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BOBBYSS
BOBBYSS, Master Technician
Category: Chevy
Satisfied Customers: 17144
Experience:  ASE MASTER CERTIFIED, AA DEGREE AUTO/DIESEL TECH
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I have a Chevy Trailblazer 2005 2WD 6 cyl. been having

Customer Question

Hi Dan, I have a Chevy Trailblazer 2005 2WD 6 cyl. been having electrical issues, windshield washers and headlights are ON while engine is OFF. Turning engine ON will turn off the headlights (I havent tested the washers yet). Light control knob has no effect on the headlights, but other lights work correctly.
Submitted: 1 year ago.
Category: Chevy
Customer: replied 1 year ago.
The battery is brand new and after swapping the BCM the next morning it wouldn't crank even once, it was just silent. Everything else worked except the engine. Tried a few times then left for work. In the afternoon it started up without any hesitation.
Expert:  BOBBYSS replied 1 year ago.

Hello, my name is Bobby. Thank you for the question. The most common causes of this are either a failing ignition switch or a bad body control module. The most common would be the module on this vehicle. There are other possible issues such as connections but those are not as common. Did you plan on fixing this yourself?

Customer: replied 1 year ago.
Hi Bobby. I surely would like to fix it myself :)
I did try to do very basic test of the ignition switch, these are the voltages I wrote down while turning the light control:With engine off (so the lights are on at all times):
knob on "lights on position": orange 0.2V, red 12V, light yellow 0.2V
parking lights position: orange 2.6V, red 12V, light yellow 2.6V
lights auto position: orange 2.6V, red 12V, light yellow 2.6VThe last cable in the row is a bit darker than the light yellow and it's ground, didn't test the 5th cable (orange?) which sits in the second row. Let me know how to test it more thoroughly.I just remembered about this yet another weirdness - this has been going on for months, when turning the engine off the doors would get automatically locked 90% of the time. Note that changing the gear to/from park controls the locks just fine, as well as my remote control and door controls.Regarding BCM - as I wrote, I replaced it with a friend's BCM from similar model (just 4WD but same year/engine) and it didn't help.
Expert:  BOBBYSS replied 1 year ago.

Thank you. From the readings that you show from the ignition switch it appears to be bad. I am assuming that it was still hooked up when you tested it. As for the BCM, one from another vehicle will not work. These are programmed to a particular vehicle and if you move it from one vehicle to another it must be reprogrammed by the dealer.

Customer: replied 1 year ago.
OK, I will test the switch again, can you explain which readings make you think it's gone bad? or what should I look for to re-confirm this before I go and order new one.
Expert:  BOBBYSS replied 1 year ago.

With the key off the red wire and the red/white wire should have voltage. With the key in the run position every wire except for the yellow wire should have voltage. In crank the yellow should then have voltage. When I say voltage I mean battery voltage.

Customer: replied 1 year ago.
Great, give me 4-5 hours before I can repeat the tests.
Customer: replied 1 year ago.
The previous measurements were taken from a socket just underneath the steering column, just behind the steering wheel. Maybe that wasn't the right place.Now I repeated all tests using a harness slightly further along the column, because I had trouble removing the top steering column cover, namely the right hand side of it. The harness has 3 thick cables (white/red, orange & yellow) on the left hand side and 4 thick ones (brown, pink, white, red) on the right hand side and some mixed ones in the center. Here is a photo I found:
http://i260.photobucket.com/albums/ii8/Rhino_66/trailblazer_harness.jpgEverything is fine with the headlight driver module removed. When I put it in I'm seeing the following:LEFT HAND SIDE, ignition off
red/white - light switch: off/park/on = 12.70 / 12.63 / 12.63
yellow - auto/park/on & off/acc/run = 0
orange -
ign off: off/park/on = 0.4 / 0.4 / 0.02
ign acc: off/park/on = 12.6/12.6/12.6
ign run: off/park/on = 12.6/12.6/12.6RIGHT HAND SIDE:brown (acc2)
off 0 / 0 / 0
acc 12.8 / 12.8 / 12.8
run 12.6 / 12.7 / 12.7pink (ign1)
off 0 / 0 / 0
acc 0 / 0 / 0
run 12.6 / 12.6/ 12.6
(crank has voltage)white (ign2)
off 0.46 / 0.46 / 0.02
acc 12.6 / 12.6 / 12.6
run 12.6 / 12.6 / 12.6
(crank has voltage)red
off 12.7 / 12.6 / 12.6
acc 12.63 / 12.56 / 12.56
run 12.55 / 12.55 / 12.55
Expert:  BOBBYSS replied 1 year ago.

Those readings appear to be fine. I do think that this is a BCM failure.

