Hi guys, I've been stuck with this problem for months and only just found out that apparently it's called relay chatter! I googled that and found you guys, looks like a great forum! I'll give you a quick background

I'm 21, been restoring a 1984 Saab 900 Turbo in Australia. I have cruise control in this car, all the sensors and that works because when I engage cruise control the car maintains it's speed however I get a high pitch buzzing from under the dash!

Upon further investigation I found that there is a relay before the Cruise Control ECU that was vibrating and appeared to be the cause of the sound.

I've been told that this is called relay chatter, but I obviously don't know how to fix it, I do have a wiring diagram but am hopeless at reading it, Can scan it and upload if you guys need it, but thought it might be a simple fix. I did try replacing it with a relay of the same rating but no fix!

I was told that installing a diode could fix this, but any greater more thorough advice would be fantastic.


Thanks in advance for and help!

 

Something is interacting between the relay and the cruise control circuit.

My best guess is the cruise control in not connected to the power circuits in the correct place. It might be as simple as moving the 12V power connection to another place in the fuse dist box.

 

Hi,

What i usually do is to solder a 10uF or 22uF electrolytic capacitor to the coil of the relay. A car use 12v so a 25v cap will do.

 

Chattering relay may be caused by lowvoltage or poor conector somwhere in the circuit. Have seen it in vehicles before due to poor earths.

 

ok thanks for the ideas guys!

um, so what are some things I can do about this, bad earth is a potential option right? So could I just check the earths of the relay with an ohm meter to see if there is resistance on the line? Is that how I would do it? Sorry I'm not really electronically familiar!

Ok low voltage is another option then is it? Does a diode do a similar thing regarding the sustenance of sufficient voltage. My belief was that it stopped voltage travelling back down the line? Would a capacitor be more useful than this?

I dont know how relevant this is but I tried a new cruise control box but this didn't work.

 

Hi,

What i usually do is to solder a 10uF or 22uF electrolytic capacitor to the coil of the relay. A car use 12v so a 25v cap will do.

Hey soda can you explain in laymans terms what those ratings etc mean regarding the capacitor and where I would solder the capacitor to?

This stuff is so hard to get your head around, I don't want to take it to an auto electrician if it's such a simple fix

 

Bet you have a leaky cap somewhere in the ECU

 

swapped the ECU out for another one, didn't solve the problem, I have a feeling it has something to do with the cruise control stalk, because Saab released a workshop fix which involved using a newer Cruise control/Blinker stalk and then splicing the 4 wires from the relay onto the wires that enter the Cruise Control ECU - this fix eliminated the relay and thus I guess the problem with the cruise control - Problem is, I don't have the new style cruise stalk.

 

To check the relay coil ground side when activated with a meter to ground , the voltage when activated should be very close to 0v. if not check any earth points related to the cruise control.

 

Sounds like maybe the stalk has a poor connection internally; like the switch may be corroded, or have burned/pitted contacts.

Some measurements with a meter would be helpful. For example, do you know whether the stalk provides power or ground to the relay coil?

 

Ok so just to reiterate that, I should turn the cruise to the ON position where the relay is buzzing (chattering) and then just use a multi meter to check the ground, what wires do I connect the probes to?

the red to the ground wire and the black to true ground?

Sorry like I said this is not really my forte!

 

Um I'm at uni at the moment but I'll check in my service manual when I get home, I think when I tested the stalk positions I was testing for continuity when the switch was in certain positions so I guess the switch provides ground? But I'm not too sure!

I will get back to you on that, So at this stage the consensus is that it is either a bad ground or insufficient voltage to keep the armature of the relay open or closed or whichever way it goes normally! Is there a way to see if there is insufficient voltage can I just test that with a voltmeter?

Hm I should probably do some definitive diagnosis before I keep asking more questions! trying to learn as much about the topic as possible!

 

Hmm - forget what I just wrote.

Many cruise controls won't engage unless you're above a certain speed, which implies that you are in the car and driving. Attempting to take measurements using a multimeter while driving is simply too hazardous for words. It would be all to easy to forget that your primary function is to drive the car, maintaining constant alertness to ever-changing conditions.

Perhaps you can try to find the replacement stalk that was recommended in the bulletin. It seems that is the easiest (and best) fix, as the components in the replacement stalk would already be rated for automotive environments.

