Jakes sticking on - buffer switch?

[sdc53]

Something weird and unexpected happened tonight - was driving my bus and when I applied the gas, I had lack of power and extra jake noise.  Flipped the jake switch off, and it stops.  So that's my workaround for the moment.

I suspect the buffer switch. I know where it is, but I'm not confident about removing it for cleaning or inspection because my understanding is that it has to be installed at a certain depth, properly, or some nasty things can happen?

Some advice from those more familiar with the procedure is welcomed..

Scott
'69 4107
Gladstone, OR

[bevans6]

The buffer/jake switch is a two part affair.  You can take the jake switch off without disturbing the adjustment by undoing the large nut at the switch end of the threaded rod.  The nut fits over a collar at the end of the threaded rod to hold the switch on.  Once you undo the large nut you can lift it away, and take out the pushrod that fits inside the threaded rod, which might be binding or gummy.  The method of adjustment for the buffer is actually to adjust it with the switch either loose or off, so that you can turn the threaded rod in and out without having to turn the switch.

Disclaimer - this is how my buffer/jake switch works, and how the Detroit manual describes the adjustment method, for an 8V-71.  Yours may vary.  

You can manually exercise the switch with the engine off by turning the engine stop lever on top of the governor all the way to stop, the jake switch should switch as the lever gets to full movement towards stopping the engine.  You can usually test with a test light by making the switch operate using the engine stop lever.  If the switch is working properly, you can test with the engine running by letting the engine idle (switch should be open), then accelerate the engine to around 1800 rpm, let it sit long enough to be sure the oil pressure is up so the jake modules work (2 - 3 seconds) then snap the speed lever closed so the engine returns to idle.  The jakes should operate while the engine returns to idle (switch closed) since the engine is at no fuel during that deceleration phase, then the switch should open and the engine should return to idle.  If the engine is cold it might stall during this test.  If your buffer/jake switch is indeed sticking on, I would not expect the engine to idle.

Edit...  Thinking about my last comment for a second (I would not expect the engine to idle) if the engine was running at all, it's more likely that one module is sticking.  If the jakes were actually being commanded on by the switch, all the cylinders would have zero compression.  If one or a pair were being stuck on, only the affected cylinders would have no compression.  If you have an 8V-71, the jakes work in pairs, one is master and one is slave, so you could get only the slave sticking on, or if the master failed you might get both the master and slave sticking on.

Edit for the second time...  Since the jake brake releases compression in the cylinders just before TDC, would a manual switch to operate the jake in the engine bay work as an emergency shut-down system?   You'd have to have electricity, but the old truism that you need two things to make a diesel run - fuel and air - kind of overlooks the fact that you also need compression...

Brian

[luvrbus]

A lot is going to depend on how it is wired if you have a manual transmission most GM's had a mirco switch on the clutch pedal also some are wired into the brake circuit,it may not be the buffer switch do you have the combo fast idle Jake buffer switch ? 

Jakes won't stall a warm 2 stroke they don't have enough oil pressure to engage fully when hot

[sdc53]

Clifford,
I have an Allison V730 and do not have fast idle.  I think I have the standard buffer switch.

Scott
'69 4107
Gladstone, OR

[luvrbus]

Does it screw into the governor housing or set on top working off the throttle linkage ?

[sdc53]

It screws in on the side towards the middle of the V.


Scott
'69 4107
Gladstone, OR

[bevans6]

I forgot that at idle hot the jakes might not come on enough to stall the engine.  Clifford, what do you think about my idea to use the jakes as an emergency stop to stop a run-away?  I think it might stop the run-away, then as revs and oil pressure falls allow it to start again. 

Brian

[pvcces]

Brian, I think you have answered your own question about the use of the Jakes in a hot engine to shut it down.

A runaway might happen because of a stuck injector or because of the engine burning it's lube oil. If it was burning lube oil, closing the rack might not have much effect as it would not reduce the fuel that the engine is running on.

I think that you would want emergency equipment to be very reliable, so the Jakes don't look like a suitable choice.

For what it's worth.

