H3 Rear Turn Signal and Tail Light Issues - BCM issue?

A couple weeks ago when turning right and using the turn signal I got the fast blinking signal. I assumed that meant I had a bulb go bad. Before I could get it changed the same thing happened to the left signal. Both front signals work, but neither rear signal works. On top of that the rear lights come on, but do not brighten when braking. Again, makes me think one of the filaments in the bulbs has gone bad. I figure I have some burnt bulbs and replaced them. Well, lo and behold, new bulbs have the same issue. The left rear turn signal does not work, but when braking the light works. The passenger rear light now stays on even when the car is turned off and does not work braking or with the turn signal.

I checked all wiring, connections and fuses and everything looks good. I don't think it is a socket since both are acting up at the same time just in different ways. So I'm thinking it is the Body Control Module (BCM) that has a glitch and needs replaced. Any other ideas?

I've never pulled the BCM and tested it before, but I can't imagine it is too difficult. Any experience with testing or replacing the BCM?

Did you check the harness for the trailer wiring below?
Mine was bad when I bought my H3.

It tends to short out.

Yes. I install rear bumpers all the time, and the harness is very prone to chafing. Check the harness where it passes over the seam of the body, under the rear hatch. It's easiest to see with the bumper removed. Also inspect the trailer harness (if you have the tow package). Both of these are quite often rubbed nearly in half!
Additionally, I see some pretty horrific aftermarket hitch installs, so if a truck accessories shop has added a hitch...it is most likely a hack-job with crimp connectors (a huge no-no around here). Lastly...squirrels and chip-monks love the taste of wire insulation!

I have taken a brief look at the tow harness and it looked good. I'll take a more thorough look though to see if that's an issue.

I just had an issue with some shorting with the wiring by the coils which was causing the ignition fuse to keep blowing so it very well could be the same type of issue here.

Wasn't there issues with the brake pedal switch? Not sure if this matched the issues Bigunit had.

Checked all wiring harnesses and brake pedal switch and everything is good. Tested BCM and sure enough it is the culprit. Ordered a new BCM today which should be here tomorrow. I'll have a repair shop program it.

For future reference, how does a person test a BCM?

For future reference, how does a person test a BCM?

Click to expand...

Replace it with a known good one. If the problem goes away...it was the BCM.

4speedfunk said:

Replace it with a known good one. If the problem goes away...it was the BCM.

Click to expand...

Haha well I suppose that's true

You can check the connections and pins on the connector with a volt meter. For the tail light/turn signal issue the pins were B1 and B2 on connector C1 I believe.