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Old 06-10-2007, 01:50 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, 03:05 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, 01:00 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, 01:07 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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Old 06-11-2007, 01:35 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, 01:58 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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Old 06-26-2007, 11:40 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, 03:36 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, 10:35 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, 01:51 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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