Jake Brakes Not Working

Glennman · July 14, 2019, 10:33:25 PM

I have a 1974 MC-8, 8V71 HT 740 Allison that apparently has Jake Brakes (the taller valve covers, the wire coming out of each to a micro-switch mounted to the governor with 3 wires, a diode, etc.). Some of the earlier literature on the bus states that it has Jake Brakes, etc. However, it sure seems it would have been wired with a switch somewhere at the driver's area.

There is a foot switch at the driver's area next to the seat, but further back from the dimmer foot switch. I don't know what it is for. Could this have something to do with the Jakes? The reason I ask is that one of the Jacobs wiring diagrams I viewed online showed a foot switch (a large square one). Were there some models that utilized a foot switch?

I know this is a lot of questions, but I have confidence that someone out there knows enough about these things.

I read on another post that these are typically wired so that the Jake only works when the transmission is in lockup. I suppose I could drive it around with the transmission in lockup, try the foot switch, other switches, etc., but I'm sure there a much easier way to find the switches for it (there are several switches that are not labeled, and don't appear to have any function).

I know... more questions! Thanks all for any input! Glennman

muldoonman · July 15, 2019, 06:06:04 AM

Does the MC-8's have breakers or fuses inline to Jakes? On my 91 Prevost, they quit working and push breaker was all it was. Schematics say there's 2 breakers associated front and back and it was the front one. Didn't know what tripped it as it had worked for 6 or 7 years and last year didn't work when we started a trip. Got back and started looking and found breaker tripped.

lostagain · July 15, 2019, 07:04:19 AM

There should be an on/off switch on the dash. From there, the power goes to the switch on the governor. Then to the solenoids in the heads.

The foot switch behind the dimmer switch is the air horn.

If you have a switch on the governor, you don't have a foot switch for the Jakes.

Look on the Jacobs website for the service manual.

I don't remember Jakes on MC8s in '74. Must've been added by a previous owner.

JC

chessie4905 · July 15, 2019, 08:03:45 AM

You may want to pull one or both valve covers to determine that it has the rest of the Jake system installed first. Some get into the process and add the solenoids last. Also, make a note of injector tag number and tag color. Check for any moisture telltale in there also. Water droplets, sludge or rust stains and the crossover tubes at each injector for washed clean compared to others. Covers aren't difficult to remove and reinstall. Reuse gasket unless the covers are leaking oil now.

Glennman · July 15, 2019, 06:07:47 PM

Thanks all... Just to address some of the comments; I have 3 foot switches at the front; one is the horn, another the dimmer, the other (?). I don't believe the Jakes are OE. They were installed by the previous owner to the person I bought it from. I'll check under the valve covers and see what they look like.

chessie4905 · July 15, 2019, 06:33:42 PM

Someone here they use a floor button for turn signal?

buswarrior · July 15, 2019, 06:43:57 PM

Foot switch operated jakes were one of the ways these things entered the bus industry...

Not hugely popular, but it was done.

'74 MCI would have jakes retrofitted, so the sky is the limit for what's been done.

I'd be figuring out what all my driver's area switches do, before I go taking anything apart and make more trouble...

trace the wires, see where they go, buy one of those electrician's beepers for tracing wires, it will save your sanity...

happy coaching!
buswarrior

lvmci · July 16, 2019, 01:10:29 PM

Hi Glennman, when you turn your master switch on and all the instrument panel, telltail lights illuminate, or if you have a test button for that, is there one for Jake's? Maybe the bulb is out? Maybe not in that era, lvmci...

akroyaleagle · July 17, 2019, 11:05:55 AM

Why don't you just apply a 12V (or 24V if that's what your bus is) to one of the wires coming out of the valve covers, with the engine running, and see if Jake activates on that side. Do it on the other side also.

chessie4905 · July 17, 2019, 12:08:26 PM

Or pop valve covers and then do it. He can then see if all senoids operate and injector size. If everything is there, he can buy a cable tracer from HF and hook transmitter to unplugged wire from head and then probe around front of coach to hear the beeping or squeel till it is located. Great inexpensive,useful item.

Bob Gilbert · July 17, 2019, 12:59:46 PM

What Joe said, but don't forget to rev it up as they needs oil pressure to work.

luvrbus · July 17, 2019, 01:04:15 PM

The Jake switch on a MCI 8 is on the left consul up towards the front they tie into the panel under the drivers window with a 5 amp breaker or 5 am inline fuse mine was a factory install

Glennman · July 20, 2019, 11:31:53 PM

Great additional ideas... Thanks.

bevans6 · July 21, 2019, 08:14:22 AM

Virtually all jake brake testing can be done with the engine off. Static tests - engine off - you can test for power at the input to the jake buffer switch. You can put the engine manually in no-fuel with the engine stop lever and test for power at the output of the jake buffer switch. If the buffer adjustment is wrong, you can have it where the buffer switch does not get switched on at no-fuel. You can take the valve covers off and test for power to the solenoids with the engine going into and out of no-fuel with the engine stop lever, and listen to hear the solenoids click. If you get that far, you then back off the oil line bridges between the slave and master heads and check that there are little square O-rings in the ends of the bridges, then you reset them to spec ( 1/16" clearance off the top of my head, but the spec is available). With that done, you can start the engine, rev it up to 1800 - 2000 rpm, release it to idle, you can watch the injector control racks go to no-fuel on the over-run, and you can hear the jake brake work, and likely stall the engine. On a manual transmission there is usually a clutch switch so the jakes only work when the clutch is out, and on automatic transmissions a neutral switch so the jakes only work when the engine is not in neutral. If you have an automatic transmission you may need to bypass the neutral relay to get power to the buffer switch to do any of this testing. A simple jumper cable would work. But if you just can't get power to the buffer switch and you have an automatic, try putting the transmission in gear (engine off so the thing doesn't drive off on you).

chessie4905 · July 21, 2019, 11:16:13 AM

When I brought my 4905 home (Jake's inoperative). Pulled valve cover and found 1/4" gap between two of the solenoids, and a 1" approximately thin metal disc lying in the head. It was off one of the solenoids. Some of the solenoids were different than others. Newer and older designs. Robbed a solenoid from another set of Jake brakes I had. Cliff told me to use a spanner wrench for removing solenoids and it worked fine. Found the gap setting procedure and proper clearance between oil passageways on the Jake brake site.