Customer: replied 1 year ago.
Please be a little more informative, I was expecting you will tell me briefly where it is and how to know for sure that this part is failing.Assuming the BCM is the "circuit board" which is vertically clipped next to the fuse box under back seat and connected with 3 connectors, I disconnected them in order (longest, medium, short) and lights were off completely, I think I also checked remote locks and they didn't work. I then put in the other board and connected in reverse order and everything worked the same as with mine. I didn't start the engine. I then went and put my BCM in the other car and that car's lights worked fine. Unless you can explain to me how this test wasn't good enough or the board I swapped is not the BCM, and the real BCM is still bad and you tell me how to verify it, I will have to ask for another expert or cancel this question because I'm not getting closer to finding the solution.
Expert:  BOBBYSS replied 1 year ago.

Lets get back really quick to the initial problem. Your previous comment about putting in the other BCM was a little confusing. So when you turn the vehicle off the washer and the headlights come on immediately and will stay on until the battery dies?

Customer: replied 1 year ago.
I waited 40 seconds after locking the car and they were still on (back washer & headlights)To summarise everything, these components are ON:
Engine off - back washer, headlights
Acc - front wipers & front washer
Run - front wipers & front washer
Engine on - front wipers & front washerFront wipers have 2 speeds, slow continuous on all positions until last position which is a bit faster.
I wonder if this "mess" is connected or is it due to a number of failures which happened at similar time. I hope it is clearer now, if you need any more specific tests just let me know.
Expert:  BOBBYSS replied 1 year ago.

Do me a favor, when these systems come on with the vehicle turned off, pull the connections from the BCM and see if they turn off. If not then the fuse box is more than likely the issue. If they turn off then it may be a short in the wiring from the BCM.

Customer: replied 1 year ago.
It's weird cause I'm pretty sure when I disconnected it few days back the lights turned off, but today no, they didn't and the back washer also remained on. So can you give me some tips about looking for the issue in the fusebox?
Expert:  BOBBYSS replied 1 year ago.

In order for these to stay on that means that their relays are either staying on or there is a short someplace else. We need to narrow it down. Can you see if they turn off if you remove their perspective relays.

Customer: replied 1 year ago.
There is only one relay for both headlights, and they turn off without it. I put back another relay with same part number and they are on again so the relay seems to be fine, unless they are both fried?I tested the voltage on the control pin (5) with the light sensor covered:Engine off (lights on)
light control position: lights auto: ***** ***** lights: 0.5V, lights ON 0.04V
pin 4 going to lights has voltageEngine on (lights off)
light control position: lights auto: ***** ***** lights: 12V, lights ON 12V
pin 4 has no voltagepin 2 (+12) and 6 (ground) are good at all times.
Expert:  BOBBYSS replied 1 year ago.

But will that headlamp relay turn off when you disconnect the BCM or does it still stay on?

Customer: replied 1 year ago.
I'm a bit confused by your question but went to check and both pins 4 & 5 have same voltage readings before and after disconnecting the BCM. Does that answer your question?
So does it now mean that the fault is on the control wire between BCM and front fuse box? ... or am I lost in the woods?
Expert:  BOBBYSS replied 1 year ago.

Let me clarify as I think we are getting closer. When you turn the vehicle off the headlamp relay turns on. At that point when you disconnect the BCM does the relay stay on or turn off?

Customer: replied 1 year ago.
Before I go to check that, is the light sensor connected to the bcm? Can I disconnect it easily because I think it may be making it more difficult to diagnose it...
Expert:  BOBBYSS replied 1 year ago.

It does go through the BCM. If you cover it up it will be the same as disconnecting it.

Customer: replied 1 year ago.
But that will trigger the lights to remain on so I don't expect any change on the relay... should I then try to turn the lights off manually and see if that changes anything?? But the light control is probably also going through BCM so is it still useful to perform such test in the dark? Maybe it's easier to point some light on the sensor, wonder if a flashlight will be enough, never tried.
Expert:  BOBBYSS replied 1 year ago.

No, a flashlight will not typically work as it needs UV. Even at night the lights should only remain on for a short time until the BCM times out. From what you are telling me the lights will just remain on indefinitely anyway, correct?

Customer: replied 1 year ago.
Well, something strange just happened but as I'm at work I couldn't diagnose it fully,
I was checking the pins on the relay after removing the BCM and I believe I heard a "spark/pop", unfortunately there are some road-works nearby so a lot of background noise and I was looking at the meter not at the relay so I am not sure where it came from, anyway... the headlights died completely (doesnt matter if engine is on or off). There is 12V on control pin and close to 0 on the one going to the lights (so no difference there). Car starts normally. Checked the other relay (fan clutch) in headlight relay slot, same readings, headlight fuse is good. Aaargh :(
Customer: replied 1 year ago.
Sorry, forgot to answer your question - yes, they would remain indefinitely (i checked up to 40s few posts before).
Expert:  BOBBYSS replied 1 year ago.

So are the headlamps not working at all now?

