1995 850 How to TEST brake light switch 9128577 on 850 wagon Topic is solved

[wheelsup]

Hello,

Shift lock must be depressed and I get no brake lights when pedal depressed.

I have removed the brake light switch from the pedal, but unsure of how to test, no real information online.

It's got 4 contacts in it, situated in rows of two each. Labeled what looks like in pairs 1-3 and 2-4.

The plug I tested and jumped the top right and bottom left contacts as instructed on another thread on here but got no brake lights. I did hear a clicking under the dash and saw a spark at the plug.

Fuse is good. I did look at one brake bulb so far, it was fine. However, if I removed the center rear bulb, the bulb out indicator does not illuminate (it does come on when you first start the car, so the dash bulb isn't burnt). I get 12v across both plug sides on the car side.

I would like to test the brake switch to rule it out as well but am unsure of how to do so. I tried testing continuity between all the contacts and got none, either with the switch depressed or off (open).

I'm not real good with electrical stuff.

[abscate]

On later models the pairs are normally open

1-2
3-4

When you remove the switch though, I believe it closes those contacts.

To nail this you need the 1995 version of this system.

[abscate]

Is the part number 9128577

[wheelsup]

Is the part number 9128577

yep looks like it

Also from an internet search it looks like that part # is used on a bunch of Volvos from the 850 to the V70/V40 and in between. My part # is missing the first "9" but it's ripped so looks like it's the same.

[wheelsup]

On later models the pairs are normally open

1-2
3-4

When you remove the switch though, I believe it closes those contacts.

To nail this you need the 1995 version of this system.

OK I'm gonna be honest, I have a hard time reading that. I know, I know. My wife got me an Arduino for my bday a short time ago so maybe one day I'll learn.

But for now, maybe talk to me like I'm 5? I really, really want to know how to read those diagrams though.

I watched some youtube videos of folks testing different switches and it seems like they all have some conductivity between pins.

I opened the switch back up like it's in the car (before you push the back in). I then ohm'd out the pins with and without the button pressed and got no feedback (does that mean "open"?).

I did order a replacement that will be here in a few days that will let me compare.

[wheelsup]

OK an update.

I started playing around with it and was able to get to 0 ohms between 1-2. It happens only if you extend the white "cover" over the button. And it's very finicky, only at the max coverage with the white cap and no touching of the button.

Pushing the switch down (ie where the brakes would be applied) pushes the white cover down and then you no longer get any connectivity.

Now, on a whim, I installed it back into the car with the white cover extended to the max amount (with 0 ohms between 1-2). Brake lights would go on with the car off and no key in the ignition.

If I press the button (as in the brakes get pressed) the cover gets pushed back, and the brake lights go out, and won't come on again unless you extend the white cover manually.

Now, with the brake lights on, however, my car still will not shift out of P without pushing the shiftlock override.


Sorry, I'm doing my best....

Thank you for your help...

[abscate]

It looks like terminals 3-4 don’t do anything as there is no connections there.

This switch performs three “functions “ but I don’t think it controls anything directly, it is a signal device because of all the stuff it does

1 tells the brake lights to come on , regardless of ignition switch position ...to be confirmed

2 tells the shift interlock solenoid to open when the brake pedal is depressed

3 tells the cruise control to drop out when the brake pedal is pressed

Have a good look at how the switch mounts. When the pedal is at rest, foot off, is the plunger switch depressed or is it open? I thought it is normally depressed so the the contacts are closed In the switch ?

That’s critical to understanding function

[abscate]

Switches are described as

NO ...normally open
NC....Normally closed

The part to understand here, though, is what the “ normal” refers to....switch inyour hand with the plunger out or as it is on the car,with the brake pedal in rest position closing the plunger?

Engineering “convention” would be to show the diagram in the latter case, in rest position

[wheelsup]

It looks like terminals 3-4 don’t do anything as there is no connections there.

This switch performs three “functions “ but I don’t think it controls anything directly, it is a signal device because of all the stuff it does

1 tells the brake lights to come on , regardless of ignition switch position ...to be confirmed

2 tells the shift interlock solenoid to open when the brake pedal is depressed

3 tells the cruise control to drop out when the brake pedal is pressed

Have a good look at how the switch mounts. When the pedal is at rest, foot off, is the plunger switch depressed or is it open? I thought it is normally depressed so the the contacts are closed In the switch ?

That’s critical to understanding function

Yes you are right, I was wrong. So, as it sits in the car, installed, the button/switch is depressed. As you press the pedal, it releases the button and extends it.

So, it is semi-working as advertised. But the only way I can get it to work is to extend the white "cap" fully out to cover the stem of the button/switch. That will close it (?) and shut off the brakes. But if you push the button (so release the pedal) it will push the white cap back, and it remains fully back and as such won't trigger brakes again when the pedal is depressed. (See video below at around 2:20)

See this video at about 0:15 in. It's the position you move the cap to remove the entire switch from the car, so I assume is not the "normal" position.



So, good news is brake lights do work (if signaled correctly). Bad news is I don't know if it's because of the switch or something else.

[wheelsup]

Ok. Another update.

It's not the white cap that causes it to "close" (and go to 0 ohms). That was just causing me to pull the button.

What you have to do is fully pull on the button with your fingernails.

I created a video. Hope this makes more sense!!