91 Civic Front Side Markers Both Flash When Operating Turn Signals

I think I have a problem with the integrated control unit (ICU) that
operates the front side markers on this 91 Civic.

Symptoms-

When either the left or right turn signals are operated, _both_ right
and left side markers flash. Only the same side is supposed to flash.
This is the problem. it sends mixed signals to other drivers.

When the parking lights are on, the orange front side markers are
supposed to be on. They aren't.on with either the parking lights or
headlights. If the turn signals are operated, both side markers
flash. The side markers are supposed to be on, and flash alternately
with the turn signal.

When the emergency flashers are operated, both sidemarkers flash at
the same time.

I have a wiring diagram and it shows the side markers controlled by
the ICU. The wiring diagram shows the ICU residing in the fuse panel.
Can anyone shed some light on the ICU? Is it part of the fuse block?
Is there something else besides the ICU that can cause these syptoms?

-RC

 

before you presume the icu is at fault, eliminate the other more likely
causes. if two bulbs go on where only one should, it's usually a bad
ground connection forcing the first bulb to ground through the second,
hence they both light. same for when one bulb goes out when another
goes on.

you should spend some time making sure all the grounding points are
clean, tight and electrically conductive. if still not working, then
you should also spend time with a meter checking the lighting circuits.
finally, if you're not getting signal, then come back to the icu, or
more specifically, the turn signal relay which is mounted next to it.
very unlikely it's at fault - it usually either works or it doesn't - it
doesn't discriminate about what it decides will light and when.

 

You're right, I should check the grounds first. Anytime in the past
I've seen bulbs light up that shouldn't, it has been a ground issue.

When I printed out the wiring diagrams I was expecting to find a
common circuit that would lead me to the problem ground. The side
markers share grounds with the front turn signals, which are working
properly (G201, G301). I'll make sure those grounds are working. The
ICU and turn signal relay also share a ground G401. I'll check that
too.

I have this sneaking suspicion it isn't the grounds though.

-RC

 

which are much higher wattage. you can ground a low wattage side marker
bulb through a high wattage turn signal bulb and be none the wiser.

i'll bet you a donut.

 

You're on. In fact, show up for this and I'll make it a dozen. ;-)

http://krispykremechallenge.com/

I should have figured it out by then. I've got a year before it has to
pass inspection again. I can't believe they let it go through this
time.

-RC

 

Headlights OFF, the side markers are supposed to flash together with the
signals. Headlights ON, they should flash alternately with the signals.

That's a different circuit; it does not pass through the ICU.

It's behind the fuse block, on the left-hand side.

My reading of the EWD says that the Side Marker Relay Circuit is bad. I'm
not sure this is a plug-in relay like the Turn Signal one. You may need to
get a replacement ICU from a wreck.

 

If the grounds are indeed at fault, you're looking inside the taillamp
assemblies for the cause. Any corrosion in the sockets?

I don't think it's the grounds either. The lights are dead unless power is
fed to them.

 

I had a vehicle awhile ago that had most of the same symtoms and then
some. It was the turn signal switch in the steering column that caused
everything. Swapped it out and everything was fixed.

 

In this case the signal switch is way upstream from the various splits
and convergences of the flasher circuit. It's a simple left-off-right
switch. There's no way it could cause these symptoms.

-RC

 

It may take a few days (or weeks) to track down the root cause. This
vehicle spends a lot of time away from the driveway where I can't get
to it. I'll post when I know more.

Meanwhile it's 35F, breezy, and I've got leaking seals to contend with
on another vehicle. Time to put on the long undies.

-RC