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PostPosted: Mon Feb 22, 2021 10:46 pm 
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Full Pressure Turbo
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Joined: Mon Sep 08, 2008 8:37 pm
Posts: 774
Location: deepest northamptonshire
Hi all,

1992 c900, FPT ( converted from LPT by previous owner), Lucas not Bosch, no cruise. Intermittent boost issues. I've tried nearly everything I can think of, scoured the UKS archives using search and I'm getting nowhere-and I'm running out of ideas.

Issue : within the period of one journey it will boost cleanly to the border of the red (0.8 to 1.0 bar/ 12 PSI), come off throttle and then later only base boost (~5psi) . 5 minutes later - full boost. Rinse and repeat.

I've gone through many ( nearly all- explanation after ) of the steps on 900aero.com , including in no order :

1. full service, new plugs, rotor cap, rotor, timing checked with strobe and set to 16'TDC, without vac hose connected to distributor. Vacuum advance/retard of distributor checked by strobe and moves from timing mark under vacuum/pressure. Known good distributer swapped also. Hall effect sensor wiring repaired properly. Timing OK?
2. All vac hoses replaced - with 4mm internal bore, with the exception of the fuel evaporation cannister circuit which I blocked for testing with no change and the circuit which enters the cabin - which holds a vacuum. Seals on the inlet manifold for the various vac lines replaced. Vac lines go the right place regarding dump valve, distributer. Missing non return valve added to vac lines which run to "vacuum bottle" in O/S wing. Non return valve present to cam cover and both in correct ( suck only during manifold negative pressure) orientation. Vac hoses good ?
3. Induction plumbing triple checked, tight and dump valve checked for operation ( and replaced with new ). Induction circuit good ?
4. APC solenoid. Only allows compressor to wastegate flow when 12v applied, otherwise vents to return. 12v is present on ignition and audible click heard. No fluttering. Connection to APC solenoid sound. Known good solenoid tried. APC solenoid good ?
5. APC box. Got four of these in total. None makes any difference. One is off my other c900 which boosts fine ( 16v turbo)
6. Turbo : with APC to wastegate feed removed ( which does not allow the wastegate to open with the action of the APC) - go for a drive and boost right into the red no problem and repeatable. ( >12PSI). Wastegate good ?


None of this has got me anywhere, so :

7. Blocked off vacuum pipe to cabin at manifold to test pressure transducer under the dash. This sends an over boost signal to the APC.
This restored boost. but when I removed it to test with a multimeter, it changes resistance with pressure ( going from roughly 100ohm to 600ohm at 2bar) - but importantly - when pressurised with a pump when connected - only made the APC solenoid click at approx >1.5 bar ( well into the red anyway). It seemed to do this reliably.



1. The issue with the pressure transducer seems contradictory ?
2. How can I test it better - anyone know what the resistance values are meant to be ?
3. is there anything else under the dash which could cause an issue and send a signal to the APC to limit boost. I know of the fuel cut off - this works at approx 2 bar as normal- and there is no cruise fitted?

Has anyone got any ideas ? I've only got the knock sensor left and I'm loathed to touch that !

_________________
92 c900 aero turbo
87 c900 vert


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PostPosted: Tue Feb 23, 2021 2:19 am 
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UKS Addict
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Joined: Tue May 05, 2009 4:36 am
Posts: 6527
Location: London
Car Models: c900 convertible, XJS
Under the dash are 3 items

Pressure transducer, which you may be testing

Overboost switch, this is an on off switch.


Brake switch


All of which may cause you to lose boost.

The overboost when operating normally, as it operates when the pressure is at maximum, cutting in will nearly send your head into the steering wheel!!

I would check your brake light switch as these can be temperamental, broken or loose. Then check the wiring and the overboost switch.

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Tarquin Fin-tim-lin-bin-whin-bim-lim-bus-stop-F'tang-F'tang-Olé-Biscuitbarrel. Silly party.


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PostPosted: Tue Feb 23, 2021 8:03 am 
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Light Pressure Turbo
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Joined: Sun Apr 27, 2014 5:10 pm
Posts: 416
Location: Wiltshire
Car Models: '89 Saab C900 T16S 'Trenty'
What anjum says about brake switch, mine was ‘sticky’ - you can get to it through one of the speakers in the dash (forget which side)

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Saab 9-5 Aero Estate (gone)
Porsche 930 (sold)
E38 740i ‘sporty shorty’ Daily driver (sold)


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PostPosted: Tue Feb 23, 2021 11:25 am 
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UKS Addict
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Joined: Tue May 05, 2009 4:36 am
Posts: 6527
Location: London
Car Models: c900 convertible, XJS
Lhs speaker cover.
On the 900aero site http://900aero.com/ under the tech tab and theory section you will find a q&a about the brake switch operation.

_________________
Tarquin Fin-tim-lin-bin-whin-bim-lim-bus-stop-F'tang-F'tang-Olé-Biscuitbarrel. Silly party.


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