My 33 Seems To Have Lost Vacuum.

[andrew]

I've been using my 33 on a fairly regular basis on quite a few runs over the past few months & noticed recently that it appeared to be more "revvy" than it had been. There seemed to be a marked reluctance to change up at about 25-30 M.P.H., as it did previously. I've done some investigating, and I think (but I'm not sure....) that I've found the problem, but before taking anything to bits I thought I'd post this, in the hope of some guidance. My understanding of the vacuum system on air-cooled Dafs is as follows. The vacuum is taken from the rear of the inlet manifold and is fed into the electric low ratio valve solenoid. From there it is fed to the mechanically operated "change up" valve unless the low ratio hold is operated or the brake pedal is depressed, in which case the vacuum is fed directly to the "change down" side of the primary. If the brake or low ratio hold is not operated, the vacuum passes from the change up valve to the "change up" side of the primary when the engine revs reach 2850 R.P.M. (in the case of Daf 33 cars) This vacuum is lost when the "kick-down" is operated.

So far so good..... I connected my vacuum gauge to the outlet at the back of the manifold and the reading was very healthy, but when I checked the vacuum coming from the low ratio valve to the change-up valve it was a fraction of the vacuum at the back of the manifold, which concerns me, as I should have expected them to be identical (or as near as). This suggests there's a leak within the low ratio valve, which is possibly more likely as I think I can hear a "hissing" from the valve, although it's difficult to confirm this-the background noise of the engine running doesn't help! I tried blowing down the pipes and felt there was a resistance inside the valve, although I think there is a restrictor in there somewhere, to prevent excessive change ups.

Any advice as to whether I'm over-reacting and is there a sure-fire way of confirming if there is a problem?

Edited: I've done the "blow test" down the pipes to and from the primary unit and there's no leaks and a healthy "blow back", suggesting the diaphragms and piping are all in place and in good order.

[bobdisk]

Not trying to teach "grandmother to suck eggs" here, I go back to when I first got my 33, not then knowing Daf details. Check that the brake lights are not stuck on, this would leave the vacuum valve stuck on, so leaving it permanently in low ratio. With the engine idling, in neutral, do the revs increase when you put your foot on the brake pedal? and also when you press the green low ratio button? With a multimeter (or a bulb) check the voltage at the vacuum valve comes and goes when it should.

[starider]

It's worth checking the brake light switch. I had replaced the vacuum valve a while back on the van and everything worked OK. Then recently the valve did not work and I realised that the rear lights also weren't working, I checked the switch connections which seemed fine but decided to clean the teminals,as I pulled the wires one of the terminals broke away from the switch and there was the problem, the terminal had completely corroded away. After a quick email to Steve a new switch arrived and brake lights and vacuum valve work OK. If the valve is faulty you could hear a hiss from it.

[dafman50]

8put of 10 times on 44/33 its vacuum is lost because of brake switch I use around 23 plus a year so give you idea.

[andrew]

Dec 12, 2021 7:54:32 GMT bobdisk said:

Not trying to teach "grandmother to suck eggs" here, I go back to when I first got my 33, not then knowing Daf details. Check that the brake lights are not stuck on, this would leave the vacuum valve stuck on, so leaving it permanently in low ratio. With the engine idling, in neutral, do the revs increase when you put your foot on the brake pedal? and also when you press the green low ratio button? With a multimeter (or a bulb) check the voltage at the vacuum valve comes and goes when it should.

Dear Bob (and others!)

Never feel inhibited about suggesting the obvious and I I'll never take offence if anyone does! I've spent today having a quick think through the car's recent behaviour and as it happens, I've worked out a few things:
1/ The brake lights aren't stuck on, but thanks for suggesting it! The "low hold" solenoid seems to be working properly and the engine revs up perfectly, just as it ought to when the brakes are applied gently on a long slow descent.

2/ Having looked at the design of the low hold solenoid I think the problem is that the solenoid is not shutting properly when the brakes are released, and so therefore air is leaking past the valve, making the mixture excessively weak. This explains the continuous stalling when the car comes to rest with the brakes applied and the lack of vacuum going to the primary pulleys means the car's not changing up as it ought.

3/ A non-closing solenoid could possibly be repaired, as I think there's a spring that holds it shut unless the brakes are applied and the solenoid is activated. Maybe a piece of grit or dirt has become lodged in it? I'll investigate tomorrow....

[andrew]

I think I've fixed it! As part of the "improvements" I've been attempting to make to my car I've fitted three 52mm gauges below the radio, in the middle of the dashboard. These consist of a rev counter in the middle and 2 vacuum gauges on either side. The rev counter was fairly easy to wire up and works perfectly now. The vacuum gauges have proved a tad more problematic. Fitting them to the dashboard bracket was easy, but routing a vacuum pipe was tricky.....

In an effort to trace the vacuum leak I started by connecting the vacuum gauge to the manifold outlet, and I recorded a vacuum of between 16-20 inches, which seemed good. The vacuum I recorded in the pipe leading from the low ratio solenoid to the change-up valve was only 10-15 inches and the level recorded leading to the change up side of the primary pulley was next to nothing! Clearly the vacuum was being lost somewhere, and I removed all the rubber hoses and blew through the valve outlets and inlets, in an effort to clear any muck. Nothing seemed to come out. I then put all the hoses back on, making sure they were all a really tight fit. The result was a vacuum of 20 inches between the change up valve and the primary unit, and, more importantly, a much happier car, cruising happily at 30-40 mph without any sign of the excessive revving I had noticed before. Fitting the vacuum gauge pipework will have to wait for another day, now!

I can only think that one of the pipes had somehow developed a small leak, which had reduced the effect of the change up vacuum. The tick-over seems smoother and the stalling after slowing down seems to have stopped, too.

All in all, a successful day's tinkering and, to finish things off, I checked the secondary pulley gap, which is a tad over 1mm-perfect!

[andrew]

To begin with, I should like to wish all fellow Daffers a Happy New Year!

This morning I filled up my 33 with B.P. premium unleaded, as £1.609 per litre (EEK!) and later (over a glass of something with which to celebrate the New Year....) I calulated the M.P.G., as I do every time. This time it worked out to be 28.1 M.P.G., which is appalling, although the earlier calculations have been 38.8, 33.2, 39.4. 39.1, 42.5 and 37.8, so I'm taking the most recent reading to be affected by the change-up valve not functioning properly. Let's hope so, anyway. I have also noticed how much more economical a Daf is, when running constantly at a high speed. Maybe it is only once the Variomatic is in the "overdrive" ratio that the engine becomes really economical, and that town driving involves the transmission changing down more than a manual gearbox would be changed down by a driver.

Over these thoughts I shall muse a little longer on this New Year's Day, and re-iterate my felicitations for the new year! Let's hope we can all meet very soon!