Post by andrewthe33 on Oct 18, 2011 12:03:55 GMT
Picture the scene…. John Humphries is one side. I’m in the black chair:
“Your name?” “Andrew Hutchings.”
“Your specialised subject?” “The vacuum system of a Daf 33.”
“Mr Hutchings, you have 2 minutes on the vacuum system of a Daf 33 starting..... NOW!”
And that’s how the last week or so has felt! Mind you, if it’s now possible to gain a degree in golf course management I don’t see why I shouldn’t have a HND in the Daf 33 vacuum system, at the very least!
My 33 is a 6 volt “K” registration one, which I totally love. I last owned a Daf in 1991, when I sold GLN 374 N, a lovely red 12 volt 33, that my daughter called “Daddy’s Noddy car” and before that I had owned numerous 33s. My 1st car was a 33, way back in 1978! Now, at the age of 51 I wanted another one, and now I have it!
The bodywork’s not too good, but it passed an MOT, so it’s nothing too bad. The car goes & stops and I’ve already been to one Daf get-together at Duxford in it, which covered 200 miles, so there’s not a lot wrong with it. That said, although my last Daf driving experience was 20+ years ago, I thought this one seemed a little noisy, to say the least. I began investigating the change up and down valves and this is where the learning curve steepened somewhat. I know Dafs were sold as being simple to drive and I agree that they are, but the plumbing that helps this simple driving isn’t at all simple. I hope this will help any other Daffers with their ratio changes.
The drive ratio of any Daf is changed by a number of factors, including the speed of the car, the engine effort, the position of the accelerator as well as a number of other factors. The vacuum produced by the engine is used to assist the belts in this, and is produced (as any engine produces vacuum) by the action of the cylinders in the bores of the engine. The vacuum is taken from an outlet at the rear of the inlet manifold from a rubber pipe, and is passed through a number of valves before being used in assisting the belts in setting the correct ratio, by being passed to either side of the primary unit, which is separated by a rubber diaphragm.
This is what happens in a late 6 volt and all 12 volt Daf 33 (and 44) cars, fitted with an electro-magnetic low hold switch on the dash (the green low hold button.) It is similar to what happens in a 55 and 66 car, although there are slight differences in the operation of the switching. (Earlier Daf 33’s and Daffodils used a hydraulic system, although the broad principles are the same.)
When the car is at tick-over & the low hold light is off, both sides of the primary units are open to the atmosphere and the vacuum is not being sent anywhere. The tick over speed must be low enough to ensure the clutch is completely disengaged. The vacuum pipe from the manifold is connected to the inlet pipe on the electro-magnetic low hold solenoid, which is the pipe that sticks out almost horizontally from the valve, towards the front of the car. The vacuum enters the valve and leaves from the pipe on the off-side of the valve, pointing downwards, and is then carried to the back of the change up valve, which is located behind & below the carburettor. The vacuum enters the change up valve via pipe number 2, and it goes no further while the engine is on tick-over.
As the engine accelerates the rod linkage from the carburettor starts to rotate the change up valve and in doing so allows the vacuum to flow out of the lowest pipe, along a rubber tube, then along a steel tube to the primary units of the Variomatic, where it assists the units in progressively changing on to a higher ratio; by sucking the air from one side of the diaphragm, the atmospheric pressure on the other side pushes the primary pulleys together, thus increasing the diameter of the pulley and increasing the ratio.
The precise point at which the above change takes place is critical to the operation of the car. In 33 ones, the vacuum just starts to flow to the primaries at 2750 R.P.M., although originally it was 3150 R.P.M. This is the point at which the drive becomes “solid” and the clutch no longer slips and also the point at which the engine produces maximum torque. Adjustment is affected by pulling the rod away from the rotary vacuum valve and either fractionally lengthening or shortening it; lengthening it makes the change-up process begin later and vice-versa. This is a crude version of the “sport/comfort/economy” switches fitted to modern automatic cars. (The purchasing of a decent vacuum gauge for about £20 is essential in adjusting the timing of the valve opening. I’ve learnt LOADS about my car by using this gauge!)
O.K., so that’s the change up. Off we drive in our Daf and we can hear the ratios changing into a higher ratio, producing good levels of economy and that pleasant “whirring” sound, so beloved of Daf drivers. If more dramatic acceleration is required (and I realise that the use of dramatic, when applied to a 750cc engine is perhaps stretching poetic licence a bit…) the accelerator is floored and the rotary change up valve rotates to its limit and cuts the vacuum to the change up side. Deprived of the change up vacuum, the primaries then slide into a lower ratio, and the car accelerates more quickly.
The above is the scenario when the car works perfectly. Unfortunately my little 33, (nick-named “Pooh”, on account of its colour…) didn’t quite behave itself, not least because the vacuum from the change up valve was being sent to the WRONG side of the primary units. In addition, a fault in the change electro-magnetic down valve meant that some of the vacuum was being lost there. I first bypassed the change down valve and fed the manifold vacuum straight to the change up valve. The result was that the car refused to change up until about 35 MPH as the now increased vacuum as acting even more strongly to hold the ratio down. Once I changed this, the effect was fantastic! The car immediately became a normal little 33 pleasantly changing up to a higher ratio once the accelerator was slightly lifted. I had covered over 250 miles with the pipes the wrong was round and with vacuum being lost through the change down valve and it returned 30.5 m.p.g. which isn’t exactly bad, but I expect a dramatic improvement now the vacuum’s working properly. I’ll keep you all informed.
Other jobs I need to complete include fitting a new electro-magnetic change down valve & checking if the low hold switch on the dash is working properly. As a test, one ought to hear a “click” as the low hold solenoid engergises. This should happen either when the green button’s pushed or when the brakes are applied. Put the ignition on & listen; if you can’t hear it, there’s a problem. Mine doesn’t click when the brakes are applied, so the fault might be in the brake switch or in the low hold button. I’ll work out which it is by joining the 2 white wires at the low hold switch, which are live when the brakes are applied, to the one green wire that runs to the valve. If this makes the valve click properly then the switch is at fault.
A question I cannot answer & would appreciate help with is this; the 2 steel pipes that carry vacuum to the primary units from the engine compartment are of differing diameters; is it critical which one is used for which side of the primary (i.e., should change up vacuum use the smaller or the larger pipe?) or are they simply of differing diameters to differentiate them and thus avoid confusion when connecting them?
I'll be posting more pieces on this thread very soon, just as soon as I sort more bits out!
