Tail lights/dash light issues in '04 GT

[elron8711]

I have an '04 GT that is having some electrical issues. Both the tail lights (running lights, not brake) and the dash cluster lights are out. I have checked all fuses and the bulbs in the tail lights. I decided to try replacing the headlight switch, but that didn't help. However, it did point me in a different direction. I realized that I could get all the lights to work using both the old and new switch if I got the switch in just the right position between park (running lights) and on (headlights). I thought it was odd that both switches would be "broken", but then I thought about it some more. My current thinking is that with both switches I am able to get them into a position where both circuits are "on", which is causing the lights to come on. This is not the way that the lights should work obviously, but I don't really know where to go from here. Is there a short somewhere that is eliminated when both circuits are live? Any input is appreciated!

Thanks!

 

[wmburns]

Please answer all questions and perform all tests. The results are important and an incorrect answer is going to result in a wild goose chase or replacing good parts.

I am confused on a couple of points. The post appears to call out problems in the exterior Lamp circuit and the interior illumination circuit.

Regarding the running lights, confirm if the front parking lights, license plate, and side markers do/don't work. I take it from the post that the rear parking lights do not work.

Regarding the interior illumination. I take it from the post that the cluster illumination is dark. What about the other smaller lights at the AC/heat function switch, rear window defrost switch, transmission control switch, and radio?

The interior illumination uses ground G204 which is located behind the dash near the center console. This ground is also shared by the radio, cluster, and the GEM.

Has any recent work been done behind the dash? Big Radio install for example?

I can see a problem with ground G204 causing many of your symptoms but not all. My gut tells me that you have two problems.

The other is a blown fuse. Have you actually checked all of the fuses in the Central Junction Box (CJB) and Battery Junction Box (BJB)? It can sometimes be hard to tell from above if a fuse is blown. There are many post of ppl that thought a fuse was good only to find it blown when pulled.

For the sake of avoiding a wild goose chase, pull fuse F2.37 in the CJB and actually test it with an Ohm meter. Pull Fuse F1.10 in the BJB and test it as well. Post.

Battery Junction Box (BJB) - located engine bay left hand side.
Central Junction Box (CJB) - located left hand driver's kick panel.

 

[elron8711]

Thanks for your reply wmburns. I will answer as much as I can now, but will need to check on a few things after work.

Regarding the running lights, confirm if the front parking lights, license plate, and side markers do/don't work. I take it from the post that the rear parking lights do not work.

None of the exterior lights that should come on when the switch is in the "park" position come on. This includes tail lights, front parking lights, and side markers.

Has any recent work been done behind the dash? Big Radio install for example?

Nothing, the car is entirely stock. Nothing other than oil changes have been done in the few months.

The other is a blown fuse. Have you actually checked all of the fuses in the Central Junction Box (CJB) and Battery Junction Box (BJB)? It can sometimes be hard to tell from above if a fuse is blown. There are many post of ppl that thought a fuse was good only to find it blown when pulled.

I manually checked every fuse in the CJB, and several in the BJB (in both cases, with a fuse tester). I can run through them all again tonight to make sure I didn't miss one but I'm pretty sure I got them all (at least in the CJB).

For the sake of avoiding a wild goose chase, pull fuse F2.37 in the CJB and actually test it with an Ohm meter. Pull Fuse F1.10 in the BJB and test it as well.

Will do

I drove to work this morning with the switch in that halfway position and the dash lights stayed on, along with the headlights as far as I could tell, for the whole drive.

I forgot to mention this before, but the headlight audio warning is also dead, in both regular switch positions, but works in that in-between position. It looks like anything that would go on when the switch is in park is dead unless I get the switch in-between positions. I did test the old switch with a multimeter and it was fine, per the table in the Haynes manual. However I still bought the new switch "just in case" and found the same behavior.

Thanks again for your reply!

 

[elron8711]

Regarding the interior illumination. I take it from the post that the cluster illumination is dark. What about the other smaller lights at the AC/heat function switch, rear window defrost switch, transmission control switch, and radio?

