For what it is worth, I am restoring a 79 F-100 and was re-wiring the instrument cluster. I checked the shop manual and there was no schematic for the instrument wiring harness. So I traced and tested the cluster and thought I’d share the results for anyone else wiring their original instruments.
The red flex-circuit on the back is marked D6Txxxxxxx, and it has all gauges
Each wire in the harness plug has a number:
1 - (red/yellow) Ground / Instrument back lights
2- Empty
3- Empty
4- (light purple wire) Power to the parking brake warning light
5- (pink wire) Fasten Safety Belt warning light
6- (green/black wire) High Beam indicator light
7- (blue/white wire) Power to Right turn signal
8- (red/pink wire) Temperature gauge sender
9- (black wire) Cluster ground
10-(blue/red wire) Power to instrument back lights
11 - (yellow/black wire) Amp gauge
12- (red/orange wire) Amp gauge
13- (green/blue wire) Power to Left turn signal
14- 2 wires (black/green wire and a capped black wire) the black/green wire is Power to Cluster
15- (orange wire) Fuel gauge sender
16- Empty
17- (white/red wire) Oil Pressure sender
18- Empty
Not sure these are the same for all the 73-79 trucks (probably not!) As with any wiring, bench test your cluster for yourself, but I thought this might help as a guide.

 

question-does the #4 slot supply continual switched power to the low brake switch? on my 1976 f150 with 351w, the brake light was working (prove out) and then one day it stopped. so if the one purple and white wire goes from the cluster plug to the switch and provides power...the other comes from the ignition and provides temporary ground for the prove out. i put a light tester on both purple and white wires and there was no power-even checked it at the back of the plug and there is nothing. any suggestions as to what would cause this? damaged printed circuit? any help is greatly appreciated.

 

Thanks John,

I cut and pasted that onto a word doc. Saving it for when I get there on my '77......John

 

The PURPLE with WHITE stripe wire is not power to the brake light; power to the brake light comes from the cluster; the PURPLE with WHITE stripe wire is the ground-out signal to the brake light warning switch (grounding it out turns the light ON).

Here is how the brake light warning switch works. The PROVE-OUT terminal of the ignition switch goes to one of the two terminals in the brake light warning switch plug. The PURPLE with WHITE stripe wire from the instrument cluster goes to the other terminal in the plug. These two terminals are shorted together in the switch, and this common point is grounded out if the switch closes. This two-terminal plug is used to prevent the PROVE-OUT circuit from making the driver think the bulb is good when the key is turned if the warning switch is actually disconnected.

Power is not actually applied to the PURPLE with WHITE stripe wire leaving the cluster. You WILL see 12 volts on this wire if it's disconnected, but it's the 12 volts from the other side of the bulb that you're seeing "through" the warning light filament. It works the same as any other warning light in that sense; however, it has a secondary mechanism for turning it on (the PROVE-OUT mechanism from the ignition switch).

Also, some corrections to your table. This is not meant to look a gift horse down the mouth (everyone is appreciative of everyone sharing information), but I'm following up for the benefit of others and to prevent false information from circulating on this forum. Please do not take offense to this as I'm sure this took some work and you're only sharing it for the benefit of others.

This is neither a ground, nor power to back lights. This is 12-volt power to the brake warning lamp.

It's actually the ground-out signal to the brake warning light; see above.

Quote:

Originally Posted by John Rowe

14- 2 wires (black/green wire and a capped black wire) the black/green wire is Power to Cluster

The BLACK with GREEN stripe wire is power to the instrument cluster voltage regulator only. It's a special 8.5-ohm resistor wire. It does not power the cluster in its entirity.

Note that a truck with indicator lights instead of gauges is set up differently as well.

EDIT: I just realized this thread is 2 years old. What's with all of the necro-posting today?

 

thanks so much for the reply...sorry for the typo-i didn't mean to say the brake light-meant the low brake indicator in the cluster was working fine and then one day it wasn't. so when this light proves out, where is the power applied from? also are you saying that when i disconnect from the switch, i will get 12v on the one purple and white. also the light was proving out before i even plugged into the switch... sorry if im not following and thanks again for your time.

 

.................

 

I'm talking about the same light you're talking about. We're talking about the light in the instrument cluster that says BRAKE.

12 volts is always applied to one side of the bulb, whether it's ON due to PROVE-OUT, ON due to the warning switch engaging, or OFF. This comes from the RED with YELLOW stripe wire that feeds the instrument cluster printed circuit. This never changes; the thing that changes is whether something is grounding out the other side of the bulb and making it come ON (either the warning switch or the PROVE-OUT circuit).

If you disconnect the warning light switch and turn the key to RUN, one of the wires going in to the switch is open circuit. The other will appear as 12 volts on a multimeter, but it's not a true 12-volt source like if you were to connect to the battery. You're seeing 12 volts through the other side of the bulb filament. Because the multimeter draws very little current, there's no voltage drop across the filament and you see the same voltage on both sides. In other words, yes you will see 12 volts, but don't think of it as something directly applying 12 volts.

Then either the wiring has been modified, something is shorting somewhere, or there's more to the story. The path between the BRAKE light and the PROVE-OUT circuit is THROUGH the brake warning light switch itself; current cannot physically flow between the two with the switch disconnected unless something else has been changed.

No worries at all.

Mike, I stole that term from you

 

thanks again...now i got it! you are the only proud author of how the hell that switch truly works anywhere on the internet. picking up that "fake" reading on the light meter when it was disconnected...and it then disappearing when connected was messing with my sanity.

 

Thanks for the kind words; glad to hear you got it worked out.

 

Wish I had seen this two days ago before I went and mapped out the cluster myself. Thanks for the write up, it definitely reassured what I had.

Quick question, the right turn indicator in my cluster stays lit solid when the parking/headlights are on. It will blink normally when signal is flipped to that side and then back to glowing solid again when signal is off. What ever could that be?

 

...the irony.

Guess this goes to show that this forum is necessary

 

That means the right front parking lamp socket needs to be replaced. Classic "ground shift."

 

That did the trick. I ended up replacing all the sockets front and back, had one in the back acting crazy too. Thanks for your help

 

Ford had a electrical manual that was separate from ther other shop manuals. I picked one up for my truck year for $20 shipped on ebay.

 

I'm working on a 79 F250 ranger right now for a guy. I need to know if these wires are going to be the same. Most of the colors are the same but I'm getting nothing but the gauge lights. I'm not getting any fuel pump reading, signals, or high indicator. The truck was in a field and all the wireing was toast. The truck has been mostly restored and a painless wiring harness mostly installed. I'm only haveing trouble with the gauges. The gauges are the ones that were in the truck when it was bough. I'm also unsure how to wire up the amp meter. Suggestions and help are all welcome.