Troubleshooting the cruise control on my 95, LT1, A4 the FSM in one section shows a two wire harness going to the vac release switch at the brake pedal. I find no connector and there is no reference to it in the trouble shooting section. So the question is - Is there supposed to be an electrical connection at the vac release switch??? Thanks

 

I just spent a few weeks on my own cruise and the same question was just posed last week.

The answer is no. There is no electrical connector on the switch body that has the vacuum line to it. The other switch body has the brake light, lock up and cruise disengage switch.

My problem turned out to be varying resistance across the disengage switch causing strange symptoms. I ended up cleaning the switch. You can check my thread to see if that is any assistance to you.

http://forums.corvetteforum.com/c4-t...-for-94-a.html

 

pcolt94,
You should submit your findings/post it in the "new" tech tips sticky. Of course leave out the part of how much fun it is to actually get to the switches. By the way, the pics you did of the switchs are awesome.

 

Paul, thanks for the response annd the info. We have followed the diagnostic trail in the FSM down to the last detail with no luck, just frustration. However, you have shed new light on the problem. Tomorrow evening we will replace the brake light switch and see if that does it. I don't like to throw parts at it, but my dealer has a new switch for $27, so after everything else if I waste $27 I still won't feel bad. I have a 1600 mile road trip coming up soon and sure would like to have a working cruise control. I will keep you updated, thanks again.

 

I have done extensive trouble shooting on the cruise recently and have a fresh mind for it. Perhaps I can assist and tell you what points to measure.

Unfortunately with out getting the cruise module out and hanging there are only a few points that can be checked. It is a real mess to get out, more work than I thought. But I did start at the servo unit and worked my way back to the module. I first wanted to know if the relays in the servo were getting any voltage. If I knew your exact symptoms, I might be able to give you some direction

 

Paul, we did pull the cruise module out, did all of the checks on it, then replaced it with another one and rechecked. Everything good. Last night my friend hooked up his scanner/reader and again all checks good.
We checked it while driving and it showed the cruise engaged when I turned it on, the scanner even confirmed when I hit set and resume buttons, but no cruise. We checked the disconnect relay and replaced it with another relay, no difference. I replaced the servo about a year or so ago with a used one, did not help. We have a few checks to do on the servo yet, and then the brake switch idea. I should add vacuum has been checked and verified, as has the vac switch on the pedal.
About 3 yrs ago it would work sometimes, sometimes not. Then it just quit altogether, which sounds like what you went thru. I just hadn't really tried to fix it until now. Thanks Again.

 

I going on the assumption the cruise module is OK as well as the basic vacuum.

You need to ascertain first if the servo relays are getting any voltage. This needs to be checked while driving the car. You will need 2 DVMs and two 5 foot lengths of wire.

You need to make 2 connections with 2 pins on the servo connector. Take nothing apart. Shove in a straight pin, hard piece of wire or anything that will make contact thru the back side of the connector. You will be connecting one wire each to pin E (vacuum valve), and the other to pin A (vent valve) (opposite ends of connector). Run the two wires inside the car and connect one wire the each + input of each meter. Ground the negatives to metal inside the car. Keep track of which meter goes to what pin.

(IGNITION KEY OFF) To verify you actually have contact with the pin, turn each meter to read resistance. This will measure the resistance thru each relay and should measure about 45 ohms. Once verified, change back to voltage.

Normal Circuit operation, Driving car.
When you engage the cruise (set switch), voltage is applied to pin E (vacuum valve) for about 1 second then released. This brings vacuum into the system. SIMULTANEOUSLY, voltage is supplied to pin A (vent valve) and closes the system to hold the vacuum. Pin A will stay up at 12 volts and pin E will be 0 volts.

This is what basically happens the first few seconds. The cruise module momentarily opens and closes the relays to adjust the speed, but this is the basic steady state configuration. If car is to fast, it will open (pulse) the vent valve. If to slow it will open (pulse) the vacuum valve adding vacuum. This servo process happens very quickly while maintaining speed. Hence it adjusts the vacuum motor.