Sorry, I missed this in my last reply. The cluster is dark, as are all lights other than the radio (environment controls, trac. control, fog light control, defrost control). These switches still work though (i.e., the fog lights still turn on and off, just the illumination of the switch is dead)

 

[wmburns]

I forgot to mention this before, but the headlight audio warning is also dead, in both regular switch positions, but works in that in-between position. It looks like anything that would go on when the switch is in park is dead unless I get the switch in-between positions. I did test the old switch with a multimeter and it was fine, per the table in the Haynes manual. However I still bought the new switch "just in case" and found the same behavior.

The head light reminder is a function of the GEM.

Recall what I said about the interior light grounding point. A bad ground G204 would cause all of the interior lights to fail.

Further, a bad G204 ground would cause problems with the GEM.

Any "rodent" activity? Any water damage? Any collision damage?

Either G204 is bad, there is a loose connector, or the GEM is bad.

I would start with a resistance test from the cluster to battery negative. Perform a resistance test from cluster pin #1 (BK) to battery negative. Post.

I foresee a lot of upside down time in your future.

 

[elron8711]

Any "rodent" activity? Any water damage? Any collision damage?

None to all of the above, that I know of

Either G204 is bad, there is a loose connector, or the GEM is bad.

The red/blue wire from the headlight switch harness is loose. I had discounted this as even when I hold it in tight the problem persists. I'm not sure how to fix this... the wire is attached to the connector, but the connector is not tight in the harness... it is easy to remove.

I would start with a resistance test from the cluster to battery negative. Perform a resistance test from cluster pin #1 (BK) to battery negative. Post.

I'll try this tonight. The electrical stuff isn't my strong point, but I've got wiring diagrams in the Haynes manual, so hopefully that will suffice.

I foresee a lot of upside down time in your future.

Sounds likely

Thanks!

 

[wmburns]

Here's another take on the problem. I am posting diagram 92-1 to show the relatively few places that the exterior lights can TOTALY break down.

Since you know how to access the head light switch (C205), here are some tests that may be easier for you.

Confirm +12 volts at C205 B1 (DB/OG). HOT at all times.
Confirm +12 volts at C205 B2 (TN/WH). HOT at all times.
Confirm +12 volts at C205 R1 (BN) with the head lights in the park position.
Confirm +12 volts at C205 D1 (DG/WH). HOT at all times.

Post the results. Be careful as many of the head light circuits are high amp and HOT at all times.

Note, I can not find a Red/blue wire on the head light switch (C205). How about some pictures?

 

[elron8711]

Since you know how to access the head light switch (C205), here are some tests that may be easier for you.

Confirm +12 volts at C205 B1 (DB/OG). HOT at all times.
Confirm +12 volts at C205 B2 (TN/WH). HOT at all times.
Confirm +12 volts at C205 R1 (BN) with the head lights in the park position.
Confirm +12 volts at C205 D1 (DG/WH). HOT at all times.

Post the results. Be careful as many of the head light circuits are high amp and HOT at all times.

Here are the results of testing C205. I tested in 4 positions: off, park, on, and the in-between park/on that allows all lights to work.

       Off     Park      On      In-between
B1:   12       12       12           12
B2:   12        0        0           12
R1:    0        0        0           12
D1:   12       12       12           12

Note, I can not find a Red/blue wire on the head light switch (C205). How about some pictures?

The red/blue wire is B1

 

[wmburns]

Here are the results of testing C205. The red/blue wire is B1

IMPORTANT. Pins B1, B2, and D1 are inputs to the head light switch.

How can this work as designed if the wire connected to one of the input power sources is broken/disconnected?

One of two things may be happening. Either when you held the wire in to test it did not make good contact or you held it to the wrong location. Perhaps there is a break inside the connector.

Or you have made an error in reading the diagram for connector C205. It does concern me that the wire colors do not match. Perhaps looking at the mirror image of the connector? If so, this make all of the above tests invalid.

Bottom line, based upon your reported symptoms and looking straight at a disconnected wire, that would be a hard fact to ignore.

Let's go back and double check the work. A mistake here will result in a wild goose chase.

 

[n0v8or]

Let me see if I can be slightly helpful here.

I design automotive electronics as a profession, and let me say from the outset that wmburns' electrical understanding, troubleshooting approach, and demonstrated clear-thinking are superior to half the Engineers in my lab. I wish I could hire him to work here. Even more impressive, his extensive automotive knowledge extends to the mechanical side of things as well. We are really lucky he is willing to spend so much of his time helping fellow StangNet members. He single-handedly solves more engineering problems a week than I do in a year.

