no power to brake (BOO) switch

spetz01

Registered User
This ones got me..

Brake lights stopped working. Newer switch installed a few years back. I remember how much fun I had doing that, and keeping the switched aligned with the MC rod. It wasn't that.

Jumped the connections on the harness, no lights. Fuse is good, #6, on both sides. I even tried another fuse and nothing.

I seem to not be getting power at all to the BOO connector? I don't think any of the under hood fuses have anything to do with that circut.

Any ideas?
 
Where are you checking for power? If you are checking at the actual connector, then try tracing the wires back a bit and checking there. What frequently happens is that the wires flex every time you hit the brakes, and eventually the wire breaks, but sometimes the insulation actually stays intact, so by just looking at it, you can't tell the wire is broken. I would strip a little bit of insulation an inch or 2 back from the connector and see if you have power there. If you do, then just cut off the connector, and splice in 2 new wires, each a few inches longer than what you cut out so they don't flex and break again, and just use 2 spade connectors on the end to connect to the brake light switch.
 
I have a couple of years EVTMs; what year is your SC?

For a 1991, the 15A STP LTS fuse is connected via a light green with red tracer wire to the BOO. Nothing inbetween. How did you check the fuse? With a meter or via Eyeball Mk I? (If eyeball, grab the meter and make sure there's continuity through it. Also make sure 12V is present on both sides if it's a ATO type fuse.)

RwP
 
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EVTM calls it 'starter interrupt relay'.

Are you sure?

Section 20 doesn't show the BOO in it at all; section 90 doesn't show any of the starting circuit in my 1991 EVTM.

(20 is the starting circuit; 90 is the brake light circuit).

What year has that in it?

RwP
 
Are you sure?

Section 20 doesn't show the BOO in it at all; section 90 doesn't show any of the starting circuit in my 1991 EVTM.

(20 is the starting circuit; 90 is the brake light circuit).

What year has that in it?

RwP


'91s don't have a BOO, AFAIK.

I'm looking at the EVTM for my '90 Anny w/auto trans, Section 20-1 (sits on the page between the ign. switch & the EEC) & component locator 151-4 D 10, left of IP.
 
'91s don't have a BOO, AFAIK.

I'm looking at the EVTM for my '90 Anny w/auto trans, Section 20-1 (sits on the page between the ign. switch & the EEC) & component locator 151-4 D 10, left of IP.

BOO is Brake On Off - I dunno about how you expect the brake lights to work otherwise.

Same switch used 1989-1992 on ABS equipped units, mechanically.

*opens up his 1990 EVTM*

Ahem.

The brake light switch feeds the COIL of the starter interrupt relay.

Look at the diagram again; battery --> 15A fuse --> BOO --> starter interrupt relay coil. Ground return of the SIR is the grounded side of the ignition switch (otherwise used to run the light test).

A bad starter interrupt relay won't cause the brake lights to not light up, nor would it kill power to the BOO.

Well, if the relay coil or the wiring to it was shorted, causing the fuse to blow, it might; but not the way the OP posted.

Still a Light Green/Red wire, too.

RwP
 
Glad you found it, thanks.

Was quoting previous thread - will need to correct ... next time he comes thru ;)

Do we think the ignition switch could be acting out?

Ken
 
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Glad you found it, thanks.

Was quoting previous thread - will need to correct frasier next time he comes thru ;)

Do we think the ignition switch could be acting out?

Ken

Ignition switch isn't in the brake light circuit; if he has no power to it, it's the wire feeding it, the fuse box, or the fuse.

Not a lot else there, save for the connectors, to be concerned with.

RwP
 
1990 5 spd no alarm. It did at one time, but they’d have tied into the park lights I think.
I pulled the fuse and replaced, checked both sides with a light type meter. Seems ok. I added power right to the switch and I do get lights.
Pulled to look apart and wires look fine.
I’m not seeing any relays?
 
I added power right to the switch and I do get lights.

"Jumped the connections on the harness, no lights."

Just to be clear, that means you jumpered the connector at the switch, right?

Since fuse is good, do you see 12v at the connector at the switch?

Nothing in that basic circuit besides fuse, switch, bulbs and harness.

There is a stop light feedthru in the multi-function switch - 4-ways working ok?

I’m not seeing any relays?

Forget them in this case...I was chasing another issue, sorry.
 
I did try jumping the connector but nothing happens. 4 ways work, in fact everything but the brake lights work.

There should be juice at one lead of the connector right? Power to the switch, switch is activated by the pedal, power gets to the lights? I have nothing at the connector.

I didn't have my probe style tester this afternoon. Car is being driven by the father in law now. I think it must be broken inside the insulation? I'll get it here and work my way up.

What I in the mean time is cut the power wire to the switch and feed it 12 v from a keyed source that was close by. Brake lights work now no problem.

I'd still like to find why the fused original has kicked the bucket.
 
Did you happen to notice if the high mount stop lights worked when the fender/main stop light don't? If so, EVTM says to check multi-function switch (the turn signal switch assembly).

If none of the stop lamps work, it lists these possible causes:
- Fuse 1 blown: check/replace
- Open in wire: Check both wires at stop light switch connector for continuity, repair as necessary
 
According to the book, no voltage at the MFS would do it. Says to check, vs. replace.

I think the deal with the fogs is either the rocker in the console gives up (happened to my '90), or the headlight switch fails over time trying to carry heat...see if there are significant white powdery traces underneath it. Might be less money to replace it than a MFS, but like so many electrical components on this car, sometimes it's just time, and even if you're not sure they're at fault, replacing is a good bet if you intend to keep the car.

If you go for a MFS, be sure to match part numbers and color code, blue vs. green.
 
What’s the degree of difficulty in replacing the MFS. I’m guessing the first step in the Haynes manual, then the follow up:

-remove entitle roof assembly
-turn car over
-pull motor
-change power steering fluid
-recover seats
-paint car
-install new MFS
 
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