Brake lights staying on

[Gunny65]

1979 Mach Dump Truck, DM685SX. Air brake system. The brake lights are staying on all the time. Even when the truck is not running and the key is off.  I can only shut it off by turning the battery power disconnect switch off. I replaced the air pressure sensor switch as the old one was rusty and one wire was disconnected. That didn't help. I have ordered a manual but it won't be in for a few more days. Suggestions on what to do next?

The low pressure air light never goes off even when the system is fully charged.  The low air alarm does not go off either, which I don't mind. I am not sure if these problems have anything to do with the brake lights staying on at all times.

Edited May 3 by Gunny65

[D-Day]

I can not remember the exact location of the brake light switch for a DM of that year but its common for the spring to break inside of these little switches allowing the contact points to stick together same thing with the low air pressure switch

[Joey Mack]

the brake light switch is on the firewall under the hood at the rear valve cover..  It is ''Hot'' all the time, key on or off..  or on the treadle valve. 

[Gunny65]

D-Day, Are you talking about a mechanical switch that is manually activated by the brake pedal? 

I did replace what I believe is called the air pressure sensor switch. That switch is activated by air pressure off of the valve assembly that connects to the brake pedal. This valve is located on the outside of the firewall right under the brake pedal. The switch is round and has two wires connected to it via two small nuts and the switch itself is connected via threads to the valve assembly. 

I will look around for another switch near where you indicated Joey.

[Joey Mack]

ok, but you may only have the one that you know of,  the other is easy to see if its there or not,  if you pull a wire off the pedal switch do the lights go out?  if you put a new air switch in that is 'Normally Closed' the brake lights will stay on all the time and go out when air pressure comes up to full. 

[mechohaulic]

'79 ,should be a brown dash having  top panel held by 4 screws ???  do you have yellow parking brake (must have ) ,remove top dash panel to see parking brake valve. seems to me some had an air valve (brake lights on when parking brakes applied. low air small white plastic having one wire which completed circuit ground on firewall as earlier posted .. there may have been an update on brake valve == 1x3/4 white plastic with 2 wires. Another post having the ; wish I could see it in person! actual brake light switch should be on the treadle valve it self. 2 in diameter  /2wires 

[mechohaulic]

does this unit have a trolley valve on steering column ? brake pedal(treadle valve) floor mounted ?? is the pedal and roller working freely ? this issue sounds to be electrical not mechanical ; don't ask/don't know.

[JoeH]

I believe there's 2 brake light switches on the truck.

One is a positive pressure switch on the treadle valve or near it. This senses when your foot hits the brake your lights come on. 

The other is an "off with pressure" switch, should be mounted on the parking brake valve. This senses when you set the parking brake your lights come on, when you pressurize your brake release circuit it turns off. But it should go off with key. 

 

Edited May 5 by JoeH

[JoeH]

If you install the wrong type of switch in the wrong location then you can get a goofy result, like you're seeing.

[Joey Mack]

yep,  'Normally open' switch is for the foot brake, and 'Normally closed' is for the low air buzzer..

[mechohaulic]

12 hours ago, JoeH said:

I believe there's 2 brake light switches on the truck.

One is a positive pressure switch on the treadle valve or near it. This senses when your foot hits the brake your lights come on. 

The other is an "off with pressure" switch, should be mounted on the parking brake valve. This senses when you set the parking brake your lights come on, when you pressurize your brake release circuit it turns off. But it should go off with key. 

 

off pressure switch mounted in line near parking brake valve is what I was referring to when I asked about the removable top dash panel. you can see it from top and it's a cramp to change. remove brake valve knob, loosen valve =still a pain to access even with plastic air lines.

[Icehole]

On 5/3/2024 at 4:08 PM, mechohaulic said:

'79 ,should be a brown dash having  top panel held by 4 screws ???  do you have yellow parking brake (must have ) ,remove top dash panel to see parking brake valve. seems to me some had an air valve (brake lights on when parking brakes applied. low air small white plastic having one wire which completed circuit ground on firewall as earlier posted .. there may have been an update on brake valve == 1x3/4 white plastic with 2 wires. Another post having the ; wish I could see it in person! actual brake light switch should be on the treadle valve it self. 2 in diameter  /2wires 

Jeez, how long did they use that dash?  That's what my 1990 has!

[Icehole]

15 hours ago, mechohaulic said:

off pressure switch mounted in line near parking brake valve is what I was referring to when I asked about the removable top dash panel. you can see it from top and it's a cramp to change. remove brake valve knob, loosen valve =still a pain to access even with plastic air lines.

Thanks for the info, I need to unhook that.  No reason for brake lights to be on with park.  If anything just blinds people at night.

[JoeH]

9 hours ago, Icehole said:

Jeez, how long did they use that dash?  That's what my 1990 has!

Mid 1970's til they discontinued the R model in ~2001. If the DM continues passed 2001 then they used the dash til the end of the DM.