As soon as I read the very first reply (msg 2 in this thread), I sensed he was on the right track, and have been following with rapt attention to see the resolution.

wmburns, I think your first inclinations were likely accurate. Two things jump out at me:
The word "fuse tester", and the meeasurement results that indicate terminal B2 goes to zero when loaded.
Consider a fuse that has corroded in its socket, or become high in resistance, but not completely open (I have experience both in vehicles I have owned). It may test "good", but represent 100 ohms of resistance. Or the fuse could be just fine, but not making good connection.

I suggest trying another fuse before climbing under the dash again. If nothing changes, go back to following wnburns' advice. You are in good hands.

I think the Blue-Orange wire at B1 looks like Blue-Red in certain light.

 

[elron8711]

The wire in question isn't actually broken, it just pulls out of the connector if I am not careful handling it. When it pulls out the headlights go off, as would be expected. For now the wire is in place and seems to be staying in place properly.
I double-checked the diagram you sent against the markings on the headlight switch itself and am confident that the correct wires were tested. I tested once with the switch removed from the connector, then realized it needs to be connected in order to test with the switch in the on position to verify the voltage at R1. All the data I posted was done with the headlight switch attached to the connector and the wires in place. With each position tested, I verified that the expected lights, interior and exterior, were lit.

 

[elron8711]

Let me see if I can be slightly helpful here.
wmburns, I think your first inclinations were likely accurate. Two things jump out at me:
The word "fuse tester", and the meeasurement results that indicate terminal B2 goes to zero when loaded.
Consider a fuse that has corroded in its socket, or become high in resistance, but not completely open (I have experience both in vehicles I have owned). It may test "good", but represent 100 ohms of resistance. Or the fuse could be just fine, but not making good connection.

This is definitely possible. The fuse tester in question is one of those cheap plastic things that comes with the multi-fuse pack. I can test these again with the multimeter to verify the correct resistance.

I think the Blue-Orange wire at B1 looks like Blue-Red in certain light.

That could definitely be the case. More lighting would definitely help.

Thanks!

 

[wmburns]

n0v8or, thank-you so much for the kind words. Real nice ego boost. :grouphug:

I have got to say that your thought about it being a corroded fuse holder certainly makes sense. Of course this is one of the problems working with ppl over the internet. Giving too much information vs giving just enough information. The recommendation to "check the fuse" is as far as it got.

I have to honestly admit to not fully looking at the OP's last post. When I saw the mismatch in wire colors, I quickly assumed there was a mistake.

Which leads to another great piece of detective work. Blue-Orange wire at B1 looks like Blue-Red in certain light.

I still have issues with the reported symptoms. Per the wiring diagrams if B1 were NOT connected, then the head lights should not work. Nor do I understand from the OP how the wire could be so loose yet so easily replaced. My gut say the wire being high amp is fairly heavy and needs a mechanically sound connection to avoid over heating.

Which leads to another possibility. It would seem to me that any electrically weak high amp connection should get hot. Perhaps hot enough to feel.

Working on these posts is a little like reading a detective novel. There are "clues" and "suspects". The clues are often incomplete. Sometimes completely inaccurate. The experience level of the OP can vary wildly. Which can present a set of challenges all on its own.

IMO, the single biggest mistake ppl make is forgetting that someone reading your post know nothing special about your car other than what is in the post. The author is working knee deep and has a full history of everything done. In effect, he/she may be too close to the problem.

The responder sometimes has to re-hash simple things because who wants to spend hours looking for a problem when it may be a very simple thing over looked.

Anyway, it's all fun when the answer is finally found. It's even better with kinds words like this.

 

[n0v8or]

In my view, the expressions of thanks were long overdue.

I have an '03. I went out and pulled fuse 1.10 (30A). It's one of those monster fuses and required a big pair of pliers to extract it. I looked like one of the Three Stooges pretending to be a a dentist Once out, I was able to duplicate all of the conditions described in the original post. I also found the "in-between" spot in the light switch where the park lamps are magically stored. What's up with that I have no idea, but the switch is fairly stable in that position and I would not be afraid to drive around that way.

