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
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.
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
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.
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.
Someone here they use a floor button for turn signal?
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
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...
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.
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.
What Joe said, but don't forget to rev it up as they needs oil pressure to work.
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
Great additional ideas... Thanks.
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).
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.
It's not always a electrical problem with Jakes the engine I am working on now had 2 broken cross over tube and 1 broken plunger
luvrbus glad to see a picture ! nice !
dave
The man in the hat Gary made it easy now to post photos 8)
I thought I would revive this thread that I started, since it is still concerning my Jake brakes.
Just to remind everyone what is happening with the Jakes... they are installed under the valve covers, wiring is there, micro switch is wired up, relay is installed, a wire feeds the relay from the rear panel near the air cleaner, but I have not been able to figure out why there is no toggle switch at the front operating the brakes, nor which wire at the front drivers window goes to the rear Jake relay.
I used one of those wire tracers that was suggested on this board and found the wire that goes from the relay at the motor area that is connected to the Jake micro switch at the governor to the front main panel under the driver's window (1974 MCI MC-8). The wire dead ends there at connector nut #56 (no other wires are connected to #56). I'm thinking that I simply need to find a power source there at that panel (again, under the driver's window), install a toggle switch to provide power back to the relay, and see if it works.
My question is this: with the bus power being 24 volts, is there any situation where there is both 12 volt and 24 volt in any of the bus wiring (not referring to the house wiring at all of course)?. My concern is that I don't know if the Jakes are 12 or 24 volt. I'm assuming that since the bus is 24, and the Jakes are already installed, then they would be 24 volt. But if the bus wiring has 12 and 24 volt, how would I know which to use? I'm new to 24 volt systems. Also, does my conclusion above sound correct (providing the power at the panel under the driver's window to a toggle switch)?
Thanks for all your help, Glennman
Looks like someone disconnected at the front end for some reason. If you don't have a switch on your dash somewhere that controlled it, is there a chance you have (or had) foot control for the Jakes? Sounds logical to add power to the system at the front through a switch if you've checked over all the items at the rear, but I'd disconnect the system at the rear temporarily so that you can test the wire in the harness for shorts (just to be sure that's not the reason it was disconnected).
Is it 12v or 24v? Could be either, in my experience. My bus is a 24v GM, and nearly every single thing on the bus is 24v. Imagine our surprise when we discovered that the electric a/c clutch was running on 12v? Luckily we caught the very small print in the wiring diagram before we ordered the new one last year. Also, in working on our speedometer recently I learned that it runs on 12v. Seems that the bus makers were good about mixing things up as needed.
Not sure how you determine the voltage for your Jakes. I'd bet they are 24v, but this is something you should know before connecting. Are the solenoids under the valve covers themselves marked with part numbers or voltage?
Any chance you have a factory wiring diagram which would show the original setup?
The older Jake's used the same solenoids for 12 or 24 volts. Now they sell different ones. I think you can check which voltage they are by disconnecting wire at one solenoid and check it's resistance. I recall Cliff mentioned in past what it should be for the voltage it is for.
Just checked. Jacob's vehicle systems has a downloadable pdf. troubleshooting manual with everything you need to know on install/ checking/ diagnosis. Also solenoid resistance numbers listed.
It is so damn easy to check with a test light just hook it up to the buffer switch and start flipping the switches or stepping on switches if you have power to the buffer and if they don't work move on,the Jakes will work without driving the bus fwiw
There are 3 foot switches at the drivers floor, 2 toward the front, and one a bit further back. I know one runs the high beams, but the other 2 (?). I suppose I can check the micro-switch for power at the governor and see if it gets hot when the buttons are depressed (and I can try some of the unmarked, seemingly unused toggle switches too). It just doesn't make sense to me that someone would disconnect them, unless as it was suggested, there is a short or something. I found an old hand written letter that is a description of the bus written by the owner prior to the gentleman I bought the bus from, and he says that the bus was equipped with Jakes. That's how I found out! I doubt that the guy I bought it from even knows what Jake brakes are (nice guy, but he is A-mechanical).
