[77newell]

Additional notes: my 77 coach with 6v92ta had single stage Jake brake operation and no "fast idle" setting. Regarding the single stage; the wiring is as I described it in my previous reply. If you have a two stage system the wiring is different in that the output of the buffer switch will have to go back to the front of the coach to the 1-2 stage selector switch before going to the rocker box covers.

The manual is pretty clear though the illustrations are really dated, some of them look pre-war and I'm not talking Vietnam.

[prairieschooner]

Steve does your dvm have a hold function? That should show you the peaks.

Forest,
What the heck is a DVM?

[encantotom]

digital volt meter

tom

[prairieschooner]

I was going to hold off until I finished but here is what I know so far about the older systems after talking to a representative from Jacobs.
These older Jacobs Brakes usually did not have a Control Box. We have a Micro Switch that is energized by a piston type unit that attaches to the Governor. This Switch is energized by a "No Fuel" situation. It looks like this.
   
Now the System changes to Hydraulic where the Solenoids (1 each per side) inside the Valve Covers are energized. In our case it appears to be an Electrical issue so I may not need to pull the Valve Covers.

I didn't get a signal out of the Micro Switch when I checked with the my Digital Volt Meter. Wasn't sure if it wasn't fast enough but the Brake still didn't work when we came home, if I was any further from home I would have repaired it there. I have ordered the new Switch and will install it later, I just didn't want to start removing engine parts while at a State Park and Dry-Camping.

[77newell]

Steve: that looks quite similar to the one on my 77 coach. With the engine shut down and the Jake dash switch on you can gently run a small screwdriver or something similar between the rod end and the switch and that should trigger the microswitch if it is functional (or not if it isn't).

If the switch checks out then you will need to do the buffer switch adjustment by turning the threaded rod into the housing 1/8th of a turn at a time until it starts functioning. If the switch is dead, then following its replacement you have to do the whole adjustment process as described in the manual. None of this is difficult (the hardest part is keeping your body parts out of the rotating fan while doing the initial adjustment to eliminate the "rolling idle" mentioned in the manual - though in my case fear of dismemberment was incentive enough to take it slow). Just get the new switch initially threaded into the governor housing, start the engine and work slowly in. If you see blood you dun sumpen rong. It really is a pretty straightforward job.

[prairieschooner]

Thanks 77 but my experience with Mirco Switches is that when they start to need an adjustment (with 30 years industrial experience) they are on their way out and since the rubber boot was damaged I bought some new ones today. I'll post the numbers later but I bought them from Cummins, the bag had a Cummins Number yet the Micro Switch had a Detroit Number!
I should have it replaced within the next few days, but hopefully the adjustment is as straight forward as everything else.

[77newell]

Steve: My experience with micro switches is the same as yours. Several years ago when I talked with Jacobs about the switch the person I was talking with was pretty adamant about buying the switch and its mounting assembly from them as they are able to accurately position the switch relative to the other operating parts. I did not have to replace any of my parts so it was moot. The point they were making is that the clearances are critical and it would be hard to replicate in the field. I don't know if that is real or just trying to sell parts. I can say based upon the tiny adjustments I did that made big differences that if I were trying to replace the Micro switch I would try to figure out a way to very accurately replicate the position of the micro switch relative to its actuation points (if you can determine them from the current switch). I don't know for sure if that matters but if it does it would be very difficult to get the position right unless you knew exactly what the "on" dimension was. Just some thoughts, feel free to disregard if they don't match your experience. My experience is several years old and my memories don't improve with aging. Although I have to say I remember being far better than others' remember me as.

[prairieschooner]

My question is how to Adjust only the Micro Switch (I bought a new one from Cummins for $16.34). The Mounting Bracket for the Micro on my Coach is Slotted so I should be able to slide the Micro into Adjustment and then Lock it into position.

Cummins #3871549 Detroit #018188.

thanks in advance!

[77newell]

This is the problem I noted in my previous posting. The only way I can figure to do it would be to measure the distance from the end of the threaded cylinder to the end of the rod sticking out at the point at which the switch "clicks". You would first have to measure on the original and then duplicate that measurement on the new one. Given how touchy these relationships are you will have to be very precise. Thankfully just adjusting mine in the engine per the manual was sufficient in my case. I never asked Jacobs about the price of the assembly, did you get a price?

[prairieschooner]

No since Jacobs no longer sells parts for these. A while back they went to selling through the Engine Manufacturer. I did get in touch with the Representative and he is the one that gave me these part numbers.

My Micro Switch had moved so there was no way to know precisely where it was. Not sure what manual you used 77, My Assembly doesn't look at all like the "Buffer Switch" in the Manuals.