The fuse element is quite a fat conductor; I can't imagine a high resistance condition without visible evidence. Contact oxidation remains a possibility though, particularly if the car is outside at night in a humid climate.

Any poor connection between the fuse and headlight switch would produce the same effect.

With the switch in the PARK position, can you measure the voltages at both ends of the fuse as well as terminal B2? If both ends of the fuse read ~12V, but B2 is low, it suggests a high resistance somewhere between fuse and switch. If one end of the fuse is low, blame the fuse. This test has to be done with the light switch ON (or PARK) to be meaningful.

Another interesting piece of data would be how low is the voltage at B2 with the switch in PARK. If you drop one range on your multimeter, you may find 0.5V or 1V rather than zero. This would confirm a high resistance in the power path rather than an open circuit.

 

[elron8711]

wmburns and n0v8or, thanks again for all the time you are putting in to this issue. It is much appreciated. Dealing with electrical systems has never been my strong suit, but I'm actually getting into it this time.

I was out of town for a couple days but am back now and will be taking a look at the fuses tonight. I will start in the BJB per n0v8or's suggestion.

Regarding wmburns' comment about not knowing the history of the car, I'll try and give some more background, though there isn't much to give. The car is a daily driver, except during the winter. Mustangs don't make great winter cars in VT. I've put less than 10k miles on it in the last 12 months, and it has had nothing done to it other than oil changes, a new air filter, and new tires. I have had this problem once before, about 3 years ago. I forget exactly how long it lasted, maybe 2-3 weeks, then it went away and everything was good up until about a month ago. At the time I was living in CA and the problem occurred during the winter. I currently live in VT, where there is far more humidity than in CA; that may play a role as n0v8or suggested.

So, I will post more tomorrow after carrying out all the tests that have been suggested.

Again, thanks for the help, it is definitely saving me time and money!
Kevin

 

[elron8711]

With the switch in the PARK position, can you measure the voltages at both ends of the fuse as well as terminal B2? If both ends of the fuse read ~12V, but B2 is low, it suggests a high resistance somewhere between fuse and switch. If one end of the fuse is low, blame the fuse. This test has to be done with the light switch ON (or PARK) to be meaningful.

Another interesting piece of data would be how low is the voltage at B2 with the switch in PARK. If you drop one range on your multimeter, you may find 0.5V or 1V rather than zero. This would confirm a high resistance in the power path rather than an open circuit.

Ok, some more updates...

I did the recommended tests this morning. Fuses 1-10 in the BJB all showed 0 resistance and read ~11V across the terminals. All fuses in the CJB other than 6 and 37 showed 10-12V, depending on if the headlights were on or not. Fuses 6 and 37 showed no voltage. I did this test with the ignition in the ON position (but the car was not running).

I retested C205 as well. With the switch in either ON or PARK, the voltage at B2 and R1 was around 0.7V while B1 and D1 were at ~12V. I also re-tested fuse 10 in the BJB after re-reading n0v8or's post. Again, this fuse tested fine. Testing between fuse 10 and C205 I saw 12V at B2 and R1 and ~0.3V at B1 and D1. This is opposite what I see when I test from C205 to the battery.

Hopefully this is helpful information to continue the troubleshooting process.

Thanks!

 

[wmburns]

A couple of things. Does zero resistance mean zero as in very low (closed circuit) or as in infinity (very high or open)? A fuse normally has some small resistance value. Zero is not realistic unless the Ohm meter is set on a large scale. Use the LOWEST multiplier for these tests. Touch the leads together to ensure the meter is calibrated and zero before making resistance measurments.

I am not positive I understand exactly how the test were run. The results have to be perfectly understood or we will miss the mark.

Set the VOM to the DC voltage scale. Put the black test lead on a known good ground point (battery negative the best).

With the head light switch in the off position and key off. Put the red lead on the head light B2 (TN/WH) terminal. Record that measurement. Put the test lead on the R1 (BN) terminal. Record.

Now put the head light switch into the park position. No half way stuff. Measure the voltage on the B2 and R1 terminals. Post. During all of these tests, leave the black terminal on the battery negative.

