Combination Switch Expert

[Randalla]

I have a 72 240Z that has a screwed up combo switch. Need to determine if it's fixable or if I need to replace it. Blinkers work fine, left and right and front and back. I have no headlights at all on low beam. On high beam, I have a bright passenger side light and a dim driver's side light. Guidance on where to start would be much appreciated. 

[240260280]

If you know basic electronics and can take apart and reassemble a toy then you can fix it.

 

 

[Racer X]

31 minutes ago, Randalla said:

I have no headlights at all on low beam. On high beam, I have a bright passenger side light and a dim driver's side light. 

The blinkers aren't controlled by the combo switch, rather they are switched on a plate at the center of the steering column under the steering wheel.

If the low beams don't come on, look for an open circuit.

The one bright, one dim high beam could be caused by a poor ground.

 

 

[siteunseen]

I lost my gauge lights first then fixed it.

I fixed mine...so it's easy.

You can flip the 2 plates and make it like new. 

Read my post towards the bottom for kamakaziracing rebuild link.

 

Edited December 27, 2020 by siteunseen

[SteveJ]

44 minutes ago, Randalla said:

I have a 72 240Z that has a screwed up combo switch. Need to determine if it's fixable or if I need to replace it. Blinkers work fine, left and right and front and back. I have no headlights at all on low beam. On high beam, I have a bright passenger side light and a dim driver's side light. Guidance on where to start would be much appreciated. 

Two issues:

1. No low beams: The high/low beam switch is not making contact on the low beam setting. Sorry, I don't have it documented about taking it apart to clean/repair.

2. Dim driver's side: The headlights have a common grounding point, so unless the connector at the headlight is badly corroded, it is unlikely to be a ground. Check the fuse for the driver side headlight. Most likely it is blown, so the passenger side headlight is backfeeding through the driver side headlight. Several years ago, I documented how there is just enough current for you to see a dim headlight with a blown fuse. 

[cgsheen1]

Randy,

The power (+12v) to the headlights runs through the two 10amp fuses at the top left of the fuse block. Stock wiring would be Red wire to the right headlight and Red/Yellow to the left headlight. The ground(s) to the headlights come through the selector switch attached to the turn signal stalk. The selector switch swaps the ground between the low beam filament in each bulb ( stock wiring = Red/Black) and the high beam filament in each bulb (stock wiring = Red/White) (I may have those backwards - you know how my memory is).

The selector switch is pretty simple and I can't see how it would deliver ground to one low beam filament and one high beam filament if the wiring to the headlights is all correct.

I would start with the 3-pin switch connector under the steering column. With a multimeter you can verify it has a connection to ground (for completeness of investigation - if the headlights come on at all, they're obviously getting a ground from somewhere) and if it's competing both circuits when switched. It should rock between the the R/W and R/B. Disconnect the 3-pin dimmer switch connector from the dash harness. Check continuity between the ground source and each of the other two wires. When there is continuity to the R/W, the R/B should be open (dead, no continuity) and vice versa. Just make sure that the switching takes place and is reliable and consistent. 

IF the switch is acting properly, then the problem is likely in the wiring (or connectors).

The path of the wiring is from the switch -> 3-pin dash harness connector -> to right side of dash in those myriad of connectors near the glove box (RW and RB are not in the same connector) -> engine bay harness -> right headlight -> left headlight.

The path is straight except for a junction off to the high beam pilot light in the speedometer from the high beam wire in the dash harness.

It's a drag but you can find and separate the connectors along the path and check wire/connector continuity point-to-point.

ONE THING: Incandescent bulbs "are a coil" (positive and negative are connected through the filament) - it's easiest to check their wiring with the bulb unplugged. When the bulb is connected, voltage passes through the filament and you'll see voltage "on the ground side" - which means with all the wiring attached, you'll record voltage all the way back to both RB and RW wires on the dimmer switch and thus to the ground connection. The ground connection here is switched on/off by the headlight combo switch.

Good luck (and know that you can just bring it in and we'll figure it out).

 

Edited December 27, 2020 by cgsheen1

[Randalla]

Thanks guys for the quick response and thank you Chuck for the very through explanation. You've given me an excellent place to start. With this crazy Covid, I've got nothing but time. Very much appreciate the advice!

[SteveJ]

37 minutes ago, cgsheen1 said:

Randy,

The power (+12v) to the headlights runs through the two 10amp fuses at the top left of the fuse block. Stock wiring would be Red wire to the right headlight and Red/Yellow to the left headlight. The ground(s) to the headlights come through the selector switch attached to the turn signal stalk. The selector switch swaps the ground between the low beam filament in each bulb ( stock wiring = Red/Black) and the high beam filament in each bulb (stock wiring = Red/White) (I may have those backwards - you know how my memory is).

The selector switch is pretty simple and I can't see how it would deliver ground to one low beam filament and one high beam filament if the wiring to the headlights is all correct.

