JAKE BRAKE

[Pete]

Woody, I think you have an 8V92 Detroit engine, unless it has been changed out.

When I had an 8V92 with a jake, I found the relay that controls the jake to be bad. It is located on the rear of the engine as you face it from the rear of the bus, and in the area above the main shaft belt pulley.

You can test to see if the jake is working by simply starting the engine from the drivers seat with the toll window open, increasing the rpm's with the transmission in neutral, then releasing the accelerator pedal. If it is working, you will hear it retard the engine.

If the relay is bad, it can be purchased at any DD service center very inexpensively and easily replaced.

The relay is not the only culprit that will cause your jake not to work. It is only a suggestion that you can try.

By the way, welcome to POG!

[Pete]

Woody, needless to say, I failed to mention to have the jake switch set to the HI position while testing....sorry. Try the test with the jake on and off.

[Jon Wehrenberg]

I'm not sure I agree with Pete on this one, but there is a test of the Jake brake while parked that I have used in the past.

A Jake is essentially a system that turns your engine from a power unit to an air compressor. When it functions as an air compressor, it has the compression stroke the engine would have when used for power, but at the end of the compression stroke, the Jake mechanisms open the exhaust valves so none of the work of compression is used to push the piston back down. That exhausted compressed air is the noise heard when the Jake functions.

What makes it work is a mechanism located in the cylinder head under the valve covers. There are two solenoid valves in each head. They have to work for the Jake to work. Your problem could be a failed switch, failed solenoid valves, broken wires so to determine if the solenoids are functioning look in the large rear electric box. I don't know the exact post number, but it is likely to be listed on the diagram for the electric terminals and it will be described as engine brake or retarder or something similar.

Do not go by my memory, but the post numbers for the Jake are going to be low, single digit, like posts #1 and #2, or #2 and #3. They varied slightly from coach to coach as Prevost made design changes. With the engine idling (be careful around the belts) use a jumper from the 12V post (on the bottom of the box, check with the voltmeter) to each of the Jake posts in the box. Leave the jumper on for a few seconds and if the solenoids are functioning properly you will hear the engine lug down as if it were under load.

Check each post by using the jumper. If only one post lugs down the engine focus on the side that did not lug to insure you have continuity all the way to the solenoids. You may have to remove the valve cover. If both sides lug the engine down then you can rule out any further investigation relating to the solenoids or the wires going under the valve cover to them and should instead focus from the switch at the driver's seat back.

Hope this helps.

[Will Garner]

Woody,

While working in the engine bay with engine running DO NOT under any circumstances wear any loose clothing, rings, or watches. It is very easy to really do damage to your body parts and it happens very quickly with an engine running 1,000 - 1,200 RPM. The engine does not even feel it but you sure as heck will when you come back to your senses.

[truk4u]

Woody,

You won't get any jake action in neutral by raising the RPM's, the transmission has to be in lock-up before any jake function. If you don't follow Jon's suggestion, just drive the bus locked in first gear and run up to 2100 rpm and then let off the throttle. You will hear and feel the jake if it's working.
While driving, you will get a very slight movement in the boost gauge when the jake is working.

Remember, the jake on the 8V is very whimpy compared to the 60 and you need high rpm's to get any benefit. On the low setting, it's just about worthless.

[Jon Wehrenberg]

Tom just jogged me out of a senior moment.

During normal driving with the Jake off your turbo boost gauge should drop to zeno when you let off the gas pedal. At that time the pyros should start to drop and if you can coast long enough, (like downwhill for a few miles) the pyros should also hit the bottom of the gauges.

When you engage the Jake while coasting the turbo boost will drop to zero, but the pyros if set on the high setting should both read about 300 degrees. If on the low setting one pyro will drop to the bottom of the scale, but the other will be at 300 degrees approximately.

If this is what you see no sense checking in the back. They are working. If you do not see those indications, then run the test I described above to determine if the problem is in the wiring or the solenoids.

BTW, if it is in the solenoid, they are available and after removing the valve covers they are accessible for replacement, a fairly easy job.

[stevet903]

Here's a link for 8v92 Jake Brake documentation:

http://www.jakebrake.com/support/par...obs_product=26

The troubleshooting manual will step you through the process to find out what is wrong:

http://www.jakebrake.com/service/pdf2/021772.pdf

If you are getting power to the valve covers, the common problems are broken wires under the valve covers, leaking o-rings on the solenoids and leaking or broken crossover tubes.

If you find that the module is bad that controls the Jake, you have a slightly bigger issue. I don't think it is made any more. However, Prevost put out this bulletin to replace the Jake module with a relay and some updated wiring.... I had to do it and it works well.

Here's the link:

http://prevostparts.volvo.com/techni...f/Is-99061.pdf