Testing Boost Control Solenoid?

The American · Postby **The American** » Sun Jul 06, 2014 9:18 am

I've had a problem emerging with my turbo NB surging/hesitating around 4800rpm. I did plugs and leads (no change) and checked for boost leaks (none) found. As part of searching for a potential boost / vacuum leak, I bought and fitted a vacuum / boost guage.

My car has a low/high boost pressure switch using a MAC solenoid. The low boost setting appears to have the solenoid 'off' allowing full manifold pressure to reach the wastegate actuator (about 7 psi).

When I switched to the high boost setting (I was expecting 12psi), the pressure built and then sailed past 15psi and kept rising fast... The engine has good quality forged internals, so hopefully no damage has been done...

So, having parked the car back in the shed, I've sat and pondered. The solenoid is getting some instructions from the ECU as it is being energised and shutting of the signal to the WG actuator, but for some reason it is not reacting or reacting but not quickly enough once the higher target boost level is reached. Here's my list of possible causes:

1) The MAC solenoid is dead (I've disassembled, cleaned, lithium greased and reinstalled it, but no change).
2) there's a problem with the map in the ECU (I can't see why this would suddenly happen, so seems unlikely. Could a flat battery cause a map loss??)
3) there's a problem with the MAP sensor signal.

Any other ideas or suggestions?

Postby **NitroDann** » Sun Jul 06, 2014 11:54 am

MAC valves are continually powered and are actuated by grounding them. This is done by the ecu at 30-80Hz typically.

If your switch circuit has a break or something that puts it directly to ground after the switch it would do this.

Also if you mix up the hose orientation you can get overboost issues.

Dann

The American · Postby **The American** » Tue Jul 08, 2014 10:23 pm

Thanks Dan,

I've checked through the wiring and could not see any obvious issues. I have ordered a replacement solenoid and will see if that corrects it.

The American · Postby **The American** » Sat Jul 19, 2014 9:20 pm

So, I've just replaced the MAC solenoid, and I can now hear it running when switched on. I don't recall hearing the old solenoid running before, which suggests the old one was not working.

However, I would not have expected to hear it running when the engine is at idle! It runs when idling, stops briefly when the throttle is first opened, and then begins again as vacuum rises and continues as the car begins to go into boost. Although I can hear it operating, boost rises uncontrolled past the target of 12 psi. The pause on throttle open suggests it is getting some kind of instructions from the ECU.

I have checked that the ports are connected correctly: port 1 vents to atmosphere. Port 2 runs to the waste gate actuator. Port 3 is connected to the intake just after the turbo compressor.

I'm wondering if there might be another sensor that is causing a problem. The car does have a small hesitation/pause/bog when the throttle is first opened. Could this be a MAP sensor problem?

Any ideas greatly appreciated!