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High Idle Quit
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Glad to help!
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NOTE: The voltages I wrote in this response proved later to be faulty do to user error. Refer to later in the thread for proper voltages.
Ok, I took the High Idle switch loose from the dash and this is what I found.
This plug is hooked to the switch and has two wires on the back.
The top space is C155 coming from 91A and connects to pin #1 on the switch. With the key on, parking brake set and transmission in neutral it has 13 volts. It should have 24 if I am understanding you and the diagram correctly. The wire on the diagram is labeled “24-91A-18”. With the key off it shows no voltage.
The other occupied space on the plug above connects to pin #5 on the switch. It has no voltage with the key on. If I understand the diagram correctly the wire that feeds that is 91B and comes from R22 and has to do with the transmission in neutral signal. I replaced R22 earlier in this process for kicks and grins.
The switch itself does show continuity between pins 1 and 5 with the switch in the on position and none with the switch in the off position.
I could not easily remove the light bulb at the moment due to fat fingers. I will find smaller fingers or something to remove it shortly. I know that I can not conclusively assume anything but I do not think it will have the proper voltage. A bad bulb would not keep the High Idle from engaging, correct?
I understand that I am not completely comprehending all of this operation obviously but I am catching some piece by piece. Thank you for your patience.
Davy
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13V. I didn’t see that coming…
What ground did you measure that to? If you were measuring from pin 1 to 5, that could make sense, as you would be measuring through the resistance of the coil in the relay.
If the parking brake tell-tale light was getting 12V instead of 24, it would still come on, but dim.
Basically, two things need to occur for R22 to close and command high idle to the ECM. One is 24V needs to pass through the dash switch to the relay, and the other is the neutral switch needs to close (ground) to complete the circuit.
According to the diagrams, pin 1 on the high idle switch is supposed to be connected to junction 69 in the front panel by a wire, so it’s surprising to see different voltages unless the ground was not a good connection when you got 13V. I would be curious to measure resistance between pin 1 in that connector and junction 69. It should be ~0 ohm.
This part also doesn’t make sense, after going back and re-reading.
“The [parking brake] sensors below, I believe these to be the sensors, show 24V on top with the key on and parking brake set. There is no voltage on bottom with the brake set. “
They should pass 24V through when they close. There should be 24V on both sides if those when the brake is set. Measured to chassis ground, not from one lug to the other. The correct sensor will have wire 85C on one side and 91 on the other.
Do you know your chassis number? Your year falls in the middle of the range so I don’t think I would have pulled the wrong diagram.
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Yeah, the 13 volts on the switch and no through voltage on the pressure switches threw me for a loop too. When I jump those pressure switches the service brake light illuminates but no light for the parking brake and no change in idle.
As to ground, outside I used what I think is a ground lug in the top compartment. Inside I used a bolt on the steering column. That may not be sufficient. Nothing else I touched for ground gave me anything but that bolt gave me 13 each time. I will try again and place the ground on the lug outside.
I will also check continuity between pin #1 and junction 69.
Last 8 of VIN are S1001536.
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BIG CORRECTION! I have been giving you faulty information on voltage due to user error. That has caused you to run rabbits for no good reason and I apologize.
The following readings are with good ground.
13V on #1 pin with the key on. This should be 24V. Junction 69 has only 12V with the key on too.
I do show continuity between Junction 69 and the #1 pin on the High Idle switch.
Junction 20 has 24V with the key on.
Pressure sensors:
I can not see any numbers on the wires going to the pressure sensors. The top sensor’s black wire has an 18 or 81 stamped marking but that could be the 18 gauge wire.
With the key off both pressures sensors have 24V on the bottom lug and no voltage on the top lug.
With the key on and the parking brake engaged there is 24V on the bottom lug and 12V on the top lug.
With the key on and the parking brake released the voltage is not changed. It remains 24V on the bottom lug and 12V on the top.
Again, I apologize for the wrong information on voltage. It was completely my fault.
Thank you,
Davy
Also, do the light bulbs on the dash of my vintage twist out, pull out or what? Even my wife’s skinny fingers can not remove it.
Jeff and Lindsay
91 Marathon XL -
Don’t beat yourself up!
As you know, the parking brake switch behavior seems wrong. According to the diagram, it’s just a switch – open or closed. It would seem that this is part of the problem, but you jumped it and there was no change in the tell-tale light or the high-idle switch functionality. The voltage drop would suggest resistance inside the switch, maybe due to corrosion. If you jump it, do you get 24V at junction 69? If not, I would question whether we have the right switch.