If you never get a voltage at pin E, the cruise will never engage. In that case the module is being prevented from turning on or possible engaging. At that point you have to make measurements on the cruise module circuit card. If you need to go there I have a lot more information and can assist you with that. If you look at my posts pictures you will see the set up I had to use.

The vacuum hose to the brake pedal can be pinched off if in question. It is a safety release (back up) in case the electronics don’t work to disengage. Not needed for cruise operation.

The troubleshooting is easy, getting to the places to check is the hard part.

Additional thought as to what I said.

If the scanner shows it on, that would mean that it probably has main power to pin A. Pin L is OK as the set switch has to work to show (scanner) on. Does your scanner show vehicle speed? The brake switch circuit , cruise control relay and 12 volts thru them to pin G are probably OK since it won’t go on without that input.

Assuming these assumptions are right, it then sounds like voltage is not getting to the servo relays or servo is defective. In any event it sounds like the cruise module is coming on (at least on the surface).

 

I assumed that you have check and adjusted the vacuum dump switch at the pedal and line? Could be a vacuum leak or switch is on when pedal is up. I would pull a vacuum from the servo to the pedal and see if it holds.

 

Paul, thanks for the proceduure on checking the servo, we will be doing that this evening. Getting to things is tough like you said. How GM designed access to the pass side under dash is amazingly stupid.

Dave, good to hear from you again, hope all is well. Yes, I have checked the vac side of the system thoroughly. I have good vac to the servo and have not only checked the release valve, but also blocked off the line to ensure it is not losing vac. I just wish it was the problem because it is such an easy fix.

 

While the cruise control topic is at the top priority, mine isn't working either and I believe it's vacuum related because I have a vacuum hose that comes from the servo up by where the headlight module is, but cannot figure out where it goes for the life of me. Any ideas?

 

The lower port on the servo is the vac supply line, it comes from the vac ball (supply storage tank) up by the HL module. It in turn gets vac from the fitting on the pass side under the fuel rail cover. The top port on the servo goes to the vac switch on the brake pedal (A4).

 

This is some great information. I also looked at the other thread that was referenced.

pcolt94 did a nice job looking into the trouble shooting.

My 94 started acting up about 2 weeks ago. Usually works, but will start losing a couple mph up hills. In the afternoon after work it is worse. It sometimes will not engage or resume. A couple of times I had it engaged and when it slowed a couple mph I felt it completely drop and could not reengage it.

I am going to start troubleshooting it this weekend and these threads give me some good places to start.

Thanks

 

YIPPEE!! Paul, the brake switch was the problem. We put the new switch in and it works like a charm. I really appreciate your help. If you are ever up my way dinner and drinks are one me. Thanks Again.
Andy

 

aminnich:
I don’t know what path you took or even if you got to starting my procedure, but you fixed the problem. So pat yourself on the back.

In my case my cruise only acted up in the heat of summer when the inside of the car was 100* and this went on for years, OK in winter. So I would have bet it was a electronic component in my case. The problem did act up enough toward the end of summer (not as hot) for me to find the problem. I would never thought it would turn out to be the switch especially since it checked good initially.

The common thread I see is that many of us are winding up at the brake switch, regardless of how it checks. So before I send anybody on a wild goose chase or detailed procedure, I am going to suggest the brake switch. To at least remove and replace or disassemble and clean.

QCVette – I had some of your symptoms also. It would drop out, loose speed and then race back to the speed I had set. Sometimes just not engage. So my advice is to play the law of averages and other peoples experience and look at the brake switch first.

 

Sounds like a vacuum loss problem check the servo hoses for cracks or rotting.

 

[QUOTE=pcolt94;1571609546]aminnich:
I don’t know what path you took or even if you got to starting my procedure, but you fixed the problem. So pat yourself on the back.
QUOTE]

Your comments about the brake switch is what led me directly to the problem (after about 6 hrs of following the FSM procedures). I am sure I would have had a lot more time in diagnostics without your help. So thanks again.

ps--I agree about checking that switch first. It sure beats getting to the dam module.