One of the "sticky wickets" we get into on this forum is people wanting a circuit to fix/enhance something in an automobile. Automotive environments are among the harshest on the planet, and a lot of engineering and testing goes on before the parts are actually approved for use.

Since there is a "known fix" for this issue, I strongly suggest that you follow that route, and track down the elusive replacement stalk. You may very well find one among Saab collectors groups, in a breakers' yard, or on an auction website.

Many novices will royally botch an autos' wiring harness using stuff like "electrical tape" or "electrician's tape" - don't make that mistake. Use heat shrink tubing in two layers, preferably the type that has glue inside. Once properly shrunk over a good, solid connection, it will be a permanent repair. "Electrical tape" will shortly become gummy, unravel/unwind, and leave bare wires dangling in the breeze, waiting to short out against something.

 

Yeah whilst that is the case, it is an older cruise control system which functions mostly on mechanical levers and things which control the throttle plate depression, I believe newer cruise control is based more purely on electronic control mechanisms, I don't think car speed has much impact on the actual functionality of the stalk communicating with the relay

The service manual for my car which is very reputable (Bentley) states that the testing procedures all give accurate results when done with ignition off and car not moving, in addition to this the official 'fix' requires splicing out the cruise control relay and splicing in a new cruise control stalk, all of which require much more invasive methods than installing a capacitor or a diode, if installing a capacitor or diode will indeed fix the problem, I think it will result in being the least intrusive method.

 

The Relay is chattering due to it pulling in & the voltage droping enough to release it @ voltage then rises enough to pullin again, this is reapeted at an audiable freq. You need to check your circuits & earths for poor conections. Ive had to check cars for chattering relays before, its been caused by what Ive described. You need to understand how the cruise control actualy works. Is it a vaccumm operated cable type, or mechanicly driven by an electric motor & gearbox with cable to throttle? Do you have a circuit diagram in the manual you can post?

 

it is electronically controlled - the vacuum pump acts on a diaphragm which then moves the throttle plate... So I guess it is electronic

I certainly do have a wiring diagram, I will post it up in an hour! It scarcely makes sense to me!

I can also post up the link for hte cruise control fix if that will help you guys diagnose what the issue is?

I can't complete the fix because I do not have the cruise stalk, and the method is very intrusive on modifying wiring (already had a spud Previous owner hack the wiring up for a tape player)

 

On an 83model BMW I pulled apart recently had cruies control on it & the vac pump had a switch in it so it only ran when the vac was too low, so it didnt run all the time only when necessary, the cruise mainly used manifold Vac. Same set up Vac actuator using cable to the throttle. Basicly it uses Vac solenoids to either aply Vac to the actuator & then hold for the set speed & if it over speeds it activates other solenoid to let cable back. Fairly simple computer module senses vehicle speed & activates the solenoids to keep set speed.

 

heres the link to the wiring diagram

I assume the relay were talking about is 128 - speed control relay

http://www.flickr.com/photos/60590887@N03/

let me know if that link doesn't work, i've never used flikr before

because my car is a 1984 I assume the 141 (selector for cruise control at direction indicator stalk) will be the top one (M84-86)

the M87-88 I assume is the newer style control stalk obviously




Found this :

On earlier 900 Switches, the SET (button on stalk) voltage goes to a relay mounted beside the ECU, it supplies pin 86 of the relay for its pull down coil, pin 30 of the relay is connected to pin 2 of the ECU and receives 12v from it.When the relay gets the 12v from the SET switch it connects pin 2 of the ECU to pin 9 of the ECU and pin 3 of the vacuum pump. This causes the ECU to set the speed in memory.

 

Link works great , have printed the pages. But can you put up the circuit thats missing on the RH side of the 141 selector. Its simmilar to a BMW. BOSCH unit Pic of the Vac pump of the BMW looks the same.

 

that looks like the exact same pump as mine , yeah sorry I didn't realise It cut off, it wasn't much but probably important!

Thanks so much for your assistance - I see your SA, I'm in Geelong

That pic is uploaded now on the same flikr page!

EDIT: Shoot it's upside down, SORRY! You can d/l and flip it though, sorry for the hassle!