Tom Caffrey

[bevans6]

My thinking was that jakes don't have anything to do with fuel source or the injector racks at all in this context, just relieving compression so the fuel can't ignite.  But the drawback is they need oil pressure to work.  They might not shut down the engine, but they surely would stop a run-away.  Regardless of what it was running on - oil, fuel, whatever, if it's turning fast enough to have oil pressure the jakes would work and it would stop running at least until the jakes stopped working.  I will have to think on this, but it's probably not worth doing.  I might try it once or twice, though.

Brian

[sdc53]

Thought of something I wanted to ask if it might be related.  I have noticed recently that when I hit my power off switch a perhaps a 0.5 to maybe 2 second delay to actual engine shutdown.  This used to be instantaneous.  Does the power-off mechanism operate in the same region as the jake buffer switch?  Reason I ask is perhaps something else is getting gummed up in there.


Scott
'69 4107
Gladstone, OR

[bevans6]

Yes, the two are in the same area of the governor.  It might be worth while taking a look to see that the shut-down lever isn't binding, etc.  Inside the governor there are a series of levers and springs.  When the engine goes to no-fuel because it's on zero throttle and over-run, the levers and springs combine to press on the end of the buffer screw, which in turn activates the jake switch.  When the shut-down lever is activated, that spring/lever setup is over-ridden and the engine is forced into no-fuel.

On my engine (MCI setup) it's very easy for me to turn the engine stop lever and see if it's sticky or moving well.  There is a return spring on it, it should move smoothly clockwise to activate and snap back on it's own when released.  With the engine off and power on, you should see power on one terminal of the jake switch, other side off, then see the power transfer through the switch when you turn the engine stop lever all the way clockwise.  I can hear mine switch, too, if it's quiet around.  If you feel the need to lubricate beyond what you can do from the outside, you can take the top of the governor off - it just lifts off and doesn't disturb anything inside.

More likely the delay in shutdown is in the air cylinder or the Skinner valve.  If you have a rear stop switch, hit it and see if the air cylinder is moving the engine stop lever promptly, etc, or if there is a delay if you move it manually before the engine shuts down, if you can reach it while the engine is running.

Brian

[luvrbus]

Which Jake switch do you have the Mirco switch 2 pc unit like Brian or the new style 1 pc sealed unit,the buffer controls the roll of the engine 

The starting aid will control the shut down if out of adjustment that is done on the #2 cylinder injector on the right bank which side that is on a GM is anybody guess lol 

Sounds like you a little maintenance like a tune up but if you have the old style buffer switch you probably have lost a nylon roller or pin, try spraying carburetor cleaner on the old style in the pin opening it may gummed up also

good luck

[sdc53]

Thanks Brian and Clifford.
I've never taken the top off the governor, the throttle, etc is on top of it, if that all lifts off as one unit without disassembly I may do it. I have an air throttle in that area as well.

>Which Jake switch do you have the Mirco switch 2 pc unit like Brian

I think it's a two piece switch.

Clifford what do you mean by the buffer controls the roll of the engine?

>The starting aid will control the shut down if out of adjustment that is done on the #2 cylinder injector on the right bank which side that is on a GM is anybody guess lol

Unfortunately on a GM I think right side means the side facing the front of the bus, which is buried behind the turbo etc.

>Sounds like you a little maintenance like a tune up
Yep I just had that done, rack adjusted etc this summer. $2000. I think it needs to go back to the shop for some tweaking, but I'm curious and wanting to learn, I've rebuilt gas engines myself but most of the DD stuff is really foreign to me.

[bevans6]

Engine roll - after the idle speed is set, the engine will tend to float around that speed, kind of up and down a bit, or "rolling".  The buffer screw is adjusted so that it just touches the mechanism at idle, and helps to stabilize the idle speed.  Left and right cylinder banks are the same regardless of rotation, per this document:  http://www.powerlinecomponents.com/literature/detroit_diesel/v71_model_chart.htm  I think you are right, the right bank is the forward bank in a GM.  I can't quite think of why you need to set the starting aid using any particular injector, but it's early and I don't have my book here to look at the procedure...

Brian

[luvrbus]

The right hand bank Id was a joke it is what most call the upper head on V drives a real pain to remove and replace seems like all head problems on a GM are always on that head it can never be the lower (left) head where it is easy to get too lol. 

Brian when you figure out why DD uses the # 2 injector on the right bank for the setting let me know,you set the starting aid on another injector they are hard to start when cool and and take a few seconds to shutdownl BTDT