I have had this problem for several years but have never really gone after it and does not happen all the time. When leaving work and the car is sitting in the sun all day, and its in the 90-100° range, the cruise will just not engage. It might starting working (or not) on the way home or engage at a wrong speed. This problem usually does not happen when its cooler or in the winter.

My first suspects are the cruise control module (berried in car), or the servo unit under the hood. All my hoses appear to be good and 3 way check valve is OK. Definitely seems to be a (electrical) heat related problem.

Since then I have connections for the 2 relays in the servo and brought them into the car so I can monitor the voltages during activation. When working I can see the 12 volts being applied when the cruse works properly. When the problem occurs the applied voltage is only 4-6 volts and varies on both relays. So obviously my problem is in the cruise module and not the servo. Seems to be a common problems to both circuits so perhaps the fix is achievable as possible regulator of feed of some sort.

I am going to try the repair the unit but just have not got it out yet. Its more work than I thought and may not get it out till the weekend. Question is, has anybody been into these units to repair and had any success. If I can’t fix it I might just have to buy one.

I have had some success repairing some of the other boxes in the car however without the schematic and the proprietary parts that they use makes some of the repairs tough even though I work on electronics on a daily basis.

 

Looks like almost none of the IC chips are identifiable on the circuit card and can’t really find any problem on it yet. Still not sure if problem is in the car or the cruise module. The module gets two input voltages, one for the transistors, ICs and operational parts, and one for the relays in the servo.

Basically diagnosing is slow as I have to drive with a couple of volt meters in the car monitoring various points and waiting for problem to occur. Each day I accumulate more information building a picture of what is happening. (Things just hanging out of the passenger side).

So far the only symptom I have found is the disengage input to pin G of the module drops to about 6 volts when the problem occurs which is probably the reason I am loosing voltage to the servo relays. I have ohmed out the brake pedal switch and cruse cut off relay where this voltage comes from (into pin G). The continuity path is about 1 ohm which is good.

So far when monitoring the main voltage feed to pin A of the module seems to be OK at least at this time. However I could not get the problem to occur while monitoring this point. I am going go stick with monitoring the 2 voltage inputs as I think something will show up here. Not sure yet if it is a supply problem or something is loading the voltage down.

Just to be on the safe side I found a deal on a cruise module so I have ordered one. Will get it next week and will try it unless I find the problem. Now the problem seems to happen when its not that hot outside.

I don’t really expect many comments on my posts. I just want to keep my progress posted as some of the more technical people might like to follow along. Pages 8A-34-0 & 1.

 

Sounds like you've got things under control. I'm not sure if this will help but it might be worth a look.

Long story short:
My switches on the brake pedal of my 89 had a problem (poor intermittent contact from use/age) and was causing the torque converter (automatic tranny) to drop in and out of "lock up". The same switch also drops the cruise control out since the cruise module "sees" the brakes have been applied....so it drops out of "cruise". Check/adjust the switch on the brake pedal for a better swipe.

 

Roger that. I understand what your saying. On the 94 the switch assembly is two separate switches and circuits. So one side can be bad and not the other. The lock up is not dropping out or a problem. I’m only working with the cruise portion of it.

You can check the brake pedal cruise part of the switch (and clutch swith if manual trans) and cruise cut off relay contacts by measuring across pins A and G on the cruise module. Its just a series circuit but checks out many connectors and lots of wiring in a big loop. This gave me less than 1 ohm and when I pressed the brake pedal it opens up. I did it 15 times and it always came back to the 1 ohm indication. So that all looks good at this time.

I am also ruling out a EBTCM or a brake event that would open the cut off relay. If that happened my relay servo voltages would be “0” volts, not partial voltage.

Now I just have to drive the car again an get some more data.

 

had a similar issue with the cruse not working all the time and it was the brake pedal switch.

 

Sounds like you have everything pretty much under control. I am assuming from your posts that your using the FSM as you listed the page numbers. I also have a '94 and had an intemittent prob with the cruise, up until the other day that is, only to find a piched wire in the steering column. Not sure if you've checked it and it would probably blow a fuse but might be worth a look.

Good luck and keep us posted. Nice and comprehensive thread
-Dave

 

Did not run the car over the weekend but did some continued testing in the garage. I removed the cruise fuse from the block and connected a wire to input end to the cruse switch. I then read the resistance from that point to pin A of the module. This gave me the basic power path thru the wiring and switch on column. This should have been a couple ohms at most but I read a varying resistance of 10 to 60 of so ohms. As I flipped the power switch it changed. I thought I had nailed the problem. However even though it is not right or perfect it is not turning out to be the problem.

Now driving the car this week as I said I am monitoring pins A and G with separate volt meters. The results got interesting. The main voltage for the module pin A always remained 12+ volts when problem occurred. However pin G (servo voltage input) dropped to 6 volts when problem occurred instead of the normal 12 volts.

This shows that the power switch, even though has some resistance in it is working satisfactory but not the problem. This throws me back into the brake pedal switch and cruise relay. This is the circuit that previously checked 1 ohm (pin A to G).

So now back in the garage I start checking that circuit again. This time it starts measuring 5-10 ohms as I step on the pedal. I switched the meter from ohms to continuity/ beep test mode. It started measuring from 6 ohms up to 90. As I switched back to ohms I got a high resistance reading.

I believe what’s happening is the switch is breaking down under load. The continuity test put a bit more current thru the contacts than the ohms mode. When the servo relays kick in that really pulls some current where the switch has a big voltage drop across it. Maybe this is the problem but until I get the switch out to check it nothing is certain. Probably should try the new cruise module when it get here just because its all apart already.

Don’t want to pull to much more apart, still have to drive the car to work. Aside from that the wife can't get in it any more due to the mess with things hanging.


Component side of cruise circuit card with clip leads.


Meters to monitor test points.

We do not have the 95-100 temperatures so car is not as hot as it was and duplicating conditions are different now.

Sounds like I am heading in your direction.

 

I received the new cruise module and thought I would try it anyway. Same problem so now I have a spare. You can’t have to many spare parts.

I re-focused on the brake pedal switch as it looked like the problem now. Took it out of the car and measured the suspect contact. The resistance did not look to bad but it has been fooling me for a while now. I opened the switch and found out all the contacts come right out. The cruise set of contacts did not look to bad, but the other two had some burn makes were there had sparked due to a higher current draw.

Took them all out, cleaned them all up and re-installed the switch. Drove for a couple days and all seems to be good. Going to get that sucker back together now and move to the next problem.

Here is a pix of the switch as I was taking it apart. All contacts just pull right out. Saved about $45, don’t tell GM, hope it holds up.

 

My cruise dosn't work at all how do you tell if the 3 way check valve is bad??
That is the valve with 3 vacuum lines going into it on the right of the manifold(is this correct)

 

pcolt94,

Thank you for a great write-up...

Whereas we don't have any problems with our 94's cruise now, if we ever do, I now know exactly where to look...

Another wonderful example of the many great members on this forum willing to share their knowledge and experience...

 

YES

Most of the time the check valve is visually broken, falling apart or has cracks in it. If your vents in you’re A/C are operating OK, and the check valve looks basically in tact, then the problem is probably some where else.

 

thanks pcolt94
I guess I will just do without my cruise

 

Did you ever find your problem? I have checked my brake switch, fuses, servo and vacuum lines and still cannot get my cruise to work on my '95. Does the dash have a cruise indicator or light?

 

I have a 94 and I do not have a cruise indicator light.

 

My electrical and vacuum connectors are on top, and I can't get either plug to rotate to the point I can remove connections. Am I correct that these should just unscrew? Is there a trick? I feel like I'm going to break something.