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Old 06-10-2007, 12:50 PM   #1
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Choke? Throttle? Buttons and knobs?

OK, I may be a compleeet idiot, but I bought a 1985 GMC diesel school bus (Carpenter body) and I have no manuals, no nothing. I know you boys don't read manuals, but being a girl, that's very frustrating. So can someone identify a few things for me and what they do?

First, there's a round button on the dash, near where the key goes, and it's labeled "choke." It's just a button, flush with the dash. Doesn't pull out or anything. What is a choke for? When do I need it?

Next to that, there a big knob labeled "throttle" that turns (I think) or maybe it pulls out? What is that for? When would I use that?

See, I warned you, lots of dumb questions. But you will be proud of me -- I did wonder what all the extra lights and buttons were on the box that has the radio and the dome light switch and all that. When I opened that up (just curious) I saw that most of the spare stuff had nothing hooked to it, so nevermind on that one.

Finally, are there any other knobs I should know about, that I haven't discovered yet? Inquiring minds want to know! Getting ready to take a long trip and probably should know what's what before I go.

Thank you all for being here and being so helpful. I don't know what I'd do without you!!

Mona

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Old 06-10-2007, 02:05 PM   #2
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Re: Choke? Throttle? Buttons and knobs?

Hello Mona,

I'll try to help you out. I am not too familiar with you type of bus, but here is a semi-educated guess. Since your bus is a diesel, there wont be a choke for it. That was probably a dash that GMC used on all of their buses at that time. The "big knob" labeled "throttle" is probably a type of throttle lock. I forget the technical term, but its basically kinda like cruise. Use it with extreme care.

HTH,
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Old 06-11-2007, 12:00 AM   #3
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Re: Choke? Throttle? Buttons and knobs?

Oh goodness, I hadn't thought of that option -- that the choke button might really be meant for a different bus altogether.

On the throttle knob -- it's like a cruise control? Can you explain any further? Like, when would you turn it (or push it?)?

OH also, there's another knob near the floor on the driver's side. It actually looks like a valve -- maybe for the heater? I know I am being vague. I guess I could just turn it and see what happens, but I thought I'd ask.....

Mona
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Old 06-11-2007, 12:07 AM   #4
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Re: Choke? Throttle? Buttons and knobs?

Mona,

For the throttle lock......I believe that when you get the accelerator pedal where you want it, the knob will screw or pull out and lock the throttle into position, thus holding your speed.

For the knob on the floor, could it be a dimmer switch? Or is there actually a valve type handle on it, kinda like a water faucet? If so, then I would think that it is a heater control valve going to the heater towards the back of the bus, same side.

Never hurts to ask before you find out that it does hurt.
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79 IH Scout II/392 (0.030 over) Hamilton Injected/727/D300/Full size axles/36" Swampers/Lockers/OBA/38 gal Custom fuel tank, roll bar, tube doors.
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Old 06-11-2007, 12:35 AM   #5
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Re: Choke? Throttle? Buttons and knobs?

Oh excellent to know. So maybe the throttle knob will make it easier on long trips to keep the speed without having to press the accelerator pedal so hard? But do you have to undo it to stop, then? Seems scary if it doesn't automatically go off like a cruise control...

And yes, the knob/valve near the floor looks like a water faucet knob. So I think you're right -- probably a heater valve.

Thanks so much! It really helps to be able to ask someone all these questions.
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Old 06-11-2007, 12:58 AM   #6
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Re: Choke? Throttle? Buttons and knobs?

I dont know how exactly the throttle lock works, you'll have to try it out before you are going down the road.

Glad I could help.
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79 IH Scout II/392 (0.030 over) Hamilton Injected/727/D300/Full size axles/36" Swampers/Lockers/OBA/38 gal Custom fuel tank, roll bar, tube doors.
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Old 06-26-2007, 10:40 PM   #7
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Re: Choke? Throttle? Buttons and knobs?

EEEEEEKKK!!!!

don't use the knob for "cruise control"

the throttle knob i've seen in most buses pulls straight out...it's easier to pull if you step on the accelerator at the same time. Once the knob is pulled out from the dash (yours may turn, i've never looked at a gmc) it makes the engine run faster without stepping on the accelerator. The purpose is to let the engine run at "high idle" mostly for winter time. Diesel engines don't heat up well at idle. If you're a bus driver I think it's common to run a bus in high idle mode when you first start it in the morning so you can make heat to defrost windows and the bus driver. High idle mode is also useful to increase the current generated by your alternator to run large 12 volt dc loads.

the purpose of the knob is not to control the speed of the vehicle while driving. This can be very dangerous. IF you had the knob pulled out all the way and had to stop in a hurry....you could slam on the brakes and the engine would still be running at wide open throttle significantly increasing your stopping distance.

Once you understand what the knob is and is not supposed to be used for then you can make up your mind about how you want to use it. I know it's not safe, but i have used my knob for cruise control along with various other methods because my foot gets too tired to step on the pedal after a few hours. But i shouldn't use the knob for cuise control, it's not safe.
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Old 06-27-2007, 02:36 AM   #8
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Re: Choke? Throttle? Buttons and knobs?

Diesel engines can suffer from cavitation in the cooling system leading to cylinder damage. By high idling you increase coolant flow speed such that it doesn't cavitate and you don't get "hot spots" in the engine. It also increases oil flow to high dollar parts like the valvetrain and the turbocharger. Both my manual and every mechanic I've talked to says to high idle at about 1200 RPM if the bus is going to be idling for more than about 5 minutes.

