Questions on checking VSS for cruise control and TCC Lock

[Rich Gio]

I've had an AVC GNX dash installed in my '87 GN for about 20 years now. After I first installed the kit my cruise and TCC lock up have not worked. (Just got around to troubleshooting this! ) I do not get any trouble codes for the VSS on my scanmaster and the speed readout to the ECM/powerlogger/Scanmaster still works dead on (3mph off of the GPS). My questions... If the Speed signal works, isn't that the same signal sent to the ecm for TCC lock up? How about the cruise control? Is there any way to test that the VSS is sending signal to Cruise control?

 

[RmvBfrFlght]

The ECM uses the same VSS/MPH signal for the TCC as for everything else. A few items to check for the TCC, there is a switch at the brake pedal that provides +12 volts to the TCC solenoid. Foot on the brake, no 12 volts, foot off the brakes then the solenoid gets 12 volts.

Many times the switch is out of adjustment, it may also be bad. Note that this is the switch w/o the vacuum line and also controls the brake lights. There are two sets of contacts in the one switch body.

The ECM provides a ground to the TCC solenoid to lock it up. The ECM may be bad as the driver for this may have fried. Note that the driver chips also drives another solenoids, such as EGR & CCP, so if the driver is fried be sure to check those before replacing the ECM.

The ECM also uses the TPS and CTS inputs for TCC lockup, so check that the ECM is seeing the corrrect values for those too.

As for the cruise, it requires the same 2K PPM from the VSS as the ECM. If this signal is missing can tap into the VSS signal to the ECM for it. Bascially share the one signal between the two devices.

RemoveBeforeFlight

 

[Rich Gio]

Can I tell by looking at the chip that it is "Fried"? Where on the ECM is this driver chip? Also, what is the CCP? I can see on the powerlogger that the ECM is commanding the TCC lock, but I can't feel it actually lock. The switches on pedal checked out ok. Jumper to ground causes TCC solenoid to energize... I can hear it click... but jumping it out while moving results nothing.

I went thru the GM trouble shooting flow chart for Cruise control and I DID get pulses as I turned the rear wheel by hand, BTW.

 

[RmvBfrFlght]

There are three driver chips in the ECM. They are located on the lower board, the three chips closest to the ECM connectors are them. Most of the time they go bad there is major physical damage. As long as it doesn't burn the PCB they can usually be replaced.

When the TCC is jumped to ground, such as hearing the solenoid click, and it doesn't lock up the TCC when moving, is not a good sign. Best to do this in third gear and watch for the RPM to drop when it locks.

If nothing, it may be an issue in the transmission or that the TCC is shot.

CCP is charcoal canister purge. Which has a solenoid on top of the can.

Cruise control, if the control box is getting VSS pulses, odds are that the servo has gone south.

RemoveBeforeFlight

 

[Rich Gio]

"Cruise control, if the control box is getting VSS pulses, odds are that the servo has gone south."

Servo checked out good. I was leaning towards the module , but I don't know how to check it. Don't wanna pay for a new module if that's not the issue. Cruise control light comes on when activated , but floors the pedal to ludicrous speed and tries to kill me! I gotta turn it off with the switch or hit the brakes to disengage.

I'll check the driver chips ASAP.

 

[GNVenom]

http://www.installationinstructions.com/FYI/vss-tech.pdf
for some reading material

 

[Rich Gio]

John,
Thanks for the interesting reading material. So, if I understand this correctly, my GN since it is equipped with cruise control, has 2 channels on the VSS. Is it possible for 1 channel to have failed and the other channel still function since my speed input still works?

 

[GNVenom]

Possible but very unlikely. If the cruise doesn't work, it's usually a broken wire on the switch stalk harness. Check to see if the cruise light comes on when you turn the switch to ON.

 

[Rich Gio]

Cruise control light comes on when activated , but floors the pedal to ludicrous speed and tries to kill me! I gotta turn it off with the switch or hit the brakes to disengage.

 

[1KWIKSIX]

I've also got the AVC Dash set up installed in my car. Although I've changed the dash panel / bezel / applique over to a GNX reproduction style...have kept the AVC's VDO gauges and it's plug n play wiring harness that originally came with this set up.

Can't comment on the cruise control not working, as my car did not come equipped with this option....however my TCC works fine and haven't experienced any issues with it.

Guess I am just lucky that everything seemed to work fine from the get go!

You may want to double check the installation instructions that came with the AVC dash set up. Maybe you'll be able to spot something simple?

Here's a copy for you in case you don't have the original one handy.

Good Luck

dave

 

[Rich Gio]

Thanks Dave!