brake lights don't light

Hi folks,

I finished parting together a Frankenvolvo for my daughter who
pretty much totalled the base car (1992 740 wagon) using parts from my
buddy's 1994 940 wagon which wouldn't pass emissions and was pronounced "not
worth fixing".

I got the whole thing done, I thought, then found that none of the three
brake lights are working. One tailight is from the original car and one is
from the donor car, but I doubt that's the problem. I checked the fuse and
that's OK. I checked the bulbs and they are okay. I did a visual on the
switch and its function looks OK. My old multimeter bit the dust so I bought
a new one today but haven't gone probing yet. All the other lights work fine
using the common ground that the brake lights use.

I think I can probably test the obvious stuff but maybe one of you knows
something that might be eluding me, especially something that crash damage
might cause. I'm stumped and get frustrated with electronics diagnosis, so
any hints will really help a lot.

Off to a big meeting tonight but I can answer the stuff I forgot to include
later on tonight.

BTW, $320 to recharge an old A/C system with 2lb 6oz of R12...

I shoulda just called it totalled and left it at that. The things we do for
those we love...

TIA,
Dean
knucklehead

 

Check the bulb failure sensor.

Baffles me why anyone uses R-12 anymore, you can convert the system to R-134
including all new seals and rubber hoses for a couple hundred bucks, then
it's only around $30 to charge it. AC is not difficult to work on, you need
a few specialized tools but even those are not expensive.

the 740/940 wagons are great cars, what better can you get for the money?

 

This may be the culprit. I'm trying to find a brake light relay and this is
as close to that as I can find, looking for relay diagrams...

The switch checks out fine but runs counterintuitive. That is, the switch is
pushed in and on, supplying 12V when the brake pedal isn't touched. Pushing
on the brake pedal releases the switch and cuts the juice. So, a 12V signal
keeps the brake lights off. Remove the 12V signal and some gizmo should
switch on the lights.

But, I don't see anything relating brake lights to a single relay. But the
"bulb failure sensor" is right there on the realy panel (behind the fuses).
Maybe the relay shorted out and fried when the taillight got crushed and
filled with snow.

There's a headlight relay, but I doubt that's connected. Anyway, I can try
those two just by parts swapping off the donor car, I think.

Well, including [parts that's not a ton cheaper unless you plan on multiple
recharges. It's a done deal. And the people considered that option, but had
seemingly good reasons to stick with the refrigerant of design. I knew
beforehand that there are very significant differences of efficiency between
the new and old stuff. The guy has been doing auto A/C for 50 years and will
just talk your ear off. He says the condenser isn't right for R-134 and a
hot day will really overwork the system and have pressures too high or some
such thing.

He did some value extra checking my work with flourescent dye in the oil,
and heated my dryer filter twide to reclaim the dessicant, and did his best
to get the oil quantity right in the system (not easy without a full flush
of the system which I didn't do.

I have been very happy with the many Volvos I have owned over the years. And
I flog them hard too.
The old rear drive cars are a pleasure to work on.

Thanks for your response, BTW.

-Dean
rambling...

 

That sounds more like power to the cruise control.

 

"James Sweet"

Bingo. Ten points for you.
Part number1362370

Must've shorted out I guess.
Now I'm wondering how the circuit works, just to understand...

Thanks.

-Dean

 

I don't think there is a brake light relay. IIRC, +12V is supplied to the
switch all the time, pressing the pedal causes the switch to close and turns
on the lights in a 240, I'm surprised if the 700s are any different. The
bulb failure sensor commonly suffers cracked solder joints, I've fixed a
pile of them over the years.

You always plan multiple recharges, the stuff has leaked out afterall, it
will leak out again eventually, and there's a good reason they don't make it
anymore.

I've heard the efficiency arguments as well, but I converted mine years ago
and it works great, cost me about $200 and I haven't touched in since. It's
rarely above the mid 80s or low 90s out here, but I've never had any
complaints about it. There is a slight difference in efficiency between R12
and R134, but "very significant" is an overstatement. I'm licensed to buy
R12 but I always convert systems I work on, I don't like to put CFCs in
automotive systems because they always leak eventually.

 

Pop the cap off and look at the circuit board, you'll probably find cracked
solder joints in multiple places.

The older sensors are simple and clever. For each light circuit, there's a
reed switch around which are two bifilar wound coils of wire. These coils
are wired out of phase with each one in series with the lamp on one side.
When current flows through the circuit and both lamps are working, the
magnetic field is equal but opposite due to the out of phase wiring and
cancels out. When one bulb is bad, the magnetic field causes the reed switch
to close and the warning lamp on the dash illuminates. Of course if both
lamps are burned out, the warning will not be on.

 

Well, the switch shows an open circuit with the plunger extended and
continuity with the plunger depressed. The brake pedal's arm contacts the
switch depressing the plunger and thus gives continuity when there is no
foot on the pedal. When you push the pedal down the arm loses contact with
the switch and opens the circuit.

The red cylindrical ones (about 47mm diameter and 5cm tall)?
I opened the bad one up and it's a three tiered stack of circuit boards.
Everything looks solidly connected and unburnt. But there's a faint smell
like somebody let the smoke out of one of the componenents.

I found a wiring diagram here:
http://volvo.techno.org.pl/wiring-diagrams.html
specifically
http://pliki.volvo.techno.org.pl/schematy/schematy_haynes_volvo_740_760_od_1987.pdf
and it seems to show both
66 Brake Light Switch G3
and
229 Brake Pedal Switch W2

so... the mystery thickens.
But the car is ready and my daughter can take it back to college, so I'm on
to the thing I really should have been doing all day.

Thanks again.
-Dean

 

Cool. This will be fun playing with this thing trying to find the bad spot.
Visually it looks good. Smells like the smoke got out though.