Rmoore45

I just got a 93 vette and am in the process of fixing all the little things that do not work. The cruise control will not engage. I have checked all the basics and have narrowed it down to the vacuum servo is not getting power. I put my multimeter on contacts B&D and did not get 8 volts. There is power at the fuses and the brake light switch is working properly. It seems to me that it must be the cruise control module but before I go digging into the dash I would like to see if any members have other idea as as to why the servo is not getting power.

pcolt94

Your theory of operation is not correct at all. There are 2 dc vacuum controlled solenoids that open and close on demand for vacuum driven by cruise circuit board. They only energie when car is in motion and back probing has to be done when driving. Your B and D terminals are not voltage input points. They are a servo feedback AC voltage to the cruise card. Never check B or D with an ohm meter for resistance.

The cruise circuit card is usually never the problem and a bear to get to. The resistance of the solenoids in the servo can be checked for resistance. Not many servos go bad.

Most problems are vacuum lines, 3 way check valve on right side of engine under the plastic cover, switch on brake or clutch pedal and on-off switch.

After that it's gets more complicated. There is a cruise relay and the EBTCM brake computer is part of the on circuit. Documentation at this point is a must.

You can also do some checking of the servo diaphragm with a hand vacuum pump and external 12 volt source to check the solenoids and operation. Have your ducks in a row and documentation if you do this.

Rmoore45

Thanks for the reply. Let me run through what I have done to eliminate those possible solutions.

-Checked all vacuum lines for leaks, put vacuum pump on lines and it held 18" for quite a while before slowly leaking down. when the engine is running it will maintain constant vacuum.

-checked vacuum check valve on engine, put vacuum pump on it and it worked properly.

- Cruise servo is free and not sticking, smooth operation of the throttle cables pulling on the throttle linkage.

- checked all fuses related to cruise control, put multimeter on on both sides of the fuse to make sure it's getting power. I also checked all the fuses in the electrical system and they are good.

-checked switch at brake pedal, it has 12v going to it and when the brake pedal is pressed the switch completes the circuit. It also dumps the vacuum held in the cruise servo. I checked that function by drawing vacuum on the hose going to the valve and then pressing the brake pedal. The vacuum was released.

-checked wires going to switch on steering column. I disconnected the connector at the steering column and used my multimeter to check for any power or grounds going to the switch. I did not get any power or ground signals at that connector. This is a problem as well.

-tested servo by removing connector and using a jumper wire from the battery to ensure that the solenoids were working. Terminals A&E engaged each solenoid with terminal C being the common ground.

- Speedometer is working properly so it's not the VSS.

I have a GM service manual for this car so I'm going through it to diagnose problems. I also found this article http://www.superchevy.com/how-to/vemp-1007-c4-cruise-control-repair/
Which has been helpful.

The cruise control servo should get power when the key is on but engine is not running. The manual says to check points B&D for about 8 volts with the key on and cruise control set to the on position. I have also read that the solenoids should click when the key is turned on but they do not. I'm beginning to think it's going to be the cruise control module in the dash, it might not be bad per se but it might have a bad connection or ground issue.

Thanks, Ranger

pcolt94

You have come a long way down the road already.

Let me first say you are right on the 8 volts as I check out your information. I have never done that test so I can't confirm results, but it is the FSM. So it looks at this time you have some sort of electrical problem rather than vacuum.

I know the FSM casually says disconnect the connector from the cruise control module. It's no walk in the park to get out and lives on passenger side of car. Do all troubleshooting you can before you pull that module out.

It sounds like may have a positive direction to go in as you said you did not get 12 volts on the connector for the on-off switch for the cruise. Wire 1239 needs to have 12 volts on it or nothing will ever work and the module will not get power.

Go back and recheck that for 12 volts with key ON. If no 12 volts is confirmed, chase problem backwards to fuse box.

Rmoore45

Well I have found an issue to try and fix, the cruise switch on the steering column is not getting power. I checked this multiple times with the key on.

I also checked the cruise control switch on the column and it works fine.

Furthermore I ran a jumper wire from the battery to the purple wire on the switch (the on that is supposed to have power with the key on) and got nothing. This tells me that not only is there a bad connection from the fuse box to the switch there is also no power going to the cruise control module. If there power did go to the module then the cruise control should have worked. That wire is the only wire that provides power. That means that my problem is somewhere near the steering column wiring.

Time to look around the steering column to hopefully five the problem.

Rmoore45

Problem solved!!!

I pulled the cover off underneath the column and saw there was a plug that was not connected to anything. After a quick continuity test I discovered that the connector was for the cruise control. I plugged it in and all is well and good in the world. It holds steady at 70 mph.
Merry Christmas

Thanks, Ranger

pcolt94

Excellent, nice work. You did good finding the problem systematically.
Have a good holiday in cruise.

mandrake48

Hi Guys, anyone out there able to help with my 86 Vette cruise issues, I have read with interest other posts hoping to glean a fix for my cruise which seems slightly different in that after messing with the cruise set switch i can get it into cruise, when I do this it increases in speed 5 to 8 Kph then settles back down to the speed i set it at. This is fine for a time, then without warning the car suddenly speeds up 15 to 20 Kph and I repeat the process only to have it go ballistic on me some time later, I have replaced the vac hoses that I can see are perished but the problem persists. Prior to this happening in the last few weeks, it was working perfectly, can any one point me in the right direction.

Cheers

Chevy1993

My cruise control on a 1986 Vette works perfectly, running for miles, and then suddenly cuts out. This happens on smooth roads, rough roads, under any conditions, any situations. Checked all connections, vac hoses, etc. Could this just be a aging switch? (55000 miles) It is 35 years old.

65Z01

Can you access the sw innards to see if maybe it could use a little cleaning?

Chevy1993

Probably would have to pull the steering wheel for that, undo the switch stalk, and check it out and clean it then. Just have to find the time. Fixed my sticking ignition key by spraying a liberal amount of silicone spray on the key and sliding it in and out numerous times. No more jamming in the switch. May have a similar problem with the cruise control, such as a dirty contact within.

jv9999

You can't clean the switch, I tried. You can't get it apart without destroying it. You can use a Camaro switch which is about $20, but it's 3/4" shorter and in white only.

When I got my '90, someone had cut off the cruise module connector (the one under the dash) and attached an incorrect one with crimp connectors. The actual module was gone. I would like to talk to that guy. :-) All fixed now.