I'll let you all know how that goes. Glennman
Highbeam switch, electric horn switch to replace the air horn button, that leaves one for...?
Are all 3 of these mounted in the left step up in the driver's area?
The one under your heel will be for the horn.
Those switches lead a miserable life, foot drippings and being stomped on, they are regular failures.
Happy coaching!
Buswarrior
I have read about Jakes being installed with a foot switch, even where the switch was a momentary one which had to be held down to engage the Jakes.
The other two? High beams and air horn?
Could have been disconnected by previous owner(s) to keep inexperienced drivers from using them in icy road conditions., especially if it was commercial operation.
Big surprises happen if a good Jake is activated in icy roads!
Well, I tested the wire connection at the Jake connection at the governor micro switch by depressing the 3 foot switches (yes, they are at the left foot area, one at the heal, the other 2 at the toe). I turned the key on, had the bus in neutral, nothing. I suppose I could try it with the auto tranny in neutral (?).
It seems to me that if the wire at post #56 at the main panel under the driver's window is all by itself, then it is dead ended there. In other words, aren't those posts used as connectors such that if there was a switched wire from a hot lead, then there would be 2 wires on the post instead of just one? Many of those studs have 2 wires together telling me that the post is a connector in lieu of wire nuts. Is this correct?
I have confirmed continuity from the Jake at the engine compartment to the #56 post at the front. I'll download the pdf wire diagram suggested in this discussion. Thank you everyone.
Can you get to the underside of the three foot switches? If you can, it should be pretty obvious which are connected to what, or at least give you a starting point for tracing things forward.
I believe you are correct about the purpose of the terminals. That post #56 would have had another wire on coming from the front of the bus. On my GM, the power feed goes from the power source, through a relay, a clutch switch, and the dash switch before going to the terminal in the driver's electrical panel. Then it rides the harness till it emerges at a terminal in the engine compartment electrical panel.
While you are tracing wires, might as well determine what the extra foot controls are supposed to do. How do you like the signal tracer? I really like mine. Makes tracing a wire easier for me.
Stock, the two toe switches were for high beam and the optional fog lights.
The heel switch was either the plunger button for the optional air horn, or was a momentary switch wired to the electric bill horns, parallel to the steering wheel mounted buttons.
Not to be confused with floor mounted turn signals, that's a whole different animal...
Fog lights had interrupt circuitry with the headlights, one or the other, not independently operated.
Happy coaching!
Buswarrior
I really like the signal tracer. I bought it from a recommendation on this board. It'll be very handy for many things.
LOL I hope you don't need these they have gotten expensive Van is still reeling this tiny box with 4 of the solenoids for $500.00 and are aftermarket too
I have 1980 Prevost with a 8V71T and HT740 automatic transmission.
I found an operators manual that specified the Jake Brake switch as off, low and high.
A previous owner did a stalled drivers panel upgrade with a two position switch on/off. When you turn it on, it sort of hardheartedly slows the bus. Nowhere as strong as I think it should be. Operator manual has all sorts of warnings about slippery roads, etc.
However looking at the electrical schematic in the Maintenance Manual; it shows a single control wire for the Jake Brake functionality from front to back.
How would have the low high functionally have worked?
Low operates half of the cylinders of the engine. High operates them all.
Quote from: chessie4905 on September 23, 2019, 04:15:30 AM
Low operates half of the cylinders of the engine. High operates them all.
I was kind of asking how this could work over one wire. Now thinking the electrical diagram must be in error and that I should trace the actual wiring.
Single wire are 1 stage Jakes
There might have been a few options available (two-stage & single-stage) and your books reference both, but in different places. Go with whichever is actually installed in your bus. I've found lots of things which are slightly different in the book than reality.
