Headlight - Parking light Question

[Rdinatal]

I'm connecting up the front lights and came upon a dilemma...

While testing, are either headlights OR parking lights on. As in one or the other.

Are they both on at any switch setting?

[John Mc]

Headlights should only come on at the far right setting on the upper switch.

[Rdinatal]

Headlights should only come on at the far right setting on the upper switch.

Yes, thank you..

But do the parking lights come at the same time? At any setting?

[John Mc]

My lighting wiring is a bit non-standard, so my own truck is not a good reference.

I've attached a light switch operation diagram (shows the various switch positions) and an internal wiring diagram for the 3 lever light switch. Tracing the connections here helps figure out how things are supposed to work.

In the switch internal wiring diagram:
15 = Power feed to the switch
B = Instrument Panel Lights (wire #40)
16 = Headlights (via the floor mounted dimmer switch)
21 = Tail Lights (rear of truck)
22 = Stop Light (a.k.a. Brake Light)
75 - (Pin A) power feed to Stop Light Switch
75 = (pin K) return from Stop Light Switch
491 = Parking Lights (front of truck)

As I read the diagram, the only way to get the Parking Lights (on the front of the truck) ON is to have the lower left switch in "PARK", and the main switch in either "STOP LIGHT" or "SERVICE DRIVE". No power is fed to the lower left switch when the upper switch is in the OFF position, so no parking lights when the upper switch is off, regardless of what the lower left switch is set for. However, switching the Parking lights on the front of the truck ON turns the Headlights off. With the stock wiring, there is no way to have both of the headlights and the parking lights on at once.

Tail lights (wire 21) are energized ONLY if upper switch is in Service Drive position

So with standard wiring, for normal driving:

• For daytime driving, the main (upper) switch should be in STOP LIGHT to enable the brake light function. (lower left switch can be in off or park, depending on whether you want the parking lights in front on or not.)

• For night time driving, the upper switch should be in SERVICE DRIVE to turn on the headlights and tail lights (this position also enables the brake light function)

If I've messed up my tracing, hopefully someone will come along and correct my interpretation.

NOTE: Edited from original post. I had confused Parking Lights (which on the M37 are in front only) with the tail lights.

[just me]

Parking lights ONLY come on when the headlight switch is on and the instrument light switch is all the way down. The headlights do NOT come on in this position. The headlights only come on if the instrument lever is off or instrument positions. Parking lights do not come on in service brake.

[John Mc]

Parking lights ONLY come on when the headlight switch is on and the instrument light switch is all the way down. The headlights do NOT come on in this position. The headlights only come on if the instrument lever is off or instrument positions. Parking lights do not come on in service brake.

Parking lights are on the front of the truck. They are powered by wire 491 (Pin L on the light switch). From the internal light switch wiring diagram, that pin CAN be powered with the upper switch in either BRAKE or SERVICE DRIVE position, as long as the lower left switch is in the PARK position.

Your comment did prompt me to go back and review my earlier post and correct the error I had made by partially confusing "Parking Lights" with "Tail Lights". You are correct that the headlights only come on if the lower left lever is in the OFF, Panel Dim, or Panel Bright positions. You cannot get the headlights and the parking lights (in the front of the truck) on at the same time.

[Rdinatal]

I just thought it odd the front yellow lights (whatever the proper name is) did not work when the headlights are on. It's either one or the other, not both.

I will eventually wire them up but that's lower on the 'ToDo List' right now. My priority is the directional signals which started this question. Since the signals are there they must be working. If they weren't installed then arm signals are lawful. Then again how many under 30 really know what arm signals are?

Ended up jumping the wires at the flasher (the 2 non ground) which helped me figure out what's wrong. With those tied together the lights will go on but not flash.

The flasher is bad, for one. The wiring was incorrect, it was also labeled incorrectly. I ran out of time with one issue left. When the right directional is on the left rear is also is on. Deep diving into the rear wiring next.

I have the wires labeled to help rewire for the front marker lights to be on when the rears are on. As said, that will some time in the future. To keep the load down I'm changing to all LED bulbs. The new flasher will also be LED compatible. Not going with the military flasher.

Thanks for all the help on this.

[just me]

Headlights are for driving.
Parking lights are for parking.
It's only modern era where they both are on at the same time. (1970's)

[Rdinatal]

Headlights are for driving.
Parking lights are for parking.
It's only modern era where they both are on at the same time. (1970's)

While very true we also:
Drive on a parkway
Park on a driveway

I get it now, just seemed weird at first.