Asiandude

Hello from a new poster on this site.
Maybe someone on here can advise with a tech issue on my 928S4 (1987 auto): The intake flap/resonator valve fails to open above 4000 rpm, but does open briefly during engine starting (as I believe it should). The vacuum, actuator and solenoid are all good of course and a mutimeter shows 12V at the solenoid only during engine starting (just a secong or so) but nothing at higher revs. I'm thinking that a rpm sensor maybe at fault (hall sender?). Has anyone come across this problem...any ideas?
Thanks for any input and all the best.

Leon Speed

You need to punch the gas pedal - i.e. depress it really quick - in order do that. Slowly raising the rpm's won't do.

Jim bailey - 928 International

Run some long wires inside the car and have a passenger use the multimeter while driving to test the power to the solenoid.

the flyin' scotsman

The flappy system is quite simple: there's a vacuum source to a switch and when the switch is activitated sends the vacuum to the flappy .........on or off.

So............is vacuum getting to the switch? is the switch being activiated? is the flappy holding vacuum?

With a vacuum pump you can check most of the system but the easiest check is the shaft movement on start up..............and you confirm that?

Asiandude

Thanks guys, have checked as you suggested but it still only opens briefly when starting and no volts at the solenoid when driving (Jim). I believe the LH control sends the volts to the solenoid (per wiring diagram) so it must get an rpm signal from somewhere to initiate opening at high rpm. No other clues from the wiring diagrams though.

ZEUS+

The LH grounds the solenoid above 4500 rpm under load.

Asiandude

Ok, so I connected the mutimeter again and went for another drive.....really hammered the accelerator this time.......with the tacho swinging upto the redline, Yo I have 12V!! So the flapper only works under 'thrash' conditions! Now i know.
Thanks for your help gentlemen.