I've got a 91 F800, L6 Brazilian engine with air brakes, that just got its rear wheel seals replaced. When I got it home I let it idle for a couple minutes and a chime started going off and the right most dash lamp illuminated, after a couple seconds the chime stopped, the lamp dimmed(but was still lit), and the speedometer seems to not work. This chime was not the low pressure buzzer, it sounded like a low coolant chime, but none of those lights illuminated. after looking in my shop manual, and I mean looking as in hours of searching because this brake warning light seems to not exist I found one table that gives a couple of causes that I think only related to the trucks with juice brakes(picture attached). The truck builds 120 psi still and releases like it should, it does everything it should do, except that dash light is still on and the speedometer doesn't work. I was just wondering if anyone else had something similar or if anyone knows what could cause a brake warning light to turn on. Thanks
Just an update, when I get the air pressure down to when the buzzer starts, the brake light (in the picture) turns off
Possibly lost a ground somewhere? The tachometer seems to not be working as well?
I’m with Birken here, this screams bad ground.
well you guys are right, except I have an unwanted ground. somehow the light buld is getting a a switched power. I took out the light bulb that keeps turning on and noticed the color of the wire matched the power wire going into the speedometer, the red with a green stripe one. That wire is usually a 12 volt power source, and when i applied 12 volts to the speedometer through that red with a green stripe wire, the speedometer started to work. But the question now is where is that circuit getting a ground that illuminates the light bulb and sends ground to the speedometer making it not work. so far I figured out power comes from a purple with a white stripped wire to the light bulb and the out of the light bulb comes the red with a green stripe wire that goes to the speedometer.(drawing representation below). The lightbulb lighting doesn't really make sense to me, the light is really dim compared to the turn signals or headlight indicator. im currently trying to figure out where the purple with white stripe wire goes. does anyone know? somehow the 12 volts from purple wire has to be wrong or something, i have no idea as to how that light bulb is lighting. the wiring was designed to have 12 volts running through a bulb and the bulb being an indication of some sort of grounding im assuming. does anyone have any ideas if they understand what im saying? im also wondering if I could just bypass the lightbulb and just run a switched 12 volts to make the speedometer work and just not care about whatever that light bulb is doing, there doesn't seem to be any sparks or wire heating up. Thanks
the tach works, I just had the key in the run position with the engine off
I think you are correct, red/gr on Ford usually means switched power, so that appears to have failed. Check for a blown fuse or something. Something else is probably also trying to ground through the vio/wh wire down the line. When it is not connected to ground by whatever switch does that, it travels back up the vio/wh wire, into the red/gr, into the speedometer and to ground that way. Thus lighting up the light bulb because it is the highest resistance thing in the circuit. I think you need to first figure out why red/gr is not getting power when the key is on. It should. The other problems will probably go away then.
This may be stretch, but try pushing the yellow tractor brake button in to release the brakes and see what happens..A lot of big truck brake systems have a warning light/alarm/dinger that goes off if the emergency brakes are set and you try to move the truck or even just rev up engine.probably to prevent twisting the drive shaft out, or tearing up your brakes, But that may explain the speedo/tach issue as well...
No matter what I do, the only way that light will turn off is if the brake warning buzzer turns on at a low psi
Here is your problem, I am almost sure of it. The 12v source to the speedometer and lamp has failed. The 12v source to the buzzer is still working. When the low air warning switch is open, power flows through the buzzer, past the low air switch, in reverse through the bulb and grounds through the speedometer. If you can find out why the red/grn wire to the speedometer no longer has power, you will have fixed it. If I guessed it right.
your right, the bulb gets its ground from the speedometer because the bulb only lights when the speedometer is plugged in. I definitely believe the red/green wire is the problem, I traced it back to a long strip of indicator lights in the middle of the dash, I believe it is supposed to supply 12 volts to those lights so that the computer (or at least what i think is a computer) can switch the grounds on and off to display the lights. At this point im pretty sure there is something up with this computer looking device which would make sense as to why everything just started acting crazy all of the sudden. since finding a new one would probably be impossible unless i can find a parts truck, my only option may be to just hook up the speedometer with a switched 12 volt and just put everything back together. here are those indicators the right most wire is the red/yellow wire that conects to all the lights, the next 2 from the right are the only ones that have 6 volts on them when the truck in in the run position and a chime is head in the lights are inserted right in the middle of the picture is the "computer", the black box that is close to the front of the dash and has a lot of dust on it. i believe this is all there is for the wiring, the only thing im uncertain of is where the red/green wire meets the red/yellow wire.
I don't think that is a computer, they did not have a computer in the light duty trucks in the 1990s and the heavy trucks were even cruder. Might be worth a dig in to see what it is and fix it, rather than hack it up.
I mean it definitely computes something, it has inputs and outputs. It's the device that makes the chime when something is wrong
Not going to be a Serious problem. You will HATE this but hand over hand the harness util find a Large Black coned tube connector with bullet connector ends. Is a Diode, should be three of the worthless damn things on the machine and REMEMBER that crap to this day. Will be a FOMOCO P/N tag on at least one and at least one will be Popped., Been a few decades but I believe will find Ford still stocks them just NOT on the Circuit one line diagrams.
When they were working on the rear end did they do any welding on the truck?
No, all they did was replace the brakes and seals
I have a 97 Ford f700 that had a leak in right rear brake line, I fixed it but now the parking brake will only release very slowly and the lights are lit up and the ding ding alarm won’t stop, anyone got a clue since it’s similar to this issue? Could it be air in the line?