1985 Mercedes 300D Turbo Diesel - Boost/Turbo Question

[marco5]

1985 Mercedes 300D Turbo Diesel
500,000KM
Non Cali version

Hello,

I have been doing quite a bit of testing with my turbo system and boost. I've replaced all boost cables that go from my intake to the switch-over valve and the switch-over valve to the ALDA. I've also replaced the cable from the waste-gate to the turbo which was cracked badly.

I've come up with the following prognosis for the vehicle:

The turbo is spooling but not kicking in at all. Even after replacing all of the cracked boost cables.

The turbo boost starts boosting just after 2000RPM and maxes around 3750RPM. The max boost is 9PSI - this is really good considering original turbo.

When I bypass the switch over valve, its night and day difference. This means directly connecting a boost cable from the intake to the ALDA and not connecting the switch-over valve. I don't just want to replace the switch-over valve without confirming it's bad. Yes I know I can leave it bypassed but I want the original setup for protection in the case of an over boost.

Here are my questions:

Is this valve known to cause issues?

Does leaving this disconnected/bypassed cause any other problems other then the over boost protection possibly with transmission shifting?

How can I now diagnose the switch being bad and not a ECU/computer issue?

What is the ECU what gives this voltage and what should the voltage be? How can I test the voltage going to this valve?

Thanks in advance - you guys are the reason I'm able to drive this beauty!

https://www.photobox.co.uk/my/photo/full?photo_id=21829094341

[Alec300SD]

No major issues from the switchover valve failing...other than losing boost and losing power (which indirectly affects the tranmission shift points).

Okay to leave it disconnected, if you are willing to give up the overboost protection.
No transmission issues should result if it is bypassed (and the ALDA is still getting a boost signal).

I believe it is a simple 12 volt solenoid valve that is always open when unpowered.
The circuit is powered when the switch is grounded.
When the boost sensor sees 1.3 bar (the pressure closes the air-gapped contact inside the boost sending switch) then the ground connesction is established from the switchover valve to the intake manifold.

If the valve is working properly, air should pass throuh un-impeded when the switch is not powered (unplug the electrical connector).

1) On your disconnected swichover valve (no electrical or boost lines attached) test to see if air flows through it.

If the valve is closed when unpowered (unplugged), you have a soot-blocked valve (needs cleaning), or valve that has seized in the closed position (needs replacing).

2) If the valve passes the previous test, see if air flows through the valve (with the car running) when you re-establish ONLY the electrical connections.

If the valve is closed (no boost passes through the valve) when powered...your valve responds as if has an overboost condition: due to a short to ground from either a bare sensor wire, an internal short in the swichover valve, or a bad boost sensor on the intake manifold).


To measure if voltage can reach the valve, unplug the switchover valve connector and attach your voltmeter to the harness connector in place of the switchover valve...you should get 12 volts when you then start the car.

[Dan Stokes]

I routinely run Mutt the Race Truck with 20+ psi boost with no issues other than the EGTs getting high at the end of the track (a 1 mile run with the skinny pedal on the floor). This is on a stock bottom end OM617 (main change is a Dieselmeken pump). I'd bypass the switchover valve and never look back.

Dan

[R-3350]

same here im running around 28-30 psi on an 8mm dieselmeken pump with all stock internals no issues as long as you watch your temps. you can even delete the alda and it wont hurt anything