Customer: replied 1 year ago.
Correct, just the headlamps, parking lights are fine.
Expert:  BOBBYSS replied 1 year ago.

I am sorry to put you through all of these tests but I have one more thing I would like to know. When the vehicle is off and the headlamps turn on what happens if you hit the brights/highbeam switch?

Customer: replied 1 year ago.
Dont worry, I want to fix it so just keep sending me your ideas.I'm afraid you are confused, the headlights stopped working altogether, they would not turn on with engine off or on. Parking lights and high beams work perfectly fine. I will test the headlights with a simple jump wire to see if the bulbs are fine but after work, in about 5 hours.
Expert:  BOBBYSS replied 1 year ago.

Let me know. At this point with the key on I would like to know I if two of the terminals in the headlamp relay show voltage and if the other two show ground.

Customer: replied 1 year ago.
Key in ON position with engine off and light control turned to ON, I've tested the HDM (46) and got voltage on pin 2 (B+) and 5 (control), ground on 6 but the one going to lights (4) has some 0.05V on it. I plugged the HDM it in the other chevy and it's working fine. Also, guess I should've mentioned this before but I complete forgot since it goes back >5 years - this car used to have xenon lights but they were replaced with standard bulbs later on, yet some extra wiring remained. There's also a small metal box, I think it's called a ballast, I had one of those go bad which caused one of the head lights to stop working. I looked around those cables, moved them around but I didn't get any power to the head lights. I' guess I'll be looking there for more clues soon, right?
Customer: replied 1 year ago.
Just to add that I still have the old xenon (proper name is HID?) high beams and that's why 2 ballasts remained but low beams (head lights) are standard halogen ones.
Customer: replied 1 year ago.
Spent some quality time with the car, found a busted fuse just next to the HDM (headlight driver module/relay). Replaced it and got my previous state back (headlights on when engine off). Then I went on to investigate the wires at the front fuse box and at the rear one. Disconnected the battery and tried to lift the front fuse box by releasing the 5 or 6 latches around it but the cables didn't allow for much movement. Then tried to check the continuity of the pink/white wire going from front fuse box to the BCM and found it in perfect condition and this is where the fun starts. I also noticed the next pin on the bcm connector also has continuity with control pin on the HDM, and the next one as well, and then a few more. ??? Noticed that B+ pin and ground pin on the HDM have continuity with slight resistance, I think around 4 Ohms. Tried to reconnect the battery and got a few sparks so I stopped. Disconncted the + cable going to the fusebox and tried to reconnect the battery and this time no sparks. Tried poking around the cables to see if the small resitatnce between B+ and ground on the HDM would change but it wouldn't. Poked under the dash and near the back fuse box, nothing. I wasn't sure if its safe to reconnect the battery so I left it. I'm very curious how "big" is the short, will it drain my battery overnight? would it be dangerous to leave the car with battery connected overnight? I'm guessing that if it was a complete 100% short, I would get much larger sparks... or not??
Expert:  BOBBYSS replied 1 year ago.

That short should blow a fuse if it is on a fused circuit. That may be the biggest issue going on here. You need to find where the wires are shorting together. It may be in the wire loom under the driver side door sill plate.

Customer: replied 1 year ago.
By looking at the wires under steering wheel and near the rear fuse box, it seems the wires go in the center of the car. Not sure how easy it will be for me to track them there. Can there be more wires going under the side door sill plate? Do you have any diagram od the wiring in this car?Also, I checked some other relays/fuses and the short seems to be all over the place. Does it make sense to look for the short between the fuse boxes? Can't it be at the front fuse box?
And finally - so why it doesn't blow any fuse, are these fuses too slow since I did have the battery connected for 2-3 seconds only. Is there any circuit which is not fused?
Expert:  BOBBYSS replied 1 year ago.

If the short is prior to the fuses it will not blow. Most of the shorts I see on this vehicle are in the fuse box under the hood, though they can happen anywhere. If the vehicle has not been in any accidents or anything like that I would suspect the fuse box.

Customer: replied 1 year ago.
It was in an accident with the passanger front door being most affected, I havent seen any other damages anywhere else.Can you tell me how to check the fuse box? Like how do I work with all those cables which don't let me move the box high enough to see what's underneath. I did see 2 thicker red wires coming in on the left side and similar ones on the right hand side but the center part is mistery.
Expert:  BOBBYSS replied 1 year ago.

You need to disconnect the battery and start disconnecting the wires and connectors as it moves further up.

Customer: replied 1 year ago.
I'm stuck on the last two connectors on the right hand side, they have some green plastic in them and letters A B on the side. they are either very stuck or something is not letting it go, any tips??
Customer: replied 1 year ago.
Do i remove the green thingy??
Customer: replied 1 year ago.
next question, how do i uncover the top part of the fuse box, just by prying on the sides?
Customer: replied 1 year ago.
Please reassign the question to another mechanic, I am not satisfied with this level of assistance.