I would start with the 3-pin switch connector under the steering column. With a multimeter you can verify it has a connection to ground (for completeness of investigation - if the headlights come on at all, they're obviously getting a ground from somewhere) and if it's competing both circuits when switched. It should rock between the the R/W and R/B. Disconnect the 3-pin dimmer switch connector from the dash harness. Check continuity between the ground source and each of the other two wires. When there is continuity to the R/W, the R/B should be open (dead, no continuity) and vice versa. Just make sure that the switching takes place and is reliable and consistent. 

IF the switch is acting properly, then the problem is likely in the wiring (or connectors).

The path of the wiring is from the switch -> 3-pin dash harness connector -> to right side of dash in those myriad of connectors near the glove box (RW and RB are not in the same connector) -> engine bay harness -> right headlight -> left headlight.

The path is straight except for a junction off to the high beam pilot light in the speedometer from the high beam wire in the dash harness.

It's a drag but you can find and separate the connectors along the path and check wire/connector continuity point-to-point.

ONE THING: Incandescent bulbs "are a coil" (positive and negative are connected through the filament) - it's easiest to check their wiring with the bulb unplugged. When the bulb is connected, voltage passes through the filament and you'll see voltage "on the ground side" - which means with all the wiring attached, you'll record voltage all the way back to both RB and RW wires on the dimmer switch and thus to the ground connection. The ground connection here is switched on/off by the headlight combo switch.

Good luck (and know that you can just bring it in and we'll figure it out).

 

Again, it's most likely the fuse. One of the idiosyncrasies of the headlight wiring is that there is one positive to each headlight and two negatives. This is why a bad fuse shows up as a dim headlight.

Here is a breakout of the circuit with arrows to show current flow with a positive to negative convention. 

Treat the filaments as resistors for modeling what it wrong, not as coils. Most of the current going through the right headlight is going through the high beam filament, to the red/black wire, to the high/low beam switch, and on to ground.

However, the saying "Current follows the path of least resistance" isn't really correct. Actually MOST of the current will follow the path of least resistance. Some of the current can find an alternate path to ground. It goes through the low beam filament, over to the low beam filament of the left headlight via the white/red wire, through the high beam filament of the left headlight, and out to the red/black wire.

The high beam of the right headlight sees enough current that the brightness appears normal. Meanwhile, the left headlight has about an amp flowing through it. If you were to measure the voltage drop across the filaments you would see something like this (with a 12VDC source):

Right High Beam: 12VDC

Right Low Beam 4.55 VDC

Left Low Beam 4.55 VDC

Left High Beam 2.90 VDC

Light output falls off drastically with voltage (https://www.thetruthaboutcars.com/2019/06/piston-slap-droppin-knowledge-on-headlight-wiring-voltage-drop/), so that is why the left headlight is dim with the loss of the left headlight fuse. 

If you don't believe me, do this test. 

1. Find a 240Z/260Z/280Z that has stock headlight wiring (no relay conversions).
2. Pull one of the two headlight fuses.
3. Turn on the headlights.

@Randalla I suggest you try my suggestion first. It is EASY to change a fuse. Change the left headlight fuse.

[Zed Head]

Before you dig too deep in to your switches you might just hose the inside of the dimmer switch down with contact cleaner while working the dimmer mechanism, for your low beam problem.  It often works.  At the base of the turn signal stalk.  If it does work you can always take it apart and make it even cleaner.

[Randalla]

Looks like I'm going to be a busy boy tomorrow. I should have said that the first thing I checked were the two fuses, which appeared to be fine. I know, looks can be deceiving, so tomorrow I'll switch them both out with new, just to be sure, before pursuing the other avenues. Will also "hose" down the dimmer switch as that's also an easy check. Steve, thank you for your suggestions and reminders as well. With any luck, I'll report back with positive results tomorrow. Thanks again guys!

[SteveJ]

9 minutes ago, Randalla said:

Looks like I'm going to be a busy boy tomorrow. I should have said that the first thing I checked were the two fuses, which appeared to be fine. I know, looks can be deceiving, so tomorrow I'll switch them both out with new, just to be sure, before pursuing the other avenues. Will also "hose" down the dimmer switch as that's also an easy check. Steve, thank you for your suggestions and reminders as well. With any luck, I'll report back with positive results tomorrow. Thanks again guys!

Don't look at the fuse. Pull and test for continuity or just replace. Fuses are cheap.

[cgsheen1]

On 12/27/2020 at 12:56 PM, Randalla said:

Thanks guys for the quick response and thank you Chuck for the very through explanation. You've given me an excellent place to start. With this crazy Covid, I've got nothing but time. Very much appreciate the advice!

I should have also mentioned that it could just be the bulbs.  When the filament blows they can do strange things. It's easy to check the filament continuity without removing the lamp from the headlight bucket. Just unplug the 3-pin connectors behind the grill on either side and test that there's continuity between the positive wire (Red on the right headlight, Red/Yellow on the left) and each of the ground wires (Red/Black and Red/White) and that there is NO continuity between the grounds. 

If you have a power supply you can also test both filaments of each bulb while you have them disconnected.