Dual brake warning diode assembly

Hey all

Recently I've made several posts about the electrical problems I've been chasing down in my truck and I have another lead as to what might be the problem...

Very long story short my 1991 F250 7.3 idi ats turbo diesel with e4od tranny is in limp mode and I've been trying for about a month now to get it back up and running. I've tried checking fuses and sensors and a whole lot of other stuff and I've discovered that I have continuity between the load side of fuse 18 and and my chassis. I don't think that is normal? I compared it to another Ford truck of the same generation and it didnt show continuity, leading me to believe there is a direct short to the chassis.

At the same time, I recently was provided an electrical wiring diagram for the instrument cluster in my truck (fuse 18) and it shows that a 'dual brake warning diode assembly' is on that circuit. I have no idea what that is, but I am quite curious about it because my truck began to have this problem immediately after a failure and replacement of my front brakes. Can anyone tell me what the dual brake warning diode assembly does and if the brake failure could have killed it, causing some sort of electrical meltdown or short circuit?

Sent from my moto g(6) play using Tapatalk

Does your brake light on the dash work? It could be triggered by the low vacuum switch, the e-brake switch, a switch in the brake system to detect loss of pressure. Disconnect all those things if you think you have a ground fault. Trace them through your wiring diagram and see where they join. Probably behind the instrument panel and wrapped in the harness.

Based on the timing of events, I'd be looking for a switch in the brake system. Other vehicles and I'd say look on the combo proportioning/differential valve. I don't know about yours. Brake systems have a switch that gets grounded when either the front or back brakes lose pressure. The fix is to bleed the end that didn't lose pressure and recenter the switch. Hopefully someone that knows your gen truck will pop in.

You can make a jumper harness to a light bulb that will act as an indicator and a safety fuse. Buy an add a fuse and a bulb socket. Put some spades on the bulb socket and stick them in the add a fuse. Plug it in in place of the fuse and it will protect the wiring due it it's resistance and it will give you a visual indicator when you disconnect the short. Just don't let it rest against any metal or you'll get a false signal.

Have you isolated the other parts on this fuse to pin point the problem? On this fuse I see;

park brake switch

Anti-lock brake module

cluster

fuel/water switch and water in fuel light on the dash

Please stop making new threads on the same issue, it makes it really hard to help you as we have to keep looking to other threads to know what you have done.

Im not sure what diagram you have but the dual brake warning switch isn’t fed by fuse 18 in any diagrams I have.

I believe in your other thread you said you removed the instrument cluster and it still blew the fuse?

Fuse 18 feeds wire R/Y (red with a yellow stripe), this wire will most likely be damaged on the engine going to the water in fuel switch, the likeliness of this wire being damaged under the dash is pretty much none existent.

Start by unplugging everything this circuit feeds, instrument cluster, door chime, low vacuum switch and I would just unplug the engine harness (which likes to melt itself hint hint) to isolate the water in fuel switch.

Thank you for the advice, I wil try to keep asking questions on the same thread in the future. I have found the low vacuum switch, the cluster (obviously) and the chime module, but i dont know where the water if fuel switch is. Is it on the fuel/water separator? Also, what do you mean by the 'engine harness? I am not very advanced in electrical knowledge or really in the anatomy of this truck

The engine harness is the wiring that is basically on top of the engine. Over the passenger valve cover there is a rectangular plug with around 8 wires in it. That should contain the water in fuel sensor wire.

The sensor is located at the fuel filter near the front of the engine. Look for the wire near there or at the plug above. The sensor screws into the bottom of the bowl attached in the bottom of the filter

Turns out that engine wiring harness deal was toast, I fixed that and some of my issues went away, thank you for that information

jonleroy2506 said:

Turns out that engine wiring harness deal was toast, I fixed that and some of my issues went away, thank you for that information

Click to expand...

Glad that helped, did that stop fuse 18 from blowing? What other issues do you have?

Fuse 18 quit blowing, but my tach still doesn't work. Transmission appears to start in 3rd gear always. My dash lights are working again but that may have been a different fuse. While test driving it after getting all that fixed up it completely died and wont start now, some fuel related issue I think. Not sure at this point, I have fuel to the injectors and the fuel filter and such, and the injector pump solenoid is fine

Sent from my moto g(6) play using Tapatalk

When the transmission goes into limp mode you will get 4th gear in D, 2nd gear in 2 and 1, reverse will still work and you will full line pressure so it will slam into gear.

The tach sensor feeds the TECA (trans computer) and the computer then relays the signal to your tachometer, if you hit the OD cancel button does it light up? This will tell us if your TECA is alive, if it doesn’t light up it may not be getting power. You can test that without the engine running.

A common issue is the wires that feed the trans on the passenger side get cooked by the exhaust and the insulation breaks off and can short together blowing the fusible link that feeds the TECA.

Okay the transmission is definitely in limp mode that is exactly what is happening. I have no o/d on and off light but when my lights are on the back light for the o/d is on. I'll have to check that plug on the passenger side, I have checked it in the past and it seemed fine but at least that's somewhere to look

Sent from my moto g(6) play using Tapatalk