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Cruise Control 92 900 vert

4K views 32 replies 12 participants last post by  JKlaveness 
#1 ·
Cruise light will come on but it will not engage. I've check everything but the relay, or is there a relay? All vacuum hoses are attached and appear not to be dry rotted.

Is there any thing else to check? Could the unit by the driver side quarter panel be bad and I just can tell it by looking at it?


Any suggestions?


Thanks===rn
 
#2 ·
Good diagnostic page: http://townsendimports.com/Web/electrical_system_folder/cruisecontrol.htm


My experience is that the cruise vacuum controlled switches can leak with time. I had temporary luck "fixing" them with some plummers silicone to make a better seal, but ended up replacing.

Also, there is a switch that limits boost when the cruise is activated that is also prone to leak. I bypassed mine completely.

Be prepared to pull the knee pad and have some spare vacuum hose on hand for tests or replacement.

Good luck!
 
#6 ·
My experience is that the cruise vacuum controlled switches can leak with time. I had temporary luck "fixing" them with some plummers silicone to make a better seal, but ended up replacing.


Are you talking about the one behind the drivers headlight behind the panel?
 
#8 ·
I've had 3 'verts ('92 & 2x94), all of them with the same problem: The cruise control would sometimes work ... and then sometimes not. It seems to be completely random. The only trend that I'm suspecting might be there, is that it helps to have the heat on high ... but it might be imagination. If it's a trend, it's weak.

I've run through the entire troubleshooting chart from the Bentley Bible (excellent chart by the way). Every single component seems to work flawlessly. No leakage from any of the vacuum switches. I've taken the stalk switch apart and put it back together. Still no improvement.

I'm mystified, and interested in all responses to this posting.
 
#9 ·
Same problem here. When it was new and under warranty we took it to the dealer at least a hand full of times to correct the problem. They always said it was fixed or "we couldn't duplicate the problem." Once, I was driving and the cruise was working fine, and all of a sudden, it just stopped working and wouldn't re-engage. It's still intermittent.

Dustin
 
#12 ·
I have a 'scrap' C900 turbo vert and that's fiited with cruise, the car will be stripped out in the warmer weather, for parts for our own(hence purchase), but non of us have cruise(5 x900's) if you have'nt been successfull in finding parts(sorting prob) give us a PM
 
#13 ·
Cruise Control Common Faults

These are the common weak points:

1988 or older- Bad vacuum lines
1989 - Bad Speed Sensor
1990 or later - Misadjusted or faulty Pedal Switches

Of course, your car may have its own individual problem.

You can manually power the Vacuum Pump and watch the Throttle open to check the mechanical portion of the system.
 
#16 ·
I took the 'vert out of winter storage a month ago, but only got around to checking the Cruise Control today. Took me 3 hours.

* There are no problems in the cable harness. I tested every single cable, and found excellent connectivity everywhere.

* The whole vacuum system works flawlessly. I can run it manually by feeding 12V into the cable harness from the CC Electronic Control Unit under the dashboard (+ to pin 1, - to pin 6 & on/off to pin 7).

* The pedal switches are reliable and do exactly what they're supposed to do.

* All voltages and signals get through to and from the cable harness for the electronic control unit, as described in the wiring diagram.

There are two things left that I don't know how to check.

1) The speed transmitter behind the instrument panel gives out a steady 12,7V (full battery voltage) on pin 2 when the car is stationary. The moment it starts to roll, the voltage drops to 6,69V. The voltage seemed to rise a little when I drove fast, but the difference was small (7,2V at 100 km/h), and there was some fluctuation.

2) The electronic control unit under the dashboard might be faulty.

I have both a spare speedometer (with a speed transmitter) and a spare electronic control unit, but both came from a car with exactly the same problem as my present 'vert, so I don't think I'll learn anything from starting to switch parts around.

Should I just give it up, and accept that I only have a Cruise Control now and then?

I drove the car for 6 hours yesterday, all the time with the heat on high. The CC started to work after about 1 hour, and kept working until I had to take a 1 hour break because of a ferry ride. When I drove off the ferry, the car was cold, and I didn't get the CC to work again ... but I had a LOVELY ride home nevertheless. It was as dark as it gets in Norway on a summer night, and I can attest that dew does amazing things to how a landscape smells.

:-J
 
#17 ·
My experience and lots of reading tells me that on '92 and newer, the electronic unit (brain) is the most common problem. The '91 and older did not have this problem, but the brains won't interchange without rewiring. My '93 turbo used to work fine sometimes, quit working when it felt like it, start working again, and so on. I swapped the brain out and it worked fine from then on. Vern
 
#18 ·
Thanks a million. I'll turn my attention away from the speed sensor for the moment, and focus on the ECU. Since I have a spare, I can open it up and look at the circuit board and connections inside. Maybe I'll find something. And if not, spares seem to go for around $50 on ebay.

:-J
Jørgen
 
#19 ·
Great reading - I will follow it with interest. My 92 Cruise Control works fine, but who knows. Mine came from Singapore.
 
