Community Forums

Kylack9 · 06-13-2025, 08:33 AM

Hi Everyone,

Does anyone have access to a wiring diagram for a 2008 diplomat? It has the ISL 400. The engine braking is very weak on this motorhome compared to my previous DPs that I’ve had. My previous DPs had pac brakes but this one has the variable geometry turbo and engine brake combo. On my switch panel it has one switch that says “engine brake”. There is no hi/low switch or a space for one to go. I pulled the valve cover off to ensure it has the engine brake solenoids which it does. My guess is Monaco only hooked it up as an on/off that’s only on the low setting? Anyway if anyone has info in regards to this I’d really appreciate it.

Thanks,

Kyle

3pedal · 06-13-2025, 10:26 AM

You think the engine brake is on the engine, but they only hooked up an exhaust brake? That's interesting.

Kylack9 · 06-13-2025, 10:30 AM

Possibly that’s why I’d like to see a wiring diagram to see where the engine brake solenoid wiring goes. It doesn’t have an exhaust brake, I know the VGT has some limited “exhaust braking” capabilities.

3pedal · 06-13-2025, 11:38 AM

This question might be better in the Monaco Owners group.

shootist · 06-13-2025, 12:24 PM

The jake on off is either a switch or a foot pedal. The hi low is 3 cylinders or six. One wire from the ecm has a breakout at the rear of the cylinder head that goes up to the hi low. If yours differs I really don't know what to say
I hate to ask but is the switch you have a three position? Or originally a three position?

Gary RVRoamer · 06-13-2025, 03:10 PM

As you probably know, the ISL 400 has an engine brake, aka Jake brake. It's a two-speed - Hi is all 6 cylinders and Lo is 3 cylinders. No VGT involved. Officially the engine brake is a Cummins factory option on the ISL, but it's rare to find an ISL without it in a motorhome. And you've already verified the solenoids are there.

That leaves the switch, which should be either a 3-position type (Hi-Lo-Off) or a 2-position (Hi-Lo) with a second switch for On/Off. Mine was the second variety.

wolfe10 · 06-13-2025, 03:41 PM

Actually, a little more complicated.

The ISL is one of the only engine that could be equipped with EITHER an exhaust brake or engine compression brake.

Yes, the chassis/coach builder choice. The engine compression brake was more $$$.

Could a coach maker have gone from exhaust to jake. Certainly. Could the wiring have not been change to accommodate the change-- possibly.

Call Cummins on Monday with your engine serial number to determine what type of brake was fit on your engine.

Then, go from there.

Kylack9 · 06-13-2025, 10:40 PM

Quote:

Originally Posted by shootist

The jake on off is either a switch or a foot pedal. The hi low is 3 cylinders or six. One wire from the ecm has a breakout at the rear of the cylinder head that goes up to the hi low. If yours differs I really don't know what to say
I hate to ask but is the switch you have a three position? Or originally a three position?

Mine does not have the foot pedal, only an on/off 2 position switch, there is no hi/low switch or wiring that indicates there should be one there under my switch panel. I will definitely look for the hi/low wire at the rear of the cylinder head. Thank you for the info

Kylack9 · 06-13-2025, 10:46 PM

Quote:

Originally Posted by Gary RVRoamer

As you probably know, the ISL 400 has an engine brake, aka Jake brake. It's a two-speed - Hi is all 6 cylinders and Lo is 3 cylinders. No VGT involved. Officially the engine brake is a Cummins factory option on the ISL, but it's rare to find an ISL without it in a motorhome. And you've already verified the solenoids are there.

That leaves the switch, which should be either a 3-position type (Hi-Lo-Off) or a 2-position (Hi-Lo) with a second switch for On/Off. Mine was the second variety.

The only switch it has is an on/off. No slot or wiring for a hi/low switch. I’ve peeled through all the wiring under my switch panel and no wires besides the ones that go to the on/off switch have engine brake printed on them. I’ve checked to ensure the existing switch has power and it does and also illuminates when turned on.

Kylack9 · 06-13-2025, 10:56 PM

Quote:

Originally Posted by 3pedal

This question might be better in the Monaco Owners group.

Thank you that’s probably where I should have started.

Lowflyer1 · 06-14-2025, 03:36 AM

Quote:

Originally Posted by Kylack9

Thank you that’s probably where I should have started.

I would have ignored 3pedal's comment about starting a new thread at this point as there is already a lot of discussion here. I would think that most reading the Monaco forum, also reads the Class A. At least owners that have a Class A and that's what matters in this discussion.
The wiring schematics are generic and the actual build can be quite different.

shootist · 06-14-2025, 03:58 AM

Your vgt is involved in the braking but it doesn't affect the jake as far as I've read. You probably should be getting six cylinders and are only getting 3. You can see the 2 solinoid wires going into the valve covers . Make sure you have voltage to both with the jake on. You may have a bad solenoid. Just feed 12 volts to them individually and listen for the click. Disconnect them from the ecm. Probably won't hurt them seeing as the ecm puts out 12 volts to them. But I don't know that for sure.

c_oneil · 06-14-2025, 07:23 AM

Quote:

Originally Posted by Kylack9

The only switch it has is an on/off. No slot or wiring for a hi/low switch. I’ve peeled through all the wiring under my switch panel and no wires besides the ones that go to the on/off switch have engine brake printed on them. I’ve checked to ensure the existing switch has power and it does and also illuminates when turned on.

https://www.irv2.com/forums/f104/two...own-55065.html
Here is a extremely detailed thread from the Alpine owners group on troubleshooting 2 stage jake brake issues and programming options for operation. It is possible that your coach was built and programmed for only single stage operation and that could be just hi or just low. That happened in a few Alpines when they upgraded the standard engine from ISC to ISL but used up some old inventory wire harnesses that were meant for an exhaust brake. Pay particular attention to the posts by EngineerMike. He is an expert and has the same year ISL400 as you I think.

Kylack9 · 06-14-2025, 08:53 AM

Quote:

Originally Posted by shootist

Your vgt is involved in the braking but it doesn't affect the jake as far as I've read. You probably should be getting six cylinders and are only getting 3. You can see the 2 solinoid wires going into the valve covers . Make sure you have voltage to both with the jake on. You may have a bad solenoid. Just feed 12 volts to them individually and listen for the click. Disconnect them from the ecm. Probably won't hurt them seeing as the ecm puts out 12 volts to them. But I don't know that for sure.

Awesome info thank you, I’ll try giving solenoids 12v