When I put on the cruise control...the car slowly accelerates! What could cause this? The cruise is vacuum-controlled, right?

 

The SET button seems to be sticking if you ask me.

 

fwiw, the cruise is controlled by the tach reading, I do believe.

 

i fucked up one day , i was driving playing around with teh cruise control cuz i didnt know if it worked. Took a while to engage then it was working fine. LIke a tard i tried to shift gear to a higher gear and the engine kept accelerating and it wouldnt shut off lol, my **** was at 8500, redlining i was shitting mypants.

 

The S5 FSM has a complete diagnostic & troubleshooting writeup in the body electrical section starting at page T-69.

The cruise control computer has it's own blink codes.

 

there is a clutch switch under the dash. Make sure that's working or the cruise won't disengage when you put the clutch in

 

The author of the thread didn't really explain EXACTLY what is happening. If he meant he was cruising and pressed the SET switch/button to set the speed but the car kept accelerating endlessly, the there is a problem with the SET switch/button.

If he meant it accelerated for a moment then settled down, that is a different thing all together.

If you'll take a few brief moments to look at the schematic it's obvious that when you select the main functions of the cruise switch, all your doing is putting a GROUND momentarily on a circuit, whether that circuit is SET, CRUISE, RESUME, WHATEVER.

IF the button on the end of the stalk STICKS the ground will remain there on that circuit whether or not you let your finger off the button or not. So if the button sticks the car keeps accelerating.

IF it is determined that that button is sticking, then remove the switch. Look at the FWD facing side of that lever and you'll see a tiny screw that holds a cover on. Remove the screw and watch the small spring and ball fall on the ground (hope you can find them after you drop them). There's a Contact board there and the spring/ball/contacts all work together and when you look it's intuitive on how it works. Make sure that it isn't sticking is my advice.

You don't have to take the thing apart. Just observe the switch/button as you press it and note whether or not the button *returns* once your finger is removed from it.

I drove thousands of miles without that clutch switch. I did what the fellow above mentioned. Was in cruise and shifted. Not a bright idea. Only did that once. That's all it took to learn to tap the brake pedal first before shifting. A switch has been installed since then.

 

Cruise control maintains a constant road speed, not engine speed. It's input is from the speedo. Pretty obvious when you think about it...

 

So, an old thread back from the dead... I have this same issue. I've tried two gauge clusters (speed sensor), two switches, two actuators, & three cruise computers. Every time I press the SET button to engage the cruise, the car just starts accellerating and continues until I press the brake or the main button on the cruise switch. So, reading Hailers post... Do you guys think it's safe to disconnect the wire for the set button at the ECM and run a test lead to ground the pin manually to test?

 

I don't see why not. On my 87 diagram it's a Green/Orange wire. Seems all one does when going to *set*, is to momentarily put a gnd on that wire going to the CCU.

You could pull the plug off the CCU and with a meters neg lead on a good gnd, then put the positive lead on the green/orange. There should be zip ohms until you press the Set switch and you should get something like 1-5ohms. Let go of the set switch and it should go back to zip ohms.

You could check the Coast and Resume the same way but get on the Blue/Red and Blue/White respectively.

You tried two switches and it's hard to believe two switches would be bad. Odd.

If memory serves, if you HOLD the Resume switch constantly it will also make the speed climb. So check it out along with the Set switch. Still hard to believe two switches would be bad.

This a series four?

 

Yeah. PUll the green/orange wire out of the plug and put it to the side. Then install a gnd wire in the socket and put the plug back on. Go drive and any time you want to SET the speed, just put the end of the wire to a good ground momentarily. You don't or should not have to hold it to gnd. IF you do the speed will increase.

Or just pull the plug off the CCU. Put a meters negative lead on a known ground place and the positive lead on the green orange wire in the plug. PUt the meter on ohms. You should have no reading , until you press the SET button whereupon you should get a reading of a couple of ohms. Let the switch go and it should read open.

Try this with the Resume (blue/white) and Coast (blue/red) buttons also. You should only read ohms when those buttons are held depressed.

Odd you have two switches that do not work. I can see why you changed the other things out. Odd.

 

Sorry 'bout the double post. The first one didn't show up after I posted it so I did the other one also. They say about the same thing.

Yes, series four 1988 model. Still a non turbo? No engine/harness swaps involved???

 

Actually you'll see 20 or so ohms if the Set switch is held in and your on the green/orange wire with a meter.

I went and got a long piece of wire and stripped both ends and stuffed one end in the back of the green/orange wire in the connector and connected the plug to the CCU. And yes when I'm driving along I can momentarily touch the other end of the new wire to the ignition switch and the cruise engages and stays engaged at a set speed.