#20 ·
My interpretation of the fluctuating voltage from the speed sensor

>> 1) The speed transmitter behind the instrument panel gives out a steady 12,7V (full battery voltage) on pin 2 when the car is stationary. The moment it starts to roll, the voltage drops to 6,69V. The voltage seemed to rise a little when I drove fast, but the difference was small (7,2V at 100 km/h), and there was some fluctuation.<<
is that the sensor is sending out pulses, with a frequency that depends on the speed of the car. If I had access to a 12V occilloscope, I could probably confirm this. An alternative, come to think of it, is to connect the signal to a loudspeaker that I don't particularly care about (or through a resistor). Might be fun to try. The worst that can happen is that the speaker burns up.

:-J
 
#21 ·
Success

Hi,
It was dead easy to open the ECU: A little prying with a screwdriver did the trick, and the circuit board slid right out. It looked pristine ... but in the middle of it sat a small electro-mechanical relay, that looked like it was meant to carry some real amps. The power for the vacuum motor?

I polished all four contact surfaces on the relay with fine sandpaper, blew the dust away, gave it a little WD-40, reassembled, and installed it back in the car. Since then it has engaged every single time I've tried it (>30 times), and it's never disengaged itself without my order.

I tried the same with the spare I had in the attic, but didn't get that one to work. Hopefully, that's because overconfidence made me do a sloppy job with the sandpaper. I'll try again. Meanwhile, my CC system is up and running, and I'm very pleased with myself.

:-J
 
#22 ·
Hi,
It was dead easy to open the ECU: A little prying with a screwdriver did the trick, and the circuit board slid right out. It looked pristine ... but in the middle of it sat a small electro-mechanical relay, that looked like it was meant to carry some real amps. The power for the vacuum motor?

I polished all four contact surfaces on the relay with fine sandpaper, blew the dust away, gave it a little WD-40, reassembled, and installed it back in the car. Since then it has engaged every single time I've tried it (>30 times), and it's never disengaged itself without my order.

I tried the same with the spare I had in the attic, but didn't get that one to work. Hopefully, that's because overconfidence made me do a sloppy job with the sandpaper. I'll try again. Meanwhile, my CC system is up and running, and I'm very pleased with myself.

:-J
;ol; NOW GO FOR A LONG DRIVE AND USE IT JUST BECAUSE
 
#26 ·
Afterword

I tried some more to get my spare to work, so I could put it in our US 'vert when I go there in June, but without success.

* I've confirmed that I now have excellent connectivity across the relay points.

* When I close the relay manually, I can see that it picks up the +12V from pin 9, and delivers it to the vacuum motor on the ECU's pin 1 (the motor's pin 3) when the relay is activated. This makes sense: It would be the master function that turns the system on or off, while the ground connection to pin 1+2 on the motor is controlled by the two big thyristors on the circuit board. When ground is passed through to both of them, the motor will run, and the accellerator will be pulled down. When it's only passed on to pin 2, the pump will release air, letting the pedal come up.

* However, when I drive the car with the spare installed, there's absolutely no response from the relay when I push the "set" button. Closing the relay manually, by squeezing it, makes no difference.

CONCLUSION: Cleaning the contact points on the relay is not a surefire way to fix these ECUs, but it might be worth trying if you like fiddling with electronics.

:-J
 
#27 ·
Success with the spare, too

Hi,

I just figured out how to fix the spare, by doing some online research. I was told that if I studied the underside of the circuit board with a powerful magnifying glass, in good light, I might find fractures in the solder around one or more of the contact points. This appears to be the most common form of failure in these ECUs

Sure enough, I found tiny circles round four of the solder points. I've tried to upload a picture as an attachment. If I succeeded, you'll see red circles around the points I found.

The solder points were covered in laquer. I removed this with a Dremel and rotary brush, so I could get to the metal and redo the solder. That made enough of the solder metal flow, that connectivity was reestablished. I tested the part, and it worked flawlessly. (I'm still going to redo the solder, to create a more permanent repair).

CONCLUSION: The Cruise Control ECU can actually be repaired on your kitchen counter. Yay!
 

Attachments

#29 ·
The solder points were covered in laquer. I removed this with a Dremel and rotary brush, so I could get to the metal and redo the solder. That made enough of the solder metal flow, that connectivity was reestablished.
You can often remove the protective lacquer with a suitable solvent such as cellulose thinners. That being said many PCB lacquers are solder through.
 
#32 ·
Agreed, the control module on '92+ is a flaky piece of engineering. I've replaced it once, it worked great for 6 months then failed again. All other components were tested and working properly in both cases. The same control module can be found on later 9000 models at the wreckers, as '92+ 900 models don't seem to turn up much at the yards!

-----Thanks for the tips JK, I'll bust mine open and take a peek at those solder points
 
#33 ·
It's amazing how thin the solder cracks were: I'd have sworn that it's impossible for two pieces of metal that close to each other not to be touching. However, evidence is evidence: The module went from non-functional to functional when I redid the soldering, and has worked since.

:-J
Jorgen
 
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