The only other thing that the parking brake pressure switch seems to control is the daytime running lights (headlights). They should turn off when the parking brake is set.
I don’t know whether they twist or pull out.
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Thank you for your kindness.
If you jump it, do you get 24V at junction 69? If not, I would question whether we have the right switch.
I do not know the answer to that but I can check it out Monday.
I will also look at the diagram again to try to answer this, but is there a way to check the continuity between the wires on what I think is the pressure switch and some where else? This would be to help confirm I have the proper switch.
I do not believe I have daytime running lamps at all.
Thanks again.
Does anyone know how the top row of dash lights are removed on a 95 XL? Do they screw out or pull out?
Davy
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You’d have to run a long wire, I’m guessing, but checking between the switch and terminal 69 should tell you.
The other post on that switch goes to terminal 20, but so do a lot of other things.
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I only had a few minutes to mess with this Monday but here is what I have.
The top of what I think is the pressure sensor shows continuity with junction block 20. When I run between the bottom of the sensor with junction block 69 the display on my digital meter fades out. I have not seen that before. Is that because it has voltage on it?
I will be back on this Wednesday morning. It just occurred to me that I can turn off the voltage from the batteries to check the continuity if voltage is a problem.
Davy
Jeff and Lindsay
91 Marathon XL -
You don’t want to measure resistance with voltage in the wire. You can blow the meter if there is enough. I learned that the hard way with a water heater element. There shouldn’t be any voltage on this circuit if the ignition is off.
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These are the switches that I think are the pressure switches. Am I correct?
I took the wires off each switch one at a time and checked continuity.
The bottom wire on each switch, which is black, has no continuity between the wire itself and junction block 69 or 20.
The top wire, which is red, shows some continuity between the wire and Junction block 69 and 20. I say some continuity because the numbers on my meter jump around indicating to me there is resistance some where between. Correct?
Maybe I am not checking the right pressures switches.
Davy
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I think those might be the pressure switches for the brake lights. I believe there’s a brake light switch for the primary and one for the secondary air circuit, but I’m a little rushed at the moment to dig into the wiring diagrams to confirm. You could have someone push the brakes and see if 24V passes through.
I’m not 100% sure, but I think the parking brake switch might be on the “Spring brake valve assembly” number 13 in this diagram
https://techpub.prevostcar.com/en/download?id=51&type=pneumatic-diagrams-search-pages&mode=view
Which would suggest maybe the steering compartment?? I wish our bus was around so I could look.
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I apologize. I completely missed the last post to this thread. I was not on the road home yet from Phoenix, but I was dealing with a broken tooth while working and trying to get into a dentist. I hate I missed it but I was a little self absorbed at the time.
I appreciate your advice, the link to the diagram and your willingness to help.I think those might be the pressure switches for the brake lights. I believe there’s a brake light switch for the primary and one for the secondary air circuit, but I’m a little rushed at the moment to dig into the wiring diagrams to confirm. You could have someone push the brakes and see if 24V passes through.
I’m not 100% sure, but I think the parking brake switch might be on the “Spring brake valve assembly” number 13 in this diagram
https://techpub.prevostcar.com/en/download?id=51&type=pneumatic-diagrams-search-pages&mode=view
Which would suggest maybe the steering compartment?? I wish our bus was around so I could look.
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First, thank you to everyone that responded trying to help me trace down the source of this problem. Jeff Rowe solved it once I was able to get it to him.
As was pointed out in the thread, the high idle needs to see the bus is in neutral and the parking brake is on. My parking brake dash indicator light would not illuminate but I could not figure out why.
There are three relays on the ceiling of the lower compartment under the driver’s seat. One of those relays is in the loop that signals the parking brake is “on”. That relay was bad.
Here is a twist. There are 24 volts to that relay and it is supposed to be a 24v relay in the socket. The relay that failed was a 12v relay. It has obviously been there a long time. I have owned the bus 10 years and only three guys have worked in that compartment besides me and Jeff’s shop. Those three guys quickly repaired air leaks and were done.
So I know that relay has been there at least 10 years. It held the 24 volts but eventually burned out. Could it have been a misplaced relay since the bus was built in 1995?
Jeff replaced the bad relay with a 24v and the high idle worked immediately! The high idle “stick” has now been retired!
Thanks again for your help and thanks to Jeff Rowe.
Davy
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Glad to hear you have it fixed now!
The wiring diagram shows a relay for the neutral switch, but not the parking brake switch. Is the diagram wrong?
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