Out today in a P-Reg Scania rigid (93M & 220 on the front) Forgive an obvious newbie question, but there were two pull out knobs on the central console. Left hand one was the engine cutout, but the right hand one remains a mystery - the symbol over it looked like a choke, but that doesn’t seem right. Didn’t seem to have any effect on engine revs when I tried pulling it - am I missing something obvious 
It’s the oil slick control for when the police are chasing you. 
Didnt you notice the lack of cars behind you after you’d pulled it?
cold start if it works
but more than likely a magic tree hanger 
simon
Might be a hand throttle that has been disconnected. Havn’t driven the vehicle type you describe but have driven the series 3 artics and the second knob was a hand throttle, pull out twist and lock. Not a good substitute for cruise control as it is locked on.
Some trucks have their tail-lift isolators or load area lights on pull-out switches.
Either cold start or idle adjuster??
Should I get my coat now??
dennisw1:
It’s the oil slick control for when the police are chasing you.
Don’t make me laugh like that I’ve got a sore back and it hurts!
I think you’ll find it’s a throttle but obviously out of commission
The other knob is almost definitely a hand throttle/idle control but it often does get disconnected as drivers have been known to try & use it as a cruise control with possible dangerous consequences.
So what exactly is a hand throttle for and when would you use it?
The driving school Merc had one but as i never needed it i forgot to ask what it was for.
Usually for when you are running PTO-driven equipment like a HIAB or tanker pump, Dennis. This sort of kit is powered by the truck’s main engine (hence Power Take Off = PTO) when the vehicle is at a standstill, and needs to have the engine revs raised to a constant level to operate. The use of a hand throttle - which holds is setting as it has no spring like it’s foot counterpart (hence no good as a form of cruise!) avoids the driver having to sit there with his foot on the pedal for the whole operation - something which would be inconvenient at best and impossible in a lot of cases.
Another use is with demountable rigids…enabling the driver to stand outside the cab and watch what’s happening whilst speeding up the inflation of the air suspension when lifting up a box.
Ahh the driving school vehicle had a fancy Heated A/C and humidity controlled box on the back for carrying antiques, so i guess it;s so you can run all that while the vehicles stationary or somthing.
Most of the PTO gear i’ve seen automaticaly pics the engine revs up, but i see what you mean.
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Well I was just clearing up my icons on my lap top, & hadnt used this link for a while, Bugger me ive forgotten how funny this can be…
Seriously though, yes it is a Manual Throttle Control, & as others have mentioned it`s no good as a Cruise Control, I found a Drum Stick wedged under the dash was much safer as while you were bashing the the steering wheel with the other Stick 
( to keep you awake) if a problem arose, you could kick it away… 
. The good old days of “Hands Free” & Cruise Control… 
Anyway, New Truck drivers & old hands should never feel embarrassed about asking for help, as you will or should find ( after the initial ■■■■ take ailbet) the driver giving help would be proud to.
I have just got back from a long Double Drive, Bucherist through Bulgaria, 9 hrs to move 3 klms to Bulgarian border, into Greece, round Thessalonika, down into Athens, 12hrs off,then back round Thessalonika & round to Ipsala the Greek/ Turkish border,11& 1/2hrs to get through then into Istanbul, via a Police T.I.R check point to get exit visa stamped, got to airport at 5 am yesterday into heathrow by 11, home by 3.30. job done… thats Rock n Roll for you… Off now until Monday, then on Low Loader to France for the week…