I have an 88 900T16 S which has developed a really annoying problem.
Sometimes it only boots to half way, or less. The next time I drive it, it
may be OK and fully boosting again. My garage has replaced the APC control
box, but this has not solved the problem. Does anyone have any ideas?
Thanks in advance.
James Borden wrote:
> I have an 88 900T16 S which has developed a really annoying problem.
> Sometimes it only boots to half way, or less. The next time I drive it, it
> may be OK and fully boosting again. My garage has replaced the APC control
> box, but this has not solved the problem. Does anyone have any ideas?
> Thanks in advance.
This seems to be coming up quite a bit lately…
Check (in rough order):
o All vacuum lines, everywhere
o Your knock sensor – remove it from the engine
block, tape it somewhere safe, and drive around
o Your APC solenoid – first try unplugging it –
you should get basic boost *all* the time. If you
don’t something else is wrong. Listen (with the engine
running) to it before and after the engine the engine
is hot – if it is clicking, you need a new one. Also,
get a 12v (9v should work) battery, and connect the
poles of the battery to the pins on the solenoid –
with voltage applied across the pins, the solenoid
should click once. If it shudders or makes many
noises, it’s shot.
o Your cruise control – if the cruise control ‘ECU’
is acting up, it might cause the APC to go into
basic boost mode (normally, while the cc is on,
you are limited to basic boost). Also check all the
vacuum lines to the cc unit in the fender.
o On the same note, check the two pressure switches
above the clutch and brake (one per pedal) to make
sure they aren’t leaking vacuum. Ideally, remove
them and check that they are not permanently shorted;
if they are, you’ll get basic boost.
o Your turbo itself (VERY unlikely) – unplug
the vacuum line between the turbo wastegate
and the APC solenoid and drive around for a
*tiny* bit – you should get uncontrolled boost
(limited to ~15psi by the fuel cutoff). If not,
there may be something amiss with the turbo
o Finally, some cars (I don’t remember what years)
will go to basic boost when you hit the brakes. So,
check the brake light switch above the brake pedal
and make sure it’s working properly. If it’s not,
BTW, your brake lights will be on all the time.
Good luck…
-Justin
Comment by admin — December 31, 2009 @ 4:48 pm
See http://www.townsendimports.com/turbo_folder/baseboostdia.htm
A chattering APC solenoid is usually NOT indicative of a bad solenoid,
the solenoid is only doing what it is told by the APC ECU from info from
either the knock detector or the pressure transducer. The blue vacuum
switch on the left fender has nothing to do with APC, it controls the
upshift light on manual tranny 2.2 LH cars. A leak in it however, could
lower boost levels. Never seen one leak however. The red capped switch
there is a part of APC, it limits the boost to base levels when the
cruise is engaged. Swapping the electrical connectors between the two
switches will cause base boost only, but this would not be intermittent.
Check the wiring to the APC solenoid. It is common for, it to loose its
insulation and if the wires touch, the signal to the solenoid is lost.
The solenoid needs power and ground to open to bleed boost from the
compressor side of the turbo from the wastegate so the wastegate doesn’t
open and reduce boost.
Comment by admin — December 31, 2009 @ 4:48 pm
In article <906215614.26712.0.nnrp-08.d4e40…@news.demon.co.uk>,
"James Borden" <ja…@jbsystems.demon.co.uk> wrote:
> I have an 88 900T16 S which has developed a really annoying problem.
> Sometimes it only boots to half way, or less. The next time I drive it, it
> may be OK and fully boosting again. My garage has replaced the APC control
> box, but this has not solved the problem. Does anyone have any ideas?
> Thanks in advance.
Your garage should not have gone to the expense (I believe around 300 pound)
of replacing the APC box until all the inputs to the box had been checked to
be OK. These are knock sensor, pressure transducer, engine rev’s of the back
of the rev counter and I think brake sensor on 88 up cars. I would guess that
the problem you describe is due to a knackered knock sensor. Unbolt sensor
from side of engine block, wrap in a rag and then go for a drive. If the
effect doesn’t occur then you know the culprit. Strangely I had a similar
problem, whereby the boost would be let of suddenly and then wouldn’t come
back on. This problem got worse and resulted in constant activation of the
APC solenoid. It turned out that the pressure transducer had been sticking
and eventually got completely stuck at 160 ohms. I replaced the transducer
but also found I had to re-adjust the trim pots on the APC box in order to
get the boost back up. In your case if this has been happening over a long
period of time I would go for the faulty knock sensor hypothesis. Yours
sincerely NJ Haughey
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Comment by admin — December 31, 2009 @ 4:48 pm