Jake Brake Circuit Breaker Size?

[richard5933]

Used two stage Jake on the 4108/8V71. ...
Bill

Did you make use of the first stage only enough to make it worth installing these as two stage Jakes? Our plan was to go with single stage Jakes - all or nothing - until this thread turned to a conversation about two-stage Jakes.

We've got good oil pressure - our engine still has under 50,000 original miles. That won't be a problem. Not too late to go with two-stage install if it's worth the effort.

[richard5933]

Here's a copy of the wiring diagram in the GM book for the 4108. Couple of interesting things I notice:

1) The system is installed single stage.

2) GM is really fond of using relays for everything. Not sure why the relay is being used here instead of just the switch. According to this diagram, the engine brake relay is to be installed in the driver's electrical panel about 6" from the switch in the side switch panel.

Seems like they could have just run the hot from the #27 circuit breaker > clutch switch > engine brake switch > panel terminal #77 > engine bay electrical panel.

Only thing I can think of is that the clutch switch isn't rated to carry the full current to the Jakes. Or possibly just in case I've got an extra relay, so I suppose I'll do it like they say.

Would be great if someone could help me understand purpose of this relay.

3) There is a wire running from the engine brake solenoid power feed to the stop light relay. Guess they thought along the same lines as Bill and didn't want that Safari driver to be confused any more.

[eagle19952]

We've got good oil pressure - our engine still has under 50,000 original miles.

Good pressure at throttle.
Detroit's have lower oil pressure off throttle.
Often to low to open a Jake.

[buswarrior]

Relays used for circuit endurance?

Only signal amperage thru all the switches?

In the hands of a careless paid driver who can't set a speed, who leaves the switch on, every throttle lift makes contact...

That's hundreds in a single day...

Happy coaching!
Buswarrior

[richard5933]

Relays used for circuit endurance?

Only signal amperage thru all the switches?

In the hands of a careless paid driver who can't set a speed, who leaves the switch on, every throttle lift makes contact...

That's hundreds in a single day...

Happy coaching!
Buswarrior

Even with the relay, wouldn't current flow through the entire system at all times if it was turned on? With or without the relay, current is also flowing through the buffer switch which will open close with throttle action?

Seems like the only thing the relay does is change what the clutch switch is controlling - instead of the clutch switch opening/closing the circuit to the buffer switch, it is opening/closing the circuit to the relay.

Didn't seem like the Jake circuit had that high a current draw to matter, but perhaps it does. I'll install the relay since I've got one.

[bevans6]

The relay is there to isolate the control switches - dash switch and clutch switch - from the inductive load of the Jake solenoids to reduce arcing and prolong the life of the switches.  The buffer switch has a diode to do the same thing.  In terms of current, a 24 volt solenoid draws roughly half an amp, there are four of them, so 2 amps.  That is a very small current load.  In the GM world many/most buses were 12 volt, so around 4 amps, so they probably felt it was worthwhile adding the relay.  Where is the relay located - if it's at the rear of the bus it's also to reduce voltage drop in the 12V application, and they probably just kept the design for the 24V bus.  Current will flow through the front half of the circuit - dash switch, clutch switch - all the time the dash switch is on and the clutch is up.  That turns the relay on and provides power to the Jake buffer switch.  Current will then flow in the back half of the circuit only when the buffer switch is activated when the governor goes to "no-fuel".

[buswarrior]

More specifically, the signal current vs the load current.

Lots of little DC arcing...

Too many bus conversions out there running load current thru their switches, when a relay would be a better choice.

Happy coaching!
Buswarrior

[luvrbus]

Richard in 1975 all they had were the 71 Jake heads they took a along time to engage and used lots of power they made upgrades for those then changed to the 71/92 heads.They used capacitors on the master for awhile to help with the engaging on the 71 heads that were not upgraded.Use a relay the later buses still use a relay on Jakes every set I install is on a relay it's  one of things what ever flips your switch.FWIW I checked a old Jake manual your wiring diagram is from Jake     

[richard5933]

I've looked this over again, and am somewhat confused about how they've got the line connected to activate the brake lights when the Jakes engage.

Wouldn't the brake light circuit activate the Jakes every time the service brakes are applied and the brakes lights come on? I don't see anything in the circuit to prevent back feeding. Only thing I can figure is that the stop light relay in the diagram is an added one, so that the line coming from the Jakes doesn't feed the brake lights directly, but rather actuates a relay which in turn lights up the brake lights.

[luvrbus]

There is a one way diode in the system to prevent that,some folks hook the Jake to the brake light to activate the Jakes and you need to remove the diode

[buswarrior]

Leave yourself facility to NOT show the brake lights under Jake use.

Few vehicles show the brake lights under Jakes. Some transit/coach properties spec to show an amber light on retarder/jake activation, but not everyone.

Here's the problem: long downgrade, you're dragging the Jakes for some length of time, and have some need to apply the brakes heavily. There will be no change to the lighting displayed on the back, you've conditioned 'em to the brighter lights, yet you are now decelerating hard...

Also, it is no fun following a vehicle with steady burning brake lights. More than sufficient reason for some crazed trucker to drag your driver's side mirror off as he passes, with that trailer he is pulling, that is not his...

Four way flashers on an up or downhill suggest a slower moving vehicle, brake lights are meant to attract immediate reaction. Jakes, when deployed properly, just let you maintain a steady speed, no warning required?

Ok, there's the top off the anthill...

Happy coaching!
Buswarrior

[richard5933]

Oh - the choices we must make...

I'll ponder a while, but I understand your message. Too bad there is not a standard of brake lights being variable - mash the pedal hard or suddenly and they go on full bright. Hold the pedal down for a while and they start to fad to half bright.

I'll leave this one sit for a while and ponder...

[Geoff]

I plan to install an LED brake light independent of the brake lights on the sides.  This would only work off the Jake active hot lead and would be centered at the top rear of the bus.  No confusion with this setup.

[lostagain]

You don't need any red light on your back for engine brake. The Jakes just hold you back. They don't slow you down so much that you have to warn traffic behind you. Save the brake lights for when you use the service brakes and really slow down.

JC

[Geoff]

Apparently some Class 8 trucks and motorhomes already have "auxiliary brakes" activating the brake lights.  But you can defeat this feature by reprograming the ECU.