What I am suspecting will happen is when the parking lights are turned on, the voltage at B2 will drop. This indicates a high resistance point upstream from B2. If the voltage drops at R1, this indicates the problem is in the connector or switch.

Now pull the F1.10 fuse from the BJB. Take a picture. Shine a light down into the fuse terminals and take a picture. Take a picture of connector C205 (both front and back).

This circuit is fairly straight forward. There are a limited number of places it can break down. It simply has to be right there.

I do have some additional diagrams. However, they are too large/many for me to post. PM your email address.

 

[n0v8or]

I'm attempting to post a simplified schematic in text form. Hopefully the lines will align OK. The word "switch should be directly under the "X", which designates the park lamp contacts within the headlight switch.


+ ----o-/\/\-o---------->>---------->>--o--X--o-->>-------------->
. . Fuse 1.10 . . . . .. . C134 . . . . .C205 . Switch . C205 . . . . . lamps

. red . . . . . . . tan-wh . . . . tan-wh . B2 . . . . . . . . R1 . . brown

Your measurements suggest an inintended resistance somewhere between the load side of Fuse 1.10 and Terminal B2 of the headlight switch connector (C205).

wmburns makes a valid point regarding the resolution of resistance measurements.
There are 8 exterior lamps in the park lamp circuit (2 front, 4 rear, 2 license plate). The resistance of the tail lamp & front park bulbs is ~12 ohms hot, but only ~2 ohms cold. Six of those in parallel is ~0.33 ohm. Factor in the license plate bulbs and interior lights, and the equivalent load resistance is about 0.3 ohms. You measured 0.7V at B2 with the switch on. Allowing 0.1V for ground potential differences, you only need a resistance of less than 6 ohms in the path to cause your problem.

Since trying to track down an intermittent resistance will not be fun, it is important to positively eliminate the easy stuff first. The next thing I would do is swap fuse 1.10 with a nearby identical 30A fuse in the Battery Junction Box, just to see if there is any difference. Next, I would flex the big harness that runs from the BJB to the firewall, and see if the 0.7V reading changes.

 

[elron8711]

I'm finally getting back to working on this after a busy week. Have had the Mustang apart in the garage all week and been driving the Jeep. Time to try to get this fixed now

A couple of things. Does zero resistance mean zero as in very low (closed circuit) or as in infinity (very high or open)? A fuse normally has some small resistance value. Zero is not realistic unless the Ohm meter is set on a large scale. Use the LOWEST multiplier for these tests. Touch the leads together to ensure the meter is calibrated and zero before making resistance measurments.

All fuse measurements showed a small resistance (0.01 - 0.03 ohm) on the 20ohm scale on the multimeter.

I am not positive I understand exactly how the test were run. The results have to be perfectly understood or we will miss the mark.

Set the VOM to the DC voltage scale. Put the black test lead on a known good ground point (battery negative the best).

With the head light switch in the off position and key off. Put the red lead on the head light B2 (TN/WH) terminal. Record that measurement. Put the test lead on the R1 (BN) terminal. Record.

B2: 12.21V
R1: 0.00V

Now put the head light switch into the park position. No half way stuff. Measure the voltage on the B2 and R1 terminals. Post. During all of these tests, leave the black terminal on the battery negative.

B2: 0.00V
R1: 0.00V

What I am suspecting will happen is when the parking lights are turned on, the voltage at B2 will drop. This indicates a high resistance point upstream from B2. If the voltage drops at R1, this indicates the problem is in the connector or switch.

B2 does indeed drop, from 12V to 0V. There was no voltage at R1 in either position (off, park)

Now pull the F1.10 fuse from the BJB. Take a picture. Shine a light down into the fuse terminals and take a picture. Take a picture of connector C205 (both front and back).

I will send you the pictures in a PM, along with my email address.

Thanks!

 

[elron8711]

Since trying to track down an intermittent resistance will not be fun, it is important to positively eliminate the easy stuff first. The next thing I would do is swap fuse 1.10 with a nearby identical 30A fuse in the Battery Junction Box, just to see if there is any difference. Next, I would flex the big harness that runs from the BJB to the firewall, and see if the 0.7V reading changes.

Swapped fuses, no effect.

I flexed the wiring harness wherever I could and didn't seen any change in the reading on the multimeter, and no indication that the lights were coming on.