T.C.U. Failure? **Paging all the Electrical gurus (schematics inside)**

[Dattebayo]

Okay, I've been tracing my "door open" warning light problem for the last couple days and I've come to a point where it's beyond my abilities...

I traced both door switches to the T.C.U. and can't check the harness in between just yet. Please look at this schematic:



If both switches are normally open when the door is closed, since both of them have to close the circuit to shut off the warning lamp, this means that either (A.) there is a short in the harness between the T.C.U. and one or both of the door switches, or (B.) that the T.C.U. is bad, right?

[MinisterofDOOM]

Every time I've ever had a door switch fail, it has failed in a way that left the dome light always OFF, not always on. So they fail "off." I'm not sure if they're normally open or normally closed, but whichever they are, they fail to "normal." So a bad door switch should not leave the dome light stuck on.

[Dattebayo]

Re-spoken: The door switches are just fine, as is the ground they attach to.

And I lost my multimeter to test for signal, this is why I can't check the harness just yet.

[Red coupe]

If it is a normally open switch would a short really mess it up?Wouldn't a short circuit basically bypass the switch, closing the circuit and effectively doing the same thing the switch would do when the door closes in the first place?

How are you testing the switches with out a multimeter?

[Dattebayo]

If it is a normally open switch would a short really mess it up?Wouldn't a short circuit basically bypass the switch, closing the circuit and effectively doing the same thing the switch would do when the door closes in the first place?

Edit: No, the door closes and interrupts the circuit. I mean "normally open" as being the door is usually closed. Sorry.

Which in this case would make it appear as if the doors were open.

How are you testing the switches with out a multimeter?

The switches are brand new OEM out of the box. Tested on another vehicle.

[Red coupe]

Edit: No, the door closes and interrupts the circuit. I mean "normally open" as being the door is usually closed. Sorry.

Which in this case would make it appear as if the doors were open.

You sure? I don't do a lot of electrical diagrams, and the ones I do do use more basic line symbols but....The one on the left sure looks like it would be normally open compared to the the one of the right which looks like it would be normally closed... It seems like it would make more sense to use a normally closed switch so that when the door opens and releases the switch it would close the circuit for the light, but it really just sending a signal for the computer to interpret anyways.

Looks pretty normally open to me.

[Dattebayo]

Thanks for being so thorough, but the door switch operation is "circuit open when door closed".

t really does make sense that the schematic would show the status with the car at rest with the doors shut.
Modified by Dattebayo at 6:32 AM 3/30/2010

[EazyBreazy]

If the switch is normally open when the door is closed, then its possible you either have a short between the two sections of wire, or a short to ground somewhere. Is this only for the drivers side? Is there any way you can isolate it to a specific switch/area of the car? Check all wires up-current from the switch to the dome light as it sounds like the problem is after the dome light.

[Dattebayo]

Thanks!

Yeah, I disconnected both switches and the "door open" light was still on. Can't get to the harness because both seats and the carpet has to come out. This is why I was asking if anybody had any issues with a TCU causing it...

[LAKE ERIE Z32]

i know my light comes on when i pop my hatch in my z32 did you also try that one?

[EazyBreazy]

Its possible datte. What's in the time control unit, is it possible that its gotten wet?. Do the foot/step lamps stay on too, or just the dome lamp? it may be possible that the illumination control relay is boogered up. Just throwing out random options that could be the problem lol

[Dattebayo]

i know my light comes on when i pop my hatch in my z32 did you also try that one?

According to the FSM, the only lights that come on when the hatch is open is the dome light and the hatch light. Thanks tho!

Its possible datte. What's in the time control unit, is it possible that its gotten wet?. Do the foot/step lamps stay on too, or just the dome lamp? it may be possible that the illumination control relay is boogered up. Just throwing out random options that could be the problem lol

The foot lamps are removed, but they would be active all the time, the same with the dome light, but I have that set on always off right now...

The illumination control relay wouldn't give a "door open" signal back to the TCU, would it? I mean would the warning light be on because of this?

[EazyBreazy]

datte I'm totally unfamiliar with what the TCU does lol, I always thought the dome lights were hard wired in without passing through anything.. without a schematic of the tcu circuitry it'd be hard to rule it out. its possible that it is just passing though from a 12 volt source, but I don't know lol.

[ZOMG.SR20]

id bet there's a wire shorted out before the switch and is grounding out is it intermittent or constant

[Ace2cool]

Start hitting things.

[s0m3th1ngAZ]

Modified by ScorchedNX2K at 6:33 PM 3/30/2010

[USMCgetsome]

if it was a short your battery would be drained by now and fuses would be popping somewhere

[s0m3th1ngAZ]

Really need a meter to check the pinouts on the TCU...But a short would cause your battery to get drained or pop that fuse up top.Dangit.

[EazyBreazy]

If it's shorted after the load on the grounding leg it will not pop a fuse it will act just like the switch is activated and ground the circuit. The dome light takes alot of time to drain a battery, even more when its switched off from the dome light itself.

There's essentially 2 switches to activate the lights, one on the doors and the one on the light, If the light switch is on and the door is closed the light won't be on, If the light is off and the door is open the light won't be on.

Datte, Is there a possibility that the 3 way switch on the dome light is broken and it will only go to either off or on instead of off, door, or on?

[Red coupe]

Why not go to the connector on the TCU and just run a bit of wire from each of the door senors to a ground.

[Dattebayo]

TBH that's why the OP is worded like it is... I was looking for an educated guess on the matters proposed.

TCU and harness leading from it are not easy to get to at all, so I ask so I save trouble based upon real experience. Thanks.

[Infinitiguy19]

Isn't the Time Control Unit only in charge of the dome light fading when the door is shut?

Look at the illumination part of the service manual not the TCU.

Which if any fuse is blowing?

Look in the service manual to see the Power Distribution Layout, That will tell you what each fuse controls...

Electrical problems SUCK and I KNOW.

[Dattebayo]

That's just it, a fuse doesn't have to blow to make this kind of thing happen...

Look at the schematic I posted, the TCU has a path to make that happen.

[ZOMG.SR20]

your probably going to have to trace the whole thing from the TCU to the switches sadly