Troubleshooting Electrical on Dual circuit brakes (88 m109a3 AF truck)

[peashooter]

Hi all,
I suspect this is going to be a tough question to get answers to. I purchased a 88 Airforce M109 with the dual circuit brakes earlier this year. Well I knew something was up when the center light (brake warning light) on the instrument cluster had been replaced with a regular gauge illumination light. Someone added a wire off of the adjacent dash light (#40) to power what used to be the brake warning light. I ordered a new brake warning light from erics but now am just trying to figure out how it all should be wired so I get the Parking brake light and the brake system warning light if one of the circuits fails.
I havent been able to find any wire schematics for the dual circuit system and would love some help either locating that schematic or someone else with a dual circuit system letting me know what goes where as some of these wire numbers dont jive with the wire number listing available on this site. Some of the wires I just want to know what they do (I'm assuming they are all brake related.

The numbers I'm trying to figure out (what they do or where they go) are:

-17 (plug is hanging unconnected from under dash), doesnt appear to be for dimmer switch as wire list states as that already works. My guess is this gets connected to the center dash brake warning light... just want to confirm this.
-16 (loose plug wire under cab) that appears to have been attached to parking brake switch. Just want to confirm this as well
-118 (run along with #16 but connected to some type of brake line splitter looking thing going from MC to Air packs, what does this do?
-175 Wire is connected to the Driver's side air pack (what does this do?)

 

[roady]

I have a truck like yours and can look at it layer tonight when I get home and try to help you out. There is also a wiring diagram in the stickys of this section that should get you close.

 

[AceHigh]

Peashooter, are you sure it will warn you if a circuit fails? On my A3 I think all it does is tell you the parking brake is on.

I added a brake light kit to my A2 (single circuit) and all it does is illuminate when the buzzer is on.

 

[AceHigh]

Wire 16: Light switch (HT) to service headlamp or dimmer switch

Wire 17: Dimmer switch to high beam service lamps

Wire 118: ?

Wire 175: ?

Are you sure on the bottom two numbers? They don't show up anywhere I can find.

http://www.olive-drab.com/od_mseries_circuits.php3

 

[doghead]

Have you looked at the TM specifically for dual brakes circuits? SMARPI 9-2320-209-14&P.

It explains what the light does, and has the parts listed/exploded views.

 

[emmado22]

I think the light also runs to the proportioning valve, and if the valve senses a problem, it illuminates the bulb.

 

[peashooter]

ROADY, thanks any help would be great and you verifying what wires are connected to the dash light and parking brake are probably my best bet.

ACEHIGH, according to the TM SMARPI 9-2320-209-14&P it says it will light up under 2 instances, for the parking brake and for a failure of one brake system. It also says it will light up briefly when the engine is first started (probably to verify the bulb works, just like how our cars work).
Also like I mentioned some of the numbers dont seem to jive with the wire listing you mentioned. I'm pretty sure my wire 16,17 isnt for the headlamp or dimmer as they all work but these wires are not currently connected. I'm positive on the 118 and 175. My guess is that these dual circuit trucks have a lot of commonality with the standard deuces, but apparently there might be some excepetions on wiring #.

DOGHEAD - yes I did go through the TM you mentioned but didnt find any info on the wiring or wire numbers. It seems like there must be another TM perhaps a higher level one that would cover this better.... I just havent found it yet and not sure where else to look.

 

[emmado22]

I wouldnt trust that wiring # chart too much....

 

[peashooter]

Okay found 2 more plugs that arent connected under the dash.
One plug is a 3 pin male with only 2 wires coming off it (127 & 174)
Another plug is a 4 pin that has 174,118,117,127 coming off it. Any help on if these should be connected to something would be great, and again what should be connected to the 2 wires comming off the dash brake light (27 and 67)

 

[roady]

Ok here is what I have for you.
17 high beam indicator
Brake light switch
127 to 27
167 to 67

 

[peashooter]

Ok here is what I have for you.
17 high beam indicator
Brake light switch
127 to 27
167 to 67

Thanks Roady,
that helps a ton! I did a little trouble shooting tonight and it looks like the parking brake switch isnt working (the contacts must be smashed inside it) so hopefully that was the only reason someone decided to remove the brake warning light.... fingers crossed.

 

[peashooter]

Okay, I traced all the dual circuit brake wires and it looks like part of this wire harness needs to be connected to something? There are 4 plugs comming off the harness under the dash. the 2 single wire ones are 127 and 167 which would go to the dash's brake warning light. There are 2 other multi pin plugs however that I think I need at least one of them connected to supply the warning circuit with the +24v which I'm guessing is 127.
Can anyone tell me what these other 2 plugs would be connected to?
The 4 pin plug has 2 male, 2 female pins numbers are 118,127,174,117
The 3 pin plug only has 2 wires going to it they are 127 and 174

 

[roady]

I should be able to get under the dash of mine this weekend and look around some more to try and help you out.

 

[wsucougarx]

Okay, I traced all the dual circuit brake wires and it looks like part of this wire harness needs to be connected to something? There are 4 plugs comming off the harness under the dash. the 2 single wire ones are 127 and 167 which would go to the dash's brake warning light. There are 2 other multi pin plugs however that I think I need at least one of them connected to supply the warning circuit with the +24v which I'm guessing is 127.
Can anyone tell me what these other 2 plugs would be connected to?
The 4 pin plug has 2 male, 2 female pins numbers are 118,127,174,117
The 3 pin plug only has 2 wires going to it they are 127 and 174

I went out and looked at my truck and I could see the 4 pin thing and the 2 pin thing. Now here's what I see:

The 4 pin deal connects directly to the momentary start switch. The leads from the 4 pin to the starter switch are 118, 117, 174, 127.

The 2 pin deal has a male and female connection. The wires to/from or from/to are the following: 127 goes to 74 / 74 (wire has two labels)
174 goes to 74 (wire has only one label).

I can see a wiring harness wrapped in electrical tape. The two wires w/127 and 174 are connected to the 74/74 and 74 then loops back up into the harness. From there the 4 wire plug loops down and connects directly to the starter switch.

 

[peashooter]

Thanks for checking that out. Once its day light again I'll have to go check to see if it looks like it will plug into my start switch..... my fear is that I'll find the dual circuit trucks have some type of "special" start switch and that mine no longer has it.