[Gunny65]

I am going to take the dash apart and check the parking brake sensor. 

Does the parking brake sensor have anything to do with the constant low pressure light staying on even though air pressure shows 120 psi? 

 

[Joey Mack]

I cant remember, but when you find it, you can pull a wire off to see if the low air light goes out. If it does, you will need a normally closed pressure switch with an 80 PSI pop pressure.

[mechohaulic]

3 hours ago, Gunny65 said:

I am going to take the dash apart and check the parking brake sensor. 

Does the parking brake sensor have anything to do with the constant low pressure light staying on even though air pressure shows 120 psi? 

 

NO == the switch on the parking brake valve is for when unit still running but parked. Pull parking brake knob ,rear lights are suppose to go on. one of those ideas from politicians for a safer world! the low air pressure light on without buzzer : taking in to consideration the buzzer does work would be wrong pressure  switch . did you have truck running and drain air tanks down checking buzzer operation?? I would go with buzzer not functioning (disconnected due to constant noise) with a faulty low air sender. one sender should control both light and buzzer . 

[Gunny65]

2 hours ago, mechohaulic said:

NO == the switch on the parking brake valve is for when unit still running but parked. Pull parking brake knob ,rear lights are suppose to go on. one of those ideas from politicians for a safer world! the low air pressure light on without buzzer : taking in to consideration the buzzer does work would be wrong pressure  switch . did you have truck running and drain air tanks down checking buzzer operation?? I would go with buzzer not functioning (disconnected due to constant noise) with a faulty low air sender. one sender should control both light and buzzer . 

The low air buzzer has been disconnected or does not work, which is totally fine with me.  Yes, I have had the truck running with no air in tanks and brake lights stay on with no buzzer.

I also plan on pulling the low air indicator light and trace the lines.  I am going to assume they should attach to the faulty low-air sender? 

[Joey Mack]

Any chance the low air bulb is swapped with another bulb socket? Like the  highbeam, and the high beams are on? 

[mechohaulic]

8 hours ago, Gunny65 said:

The low air buzzer has been disconnected or does not work, which is totally fine with me.  Yes, I have had the truck running with no air in tanks and brake lights stay on with no buzzer.

I also plan on pulling the low air indicator light and trace the lines.  I am going to assume they should attach to the faulty low-air sender? 

you might consider hooking back up/ replacing the buzzer. 1= while you are diagnosing the system as wires are disconnected you will know when right sender is found when buzzer stops ; unless you have a helper watching light. 2- for a safety issue having a properly working buzzer could alert you while concentrating on driving. low air indicator should NOT be on brake valve- that would be stop lights. after finding faulty sender and waiting for new sender you can disconnect the buzzer.

[Gunny65]

There are no switches, that I can see, that are directly connected to the main parking system push knob. I took the dash apart and can see in there.  There is a threaded opening on the back of the knob assembly with nothing in it.  No leaking though.  There does looks to be a pressure type switch on the emergency brake knob.  For now, these pics show what is connected to the low pressure switch.  I can not find the low pressure buzzer. The manual I got is not very helpful. It shows a low pressure switch but I see nothing that resembles that switch.  The book pic has an electrical connector on each end. Everything I see has connections on one side. I just don't know what I am looking at. Anyway, pics.

The first pic is of the switch I replaced. It is directly under the brake pedal and connected to the brake valve assembly.  That assembly is on the outside of the firewall.

The second pic shows a switch on the left that has a wire that connects to the switch on the right and that switch wire goes to the low pressure lamp. The other wire connected to the right switch it goes into the wiring loom. Both of those assemblies are directly behind the brake pedal and mounted on the inside of the firewall.

[Gunny65]

18 hours ago, mechohaulic said:

you might consider hooking back up/ replacing the buzzer. 1= while you are diagnosing the system as wires are disconnected you will know when right sender is found when buzzer stops ; unless you have a helper watching light. 2- for a safety issue having a properly working buzzer could alert you while concentrating on driving. low air indicator should NOT be on brake valve- that would be stop lights. after finding faulty sender and waiting for new sender you can disconnect the buzzer.

I don't know where the buzzer is. I can't see anything that looks like one. It is a mess in there (in the dash).  Lots of disconnected wires, cut wires.  Where, in general, might it be mounted?

[Joey Mack]

Is the buzzer mounted to the triangular brace above the clutch pedal? 

[JoeH]

Don't think so, buzzer is gonna be an aqua green thing in the glove box.

[JoeH]

1980 dm686sx

Low air buzzer is one of these 2^^

I think this one is Service brake lights^^^

T'd into the parking brake knob there's the parking brake light switch.^^^

Edited May 9 by JoeH

[Joey Mack]

yep, That's what I get for not working on a 70's model Mack in decades..  Thanks JoeH..  I put that back in my brain..