truck is running and driving just fine, VSS connected, speedo working etc

Things I have done/checked
12v applied to pin 33 of the blue PCM connector
Power to the TAC
Cruise buttons operating properly

Tune file attached

Any help is much appreciated

[Jon S.]

Sounds like a brake input issue.

The PCM receives 12 volts on its brake input circuit, when the brakes are NOT being applied. When the brakes ARE applied, this 12 volt circuit is interrupted (i.e. no voltage). This circuit is PCM C1 (Blue) pin 33.

The TAC Module (which is also the Cruise Control Module in DBW applications) also has a brake input circuit, which works inversely to the PCM's brake circuit. When the brakes are NOT applied, the TAC Module receives no voltage. When the brakes are applied, the TAC Module receives 12 volts on its brake input circuit. This circuit is pin 6, in the 16 pin TAC Module connector.

[Mullet5]

Both of those are hooked up. However, for testing I have pin 33 of the PCM quoted directly to power, rather than the NC switched power from the brake switch. Could this be causing the issue, i.e. the system wants to see that logic "when TAC receives 12v PCM should lose 12v"?

[Jon S.]

Yes - needs both brake connections - needs to see inverse logic occur (brake pedal depressed at least once) - before it will allow the cruise to engage.

Also, with PCM C1 pin 33 connected to constant 12 volt power (not cycling with the brake pedal), the TC will not lock.

[Mullet5]

Beautiful! I will give this a try I'm the next couple hours and report back, thank you for the help!

[Mullet5]

It works! I have working cruise control! Jon S., you, sir, are a gentleman and a scholar, and quite possibly a badass. Thank you!!

[HartRod]

Good info. Isn't cruise also "locked out" until the PCM sees a minimum speed of 25mph?

[5FDP]

In most applications yes, somewhere around that speed.

[HartRod]

In most applications yes, somewhere around that speed.

Is there a way to change that speed?

[Jon S.]

Not with P01 and P59 PCMs - guessing this is controlled by the external TAC Module / Cruise Control Module.

Yes - with E38 and E67 - although there seems to be "hard" limits (at least for the maximum speed setting - never tried lowering the enable speed).

[HartRod]

Not with P01 and P59 PCMs - guessing this is controlled by the external TAC Module / Cruise Control Module.

Yes - with E38 and E67 - although there seems to be "hard" limits (at least for the maximum speed setting - never tried lowering the enable speed).

Thanks. I have a P59 and was just curious. So if I'm scanning, "Cruise Lockout" should go from yes to no upon reaching the prescribed speed within the computer, correct? TIA

[HartRod]

I checked all of my connections but can't get cruise lockout to go to no. Here's a log and my tune. Any suggestions would be helpful. TIA

[Jon S.]

Looks like you are running a 2006 OS. Don't know if this is a complete "take-out" (engine / trans / wiring harness / TAC / APP) or a Frankenstein type collection of parts.

With the 2006 and 2007 P59s, I always make certain that the TAC and APP are from these same model years (2006 or 2007). The 2003 to 2005 P59 TAC and APP are slightly different, comparatively speaking.

Make sure you have both brake signals wired (they use opposite logic), the UART comms, a VSS signal, and a gear position signal.

Check for Cruise Control related DTCs. Confirm that these DTCs have not been disabled in the P59.

[HartRod]

Looks like you are running a 2006 OS. Don't know if this is a complete "take-out" (engine / trans / wiring harness / TAC / APP) or a Frankenstein type collection of parts.

With the 2006 and 2007 P59s, I always make certain that the TAC and APP are from these same model years (2006 or 2007). The 2003 to 2005 P59 TAC and APP are slightly different, comparatively speaking.

Make sure you have both brake signals wired (they use opposite logic), the UART comms, a VSS signal, and a gear position signal.

Check for Cruise Control related DTCs. Confirm that these DTCs have not been disabled in the P59.

Its a late 05, early 06 complete take out, motor/tranny/PCM/DBW pedal/tach module with a speartech standalone harness. No DTCs. All of the channels related to cruise control work (CC power, set, enable, accel, resume). I think I might have figured the problem out. I'm running LED tail lights and think that is messing up the ground signal to the tach module. I'll report back after I troubleshoot some more.

[HartRod]

It was the LED tail lights!!!! Installed regular bulbs and my cruise works perfect!!