 

Thanks Hailers, I'll give this a shot as soon as I get a chance. Yes the car is a 1988 N/A

 

Ok, I just gave this a try... I used a cheapo DMM that's not auto switching and set it to read 200 Ohms. This is what I got:

SET BUTTON: 0.6
RESUME: 1.2
COAST: 0.9

All connections were open until the switch was pushed and went back to open as soon as I left off the switch.

I'm really lost here....

*Edit* I just took the DMM to all three of the actuators that I have and they all read between 55-60 Ohms from c-a. According to the manual spec is 20-40. Can anyone with working cruise verify this?

 

I'll look tomorrow. But your switch seems to be working right. No continuity until the switch (your choice of switch) is pressed, then continuity goes away as you release the switch(s).

Once you let go of the SET switch the cruise should have set a steady cruising speed and not gone up/down in speed after you released the switch. It's obviously not the set switch from what I read above. Then again you removed and replaced the CCU. so it shouldn't be that. I give up. Send me your CCU and switch and I'll try them in my 86 na (part humor, part not humor. Depends how much you want to know if they're any good).

 

hehe I may do that if the Actuator checks out ok. I suppose it could be a wiring issue to the actuator or speed switch but honestly, the cruise circuit diagram in the FSM is a little over my head.

 

Does your car have AAS, POWER STEERING, or AT CONTROL???

Is this a manual car or has it ever been an automatic?

Speedomenter work? Frankly I don't understand how the Speed sensor works. You have a gnd wire going to it and another wire going to the cruise control. Makes little sense just as shown in section C. Maybe speedomenter cable sensing???

Did you ever take a meter and ohm out the GREEN/RED wire from the plug on the combination meter to the cruise control unit?

 

It's a GXL. The AAS unit was unplugged and the motors removed when I upgraded the suspension. I have power steering... not sure what AT control is, I assume that's an auto tranny thing? I have a manual transmission. I've not tried putting the meter on the wire from the back of the meter. Speedometer works.

 

since you removed all that stuff, are the metal cables adjusted correctly?

 

you mean the cable from the actuator to the TB? Yes, I've tried loosening it more and more just to be sure and it does the same thing... just takes a split second or so longer to engage after you press the set button.

 

I pulled the wire from the speed sensor and the car won't engage cruise, so don't bother ohming out that wire. I did tap into that wire with a meter and could read Frequency/hz, but I doubt you have a meter with that capability. BUT if you do, look at the freq when at a steady speed and see what it is and if it climbs all by itself when the car is at a steady speed. Actually I don't think the problem is in this area.

When you get a chance, write down the CCU's part numpber. I think it'll be something like FB-01 or something similar.

When the speed climbs, will the car continue to gain speed. Like you engage at 40mph, then it keeps climbing til 70 mph plus??????

 

For testing, I've been engaging the cruise at 40Mph and I let it climb to about 55-60 before I disengage it. I'm not sure how high it would go if I let it. I have a FLUKE 87 DMM that I use for work.. I don't know what it's capable of honestly, I just use it for DC/AC voltage & continuity checks mostly. If it gets down to it I can follow instructions pretty well.

 

I looked online at http://www.testequipmentdepot.com/fl...FRIygQodzUq0LQ

and downloaded the manual. You would put the neg lead on a known gnd. Stuff the positive lead in the back of the GR wire in the CCU plug with the plug attached to the CCU. Put the meter on DC volt.

Then it appears from what I saw, there is a button that says HZ/% on it. So while driving and the meter on DCVolts, press the HZ/% button, and it should read the freq. I flat forget what I was reading. Maybe tomorrow morning I can take a few moments to do that over again, and this time hold a steady 40mph while reading the freq.

That meter also has a duty cycle. It's interesting to put the meter on a known gnd and then put the pos lead up the back of one of the Primary injectors and look at the duty cycle as you drive along below 3500rpm and then stomp the pedal and watch the duty cycle drop in half as the secondarys come online over 3800 rpm. As you can see, I'm easily ammused.

About the HZ/%........If your measuring DCVolts then hit that button once you see freq. IF you hit again it'll show the Duty Cycle.

I still have not much of an idea WHAT is wrong that's making the thing keep climbing in mph.

 

Did you ever try depinning that wire that runs to the SET switch and then installing a new wire in that socket and while driving momentarily touching it to a known gnd???? I use the ignition switch ( not a very good gnd point but worked). Key word *momentarily*.

I'll check the coils in the motor tomorrow. Forgot.