A word of caution though. When you first start the bus you need to let the idle smooth out and you need to make sure that you build oil pressure before you touch that throttle lock. You should be looking to the oil pressure guage first thing when you start it anyway. After about a minute or so in warm weather I will start to inch my throttle up, but I sit and watch all the gauges while I'm doing it. I need to be checking air pressure anyway.
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Old 07-01-2007, 09:35 PM   #9
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Re: Choke? Throttle? Buttons and knobs?


Big "Oh Yeah Buddy" on the DO NOT USE THROTTLE LOCK WHILE MOVING. Strictly for idling while stationary. Holds position when twisted to the right (probably), can be pulled in and out when twisted the other way. Squeeze pedal down to desired fast idle, then lock with cable knob.

Garden hose faucet by left foot = heater, yup, on both my Bluebirds.

Button labled choke could be glow plugs? Need to know if you have glow plugs at all.

There are no dumb questions; only dumb mistakes made because you didn't keep asking until you learned.

Sounds like you might want to find a local truck mechanic or some such person who can give you a tour of the thing.

Welcome aboard!
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Old 01-21-2011, 12:51 AM   #10
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Re: Choke? Throttle? Buttons and knobs?

I want to thank EVERYONE for this information. SO very helpful. I know 3+ YEARS have passed and I'm just now finally getting around to posting my gratitude, but it's because of all of you and your help that I still have my bus and I just LOVE it. Took my long trip - California to Georgia and back - discovered several things I'd like to work on, but should probably post as separate questions. Again, thank you SO much!

Mona
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Old 01-21-2011, 11:32 AM   #11
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Re: Choke? Throttle? Buttons and knobs?

You know, I never did mess with that throttle knob. BUT, now that I understand more about the "high idle" I'll check into it. Today I'm finally going to take apart my dimmer switch to see if it can be fixed - or at least to take to the store to get a new one. My last ride, the lights wouldn't come on for a long time, making everyone aboard rather nervous. Usually I just have to kick the switch a bit and then leave it on high beam. But I digress...

If I discover what that throttle thingy does, I'll come back and post. Wish I knew more about so many things...

m
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Old 01-21-2011, 02:05 PM   #12
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Re: Choke? Throttle? Buttons and knobs?

UPDATE: I fiddled with the throttle knob a bit, discovered if you turn the silver knurled nut at the base then the throttle button can be pushed so that the knob then can be pulled in and out. BUT, that said, when I started the bus and tested it out, there was no noticeable difference in RPMs no matter what I did to that throttle knob. And anyone else following this post - the other person who replied earlier was right. The choke button has nothing to do with my bus at all. It's just there.

In fact a LOT of my buttons and knobs seem to be inoperable and inconsequential. Now the "faucet" valve on the side that might have something to do with the heater -- I still don't know. A LOT of heat comes in through the floorboard. Almost impossible to drive in very hot weather (think Georgia in the summertime) because your feet/legs get so hot. But then when it's cold out, no heat at all. So I'm thinking I need to fiddle with that faucet a bit to figure out what exactly that's supposed to do.

Any clues anyone?
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Old 01-24-2011, 01:45 PM   #13
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Re: Choke? Throttle? Buttons and knobs?

Hmmmm, interesting to learn all these things. My "faucet" valve is inside the bus, on the panel left of the driver's seat, near the floor. Same panel area that houses the various switches for dome lights, windshield wipers, fans and radio. Makes sense that it somehow controls the heat...but after we took out all the seats, there was nothing indicating that any heat was ever planned to be passed back to the rear of the bus. Poor students! Brrrrr.

All the stuff on the panel seems to only affect or involve the driver area - except the radio - that plays all the way back through speakers in the ceiling. But again I digress. As for the possible heat faucet thingy, I have yet to notice any difference in the temperature based on the rotation of that knob. Wouldn't mind a solution to all the extra heat coming up through the floor/firewall from the engine in HOT weather, though. In cold weather, it's kind of nice.
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Old 02-01-2011, 08:38 AM   #14
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Re: Choke? Throttle? Buttons and knobs?

Bus heater systems usually use multiple shut-off valves. There should be a pair of rubber heater hoses in the engine compartment that run into the firewall. Usually, these have valves to isolate the heaters. The inside valve by your left foot does the same thing, but can be reached while driving. If you open all of the valves (counterclockwise), once the engine warms up, hot coolant will be circulating through whatever heaters are in the bus. Closing these valves will isolate the coolant in the engine from the heater lines, although it will take a few minutes to notice a difference if they're already hot. The heater switches control the fans that blow air across the hot water to circulate the heat. With the switches off but the valves open, you'll still have hot coolant running through the heater, and the heaters will give off some heat. Standard equipment only includes the driver's heater (under the switch panel on your left) and defroster. Most buses have additional passenger heaters in the rear, but they were optional and sometimes omitted on buses used in warm climates.
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Old 02-01-2011, 10:47 AM   #15
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Re: Choke? Throttle? Buttons and knobs?

Oh thank you!! That explains everything. This weekend I will try turning the knob fully counterclockwise and search for a switch that is marked heater. There is one marked "fan" but it turns on a VERY noisy, albeit cute, little tiny fan above the windshield. And I'll tell you, in hot weather, it sure is better than nothing!

Anyway, I'm excited to check out the heater. I'm going to be totally embarrassed if I learn that I crossed the desert in the middle of summer with the heater ON. Thank you again for your very detailed and explanatory post. Knowledge is everything.
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