Question

[Parrot of Doom]

Is it normal, after starting the engine, for the revs to drop by 100 or so when I depress the brake pedal before moving from P to D?

I can turn the engine over, and sit there for 5 minutes with it happily idling away at 600-700rpm, only for that to drop down to 500 the moment I press the brake.

Just wondered if it was some kind of engine management (obviously lower revs while engaging gear is preferable).

 

[Hibbo]

Does it do it suddenly, as soon as the brake pedal switch is made, or steadily as you apply more pressure?

 

[turnipsock]

Does it do it suddenly, as soon as the brake pedal switch is made, or steadily as you apply more pressure?

I think I was thinking along the same lines.

Does turning on the lights cause the revs to drop?

 

[Parrot of Doom]

The longer I hold the pedal, the more the revs drop. If I tap the pedal lightly, they drop slightly.

The revs don't go back up, and they don't drop too low. It feels as though its meant to happen, I just wanted to confirm that.

Battery and alternator are fine btw She never fails to start, starter motor is nice and spinny

 

[jberks]

I'd say no it isn't normal - I haven't experienced it on my previous cars.
The only place the engine and brakes are related is on the vacuum side so slight vacuum leak developing on the brake servo perhaps?

 

[television]

As JBerks, has to be a vacuam issue, there is nothing else to connect the engine to the brakes.

Malcolm

 

[Parrot of Doom]

It only ever does it when the engine is first started - in normal operation it never happens, although saying that I don't ever coast to a halt!

Its almost as though the engine is revving slightly higher to aid warming, and when I press the brake pedal its thinking "ah, here comes the gearchange, better step down a bit". It doesn't actually feel like much of a fault.

I wonder if the accelerator cable is set correctly - theres a small plastic screw near the potentiometer that sets the cable tension, just like on a bicycle brake cable.

Holding the pedal down causes a very slight momentary drop in rpm, only 50rpm or so, but its only on the same scale as in any other car when you push the steering past full lock, or turn the aircon compressor on.

 

[type49]

I seem to remember a problem with the one-way valves on the servo hose but I can't remember what it was. Jolt any memories anyone??

 

[television]

It only ever does it when the engine is first started - in normal operation it never happens, although saying that I don't ever coast to a halt!

Its almost as though the engine is revving slightly higher to aid warming, and when I press the brake pedal its thinking "ah, here comes the gearchange, better step down a bit". It doesn't actually feel like much of a fault.

I wonder if the accelerator cable is set correctly - theres a small plastic screw near the potentiometer that sets the cable tension, just like on a bicycle brake cable.

Holding the pedal down causes a very slight momentary drop in rpm, only 50rpm or so, but its only on the same scale as in any other car when you push the steering past full lock, or turn the aircon compressor on.

I think that you have given the answer, the cold start devices could be part
vacuam operated, this means that you can go some more happy motoring.

If there was a leak,then you would lose the vacuam assistance on the brake as you kept your foot on it.


Malcolm

 

[eric242340]

I dont think it is normal, and the non return valve on the pipe to your servo can cause this. It wont affect the brake action much but it will affect the rpm. This is not a common fault but we have had to change some. And most of thos were on the S Class 140.:???:

 

[Parrot of Doom]

Could you give me an idea as to where I would find this pipe? Leaky vacuum pump or leaky pipe? All other vacuum devices seem to function normally so I'd guess its just one pipe?

 

[eric242340]

Could you give me an idea as to where I would find this pipe? Leaky vacuum pump or leaky pipe? All other vacuum devices seem to function normally so I'd guess its just one pipe?

Its located on the main vacuum pipe which runs from the manifold to your servo unit, the servo unit is the big round cylinder to which the brake master cyl is attached. Hope this helps.

 

[Parrot of Doom]

Thanks, I'll have a poke around in the morning.