The 2 stage brakes will have a wire from each head back to the control switch,1 to power the buffer switch from the control switch up front and 1 to power the switch from the buffer switch (4 total) if you have a buffer switch,sorta a waste of good wire on a 2 stroke engine
Quote from: richard5933 on September 23, 2019, 08:56:19 AM
There might have been a few options available (two-stage & single-stage) and your books reference both, but in different places. Go with whichever is actually installed in your bus. I've found lots of things which are slightly different in the book than reality.
OK, with the dash conversion not really sure what I have. So I should determine this by:
1. Pulling a valve cover? Stage one will cover fewer cylinders ? I have a V8...
2. Checking if there is separate wires to each head that go to the junction box?
3. Look for the old switch hidden away under the dash?
Thanks everyone!!!
Quote from: freds on September 24, 2019, 09:52:05 AM
OK, with the dash conversion not really sure what I have. So I should determine this by:
1. Pulling a valve cover? Stage one will cover fewer cylinders ? I have a V8...
2. Checking if there is separate wires to each head that go to the junction box?
3. Look for the old switch hidden away under the dash?
Thanks everyone!!!
I'd go with what Luvrbus suggested and start by looking at the wiring. It takes one wire front-to-back to power the single stage Jakes. There will likely be some unused wires if the system had a multi-stage setup at some point in the past.
Your bus may not have had factory installed Jakes, and when they were installed they were installed as a single-stage setup. That would make the information in the Operator's Manual a red herring.
Quote from: richard5933 on September 24, 2019, 10:08:28 AM
I'd go with what Luvrbus suggested and start by looking at the wiring. It takes one wire front-to-back to power the single stage Jakes. There will likely be some unused wires if the system had a multi-stage setup at some point in the past.
Your bus may not have had factory installed Jakes, and when they were installed they were installed as a single-stage setup. That would make the information in the Operator's Manual a red herring.
Thanks everyone!!!
OK it is looking more like I had stage one; but was operating at stage 0.5 in the picture below:
(https://www.crystalpoint.com/cpdownloads/public/outgoing/Freds/JakeBrakeSwitch.jpg)
Looking from the back of the engine; yellow is a wire going to the left bank, red is a unused terminal and blue is a wire going to the right bank of the engine that isn't hooked to anything. Below is picture of the wire coming out of the right bank and there is similar one on the left bank.
(https://www.crystalpoint.com/cpdownloads/public/outgoing/Freds/RightHandJakeBrakeWire.jpg)
Any electrical tests to perform before I hook it back up? In case it was disconnect for a reason?
Thanks again...
Effectiveness of the Jake also depends on the setting.
Many are set at .065
Some are set to .060
Mine are set at .055 They work great!
Be advised that when setting them at close tolerances you could cause major damage. Be careful!
So far, that is unknown till someone measures piston to valve clearance at tdc. I would think that Clifford would have checked one out of curiosity, due to all the engines he's rebuilt/ repaired.
I set mine at .059". Works good.
Do not think that Jakes will put you through the windshield. That is fantasy. They slow you down and hold you back, in the right gear for the circumstance, but it is not like the service brakes. That is why I don't agree with wiring the brake lights to the Jakes. Mine in the bus feel about the same as the engine brake in a semi loaded at 80,000 lb GVW.
JC
Quote from: chessie4905 on September 28, 2019, 12:24:15 PM
So far, that is unknown till someone measures piston to valve clearance at tdc. I would think that Clifford would have checked one out of curiosity, due to all the engines he's rebuilt/ repaired.
I will do that for you Chessie since I am starting on another 8v92 lol I getting a little tired of looking at 8v92's
Thanks!
Hi Everyone
And thanks everyone for your help, I measured the resistance on the connected bank and the disconnected bank.
The connected bank was 16 Ohms and the disconnected bank was 24 Ohms.
Simply attaching the wire did increase the effectiveness, however still not a robust response or change in the exhaust sound when engaged.
However when I turn if off while engaged it is more a definite effect/feedback.