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Old 10-19-2020, 11:07 PM   #1
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Jacobs Brake low setting not working

I have a 2004 ISL 400 equipped with a two stage Jacobs brake that has the high/low rocker switch. Regardless of which position I have the engine brake switch in it is on high providing an aggressive braking action slowing the coach down quickly. Works really good in fact too good and I have to toggle the brake on/off coming down a hill as it is very effective. Each off toggle causes a lurch as it releases. It would be real nice to have the low function that I could leave on for a longer period coming off a hill.

I have pulled the switch out of the consul and all the wires are connected. The transmission is downshifting correctly. When I leave it on and come to a full stop it does not get the Cummins shutter before disengaging indicating all 6 exhaust valves are being held open. It just smoothly disengages off at about 1200 rpm.

I have not traced the Jake wires coming out of the valve cover to see if they split.

Any suggestions of where to start to look to get the low setting to function?
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Old 10-20-2020, 12:39 PM   #2
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My experience with jake brakes is really old (early 80’s) but I doubt much has changed. Sounds like you are getting six cylinders worth of braking whenever you use it, rather than two in low, which means (to me) the problem is with the controls and not the jake head(s). I would consider this good news, but it should be corrected and in the mean time limit used of the jake in slippery road conditions.

There are two types of jake brake level selection systems - one selects two, four or six cylinders and has a high/med/low selector - the system I’m familiar with . The other (high and low) releases all combustion gasses on high, and less on low. I think this is what you have, so again, it’s most likely an external control issue.

Here’s some casual reading on the subject:
https://www.jacobsvehiclesystems.com...ting-Guide.pdf
Enjoy!
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Old 10-22-2020, 08:05 PM   #3
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Can you take out your switch and test the contacts via a electrical test meter to see if the values change from the low setting to the high setting. My assumption is that the low side of the switch is shorted out to the high setting of the switch.
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Old 10-23-2020, 12:11 AM   #4
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That's a great idea. My challenge is the wiring diagram I have is not the same as what is in the coach. When I had the switch out I checked for voltage but didn't find any as I assume the vehicle must be in gear and above 1200 rpm to activate the brake. As near as I can tell that enable voltage comes from the engine ECM if enough conditions are met.

I'm not clear on which is the power source to the switch? With electronic modules of the transmission and ECM connected to the switch I'm not going to chance powering the switch to see if the voltage coming out of the high/low terminals are different. There is a resistor looking device going between one terminal and what appears to be a transmission terminal but not the high/low terminals. I will have to try ohm testing with no wires connected to see what I can find.

I found another wiring diagram which shows only one wire to the solenoid of the Jake brake so I really need to pull up some floor boards to get to the valve cover and wiring. If I can see the two connections to separate Jake solenoids it would confirm a dual range brake.

I could also just find a new switch and try the simplest route??
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Old 10-23-2020, 04:07 AM   #5
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I could also just find a new switch and try the simplest route??
I thought about that if the switch is not too pricey and easily obtainable. I’m not a fan of parts swapping for diagnosing but in this case it might be worth a try.
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Old 10-23-2020, 07:58 PM   #6
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There are two engine brakes on my ISL with two connections on the valve cover. You could unplug one to confirm what your feeling. I would also check the wiring harness from where it connects to the valve cover (check for a short).

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Old 10-23-2020, 08:54 PM   #7
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As silly as this is going to sound: Our Jake brake on our 2014 2500 Dodge Ram diesel engine quit when the turbo went out on it. So we had the turbo replaced(thank goodness for a extended warranty on the truck or $4,000.00 for a new turbo was a wee bit expensive.. we only paid 100.00 with the warranty) and our Jake started working again. I know as silly as it sounds .....
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Old 10-23-2020, 09:11 PM   #8
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As silly as this is going to sound: Our Jake brake on our 2014 2500 Dodge Ram diesel engine quit when the turbo went out on it. So we had the turbo replaced(thank goodness for a extended warranty on the truck or $4,000.00 for a new turbo was a wee bit expensive.. we only paid 100.00 with the warranty) and our Jake started working again. I know as silly as it sounds .....
That’s not silly at all. The engine brake on the dodge is an exhaust brake built in to the turbo - VGT : variable geometry turbo. The vanes change pitch to create the back pressure to slow the engine and hence the truck. So if your turbo fails, you won’t have an exhaust brake, and if your turbo is working properly, the exhaust brake will work.

A jake brake is entirely different technology - uses engine oil pressure to open the exhaust vale at the top of the compression stroke “wasting” the energy used to compress the air/fuel, and thus slowing the motor and not providing any power. But this a complicated piece of equipment and only avail on larger engines. No ISB Jakes.

There was an after market one for a while (Pac Brake Load Leash) and still is for the 5.9, but the 6.7 version is no longer available. Must have had problems.
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Old 10-23-2020, 09:35 PM   #9
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Two Jacobs 3 cylinder assemblies was my understanding for the 6 cylinder ISL. Both on high and only one on low. I need to uncover the engine and trace those wires out to confirm. I'll unplug one to see if it will then do the Cummins shutter with 3 cylinders braking.

Haven't found a new rocker switch yet??
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Old 10-23-2020, 09:56 PM   #10
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Quote:
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Two Jacobs 3 cylinder assemblies was my understanding for the 6 cylinder ISL. Both on high and only one on low. I need to uncover the engine and trace those wires out to confirm. I'll unplug one to see if it will then do the Cummins shutter with 3 cylinders braking.

Haven't found a new rocker switch yet??
Probably so as three stage uses 2, 4, and 6 cylinders. The switch is probably too much to hope for anyway but a good Cummins shop should be able to test the existing one. Actually someone here probably knows how many ohms in what position it should read. Just beyond my scope. I’ve replaced jake heads but that was pre-electronics.
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Old 10-23-2020, 11:34 PM   #11
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Thanks for the expplaination!!! Just tell some people that and they think you are nuts ..Why that would never happen. Ummm Ok .. Its your universe have fun living in it !! :-)
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Old 10-24-2020, 10:34 AM   #12
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Quote:
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Two Jacobs 3 cylinder assemblies was my understanding for the 6 cylinder ISL. Both on high and only one on low. I need to uncover the engine and trace those wires out to confirm. I'll unplug one to see if it will then do the Cummins shutter with 3 cylinders braking.

Haven't found a new rocker switch yet??
Here’s one of the two brakes. I wouldn’t waste my time taking off the valve cover since both are working. This sounds like an electrical issue.

Bill
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Old 10-24-2020, 12:19 PM   #13
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Here’s one of the two brakes. I wouldn’t waste my time taking off the valve cover since both are working. This sounds like an electrical issue.

Bill
I agree.
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Old 10-24-2020, 01:58 PM   #14
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Great pic of the unit under the valve cover. The wire near center coming thru the case must be the solenoid actuator of which there are two of. This is what I need to track and ensure they are not somehow shorted together.
Thanks for the great pic!
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