1975 620 Headlight Relay?

[H3nleY]

It's me again!

 

Now that the truck runs and actually moves, I've been trying to figure out the headlight situation.

 

Since it was manufactured 1/75, I've read that it's supposed to have a four prong headlight relay under the hood. Except I cannot for the life of me find it. There are loose wires all over the place so it wouldn't surprise me if someone removed the relay and tried to bypass it but failed at some point. I know on a 74 it's supposed to be on the radiator support, but any idea where it would be on a 75?

 

Thanks!

[datzenmike]

I believe through '76 the headlight relay is located on the engine side of the radiator support, between the radiator and the right side high beam. From '77 on it as moved to the right (passenger) side inner fender.

 

Red

Red/Yellow

Red/White

Red/Black

[H3nleY]

Hi Mike!

 

There is nothing on either side of the rad support. Here are two images 

 

https://ibb.co/M8C2WwG
https://ibb.co/1QJJtws

[hobospyder]

White king rat said for location. It's a flat 4 prong not like the new box style relays. 

[hobospyder]

Hold on let me walk to the shop

[H3nleY]

Someone must have removed it then or I need an entirely new eyeglass prescription. Here, I figured out how to upload pics

 

 

[datzenmike]

See if you can find the plug and the wiring. Should be right there.

 

 

If cab was swapped look over on the passenger side inner fender (if a '78) there was no relay on the '79.

[hobospyder]

No it's not there

 

Passenger side just off the headlight bucket

[hobospyder]

I would almost say follow the wires and see if you find a relay anywhere on it because it looks like the bundle comes from the passenger fender straight to the lights

[H3nleY]

Yeah my end looks nothing like yours. I found a few loose wires but they're only red black. Check out this mess

 

 

 

[datzenmike]

Passenger side inner fender. There may be other relays and a large black plastic box to cover them.

[H3nleY]

This is what my fender looks like. The box closest to me is a voltage regulator and I don't know what the one closest to the firewall is since one of the screws is stripped.

 

 

I took off some of the electrical tape to look at the wires and found three splices like this. Are these factory or did a previous owner do this?

 

[KELMO]

That looks like a factory connector. I have seen this on 1200's and 510's.

[datzenmike]

 

Crimped.... definitely factory.

 

All I see is the voltage regulator and the EGR? counter. Where's the horn relay and the auto choke heater relay????????

 

Diagrammatically, the '75 shows the light relay inside the cab. Sorry.

[H3nleY]

Yeah I have no clue where those would be either. Unsurprisingly, the horn doesn't work either so that'll be a problem for a different time.

 

How can you tell what the diagram says is inside the cab and what's under the hood?

[Wildcat Walker]

I recommend the headlight relay upgrade. Use the old hots from factory harness to trigger 4 pin relays that have source voltage from under the hood. You will reduce voltage drop through original harness this way, and can use modern relays. 

Edited December 21, 2023 by Wildcat Walker

[Stoffregen Motorsports]

Wiring diagrams have one major fault; they almost never explain the location of the devices.

 

Factory assembly manuals or even parts manuals may have locations.

[H3nleY]

Brilliant. I'll look in the cab when I go back to work on Tuesday.

 

If not, I might rewire the headlights with my own wires and a relay 

[victhewhimpykid]

On 5/26/2023 at 3:04 PM, datzenmike said:

I believe through '76 the headlight relay is located on the engine side of the radiator support, between the radiator and the right side high beam. From '77 on it as moved to the right (passenger) side inner fender.

 

Red

Red/Yellow

Red/White

Red/Black

I have a 76 and relays are all under the dash, non in the engine bay. 

[H3nleY]

Just now, victhewhimpykid said:

I have a 76 and relays are all under the dash, non in the engine bay. 

Okay this is good confirmation thank you! Driver's or passenger's side?

[datzenmike]

On 5/27/2023 at 8:24 AM, H3nleY said:

Yeah I have no clue where those would be either. Unsurprisingly, the horn doesn't work either so that'll be a problem for a different time.

 

How can you tell what the diagram says is inside the cab and what's under the hood?

 

The diagram has the headlights laid out on the left and continues through the engine bay, the cab, the rear and then the tail lights on the far right. Locations are not exact.

[H3nleY]

Alrighty boys so I looked under the dash and found this red and yellow wire hanging next to a red and black wire. I'm getting the feeling the relay is no more

 

[Ooph!]

There's two missing a Red/white and a red according to the 74 wiring diagram

[H3nleY]

Update time!

 

I got deep under the dash and found what I think are the right wires according to the diagram I got (red white, red black, red yellow, red) and a relay they're connected to. Except when I looked up the part number on the relay, that's apparently the EGR relay. I also don't have an EGR relay on my diagram.

 

The EGR stuff from the under the hood is all gone. Could someone have tried to repurpose the EGR relay for a headlight relay? Or am I looking at something completely different that just so happens to have the same wires?

 

[datzenmike]

25230-B8000 is the light relay. I tried 88000 but no luck so tried B8000 and bingo. 09 '74 to 07 '76 620.