Troubleshooting Vacuum switch boost control

[brherrmann]

I have a 95 4x4 2500 (no EGR)
I have been troubleshooting a boost control issue and found no voltage across the vacuum solenoid. I have 12volts at the red lead and when I ground out the yellow lead the solenoid activates. Before I start taking the wiring harness apart, do I have a bad ground? Is the solenoid controlled via the yellow lead (common 12volts) or controlled via the red lead (common ground)?
Where could I get a decent wiring diagram? My Haynes book is not very helpful as it is geared to gas engines.

Thanks BERT

[Robyn]

The waste gate solenoid is controled via the ECM and they modulate the ground side of it.

The control is called "Pulse width modulation"


Before we rip the wiring al apart. I am not convinced that the issue is the electrical side of the thing.

Have you had any codes pop up (SES light coming on) ??

Whats the vacuum reading at the vacuum pump (direct connection to the pump[ with a gauge ) ???

This should be 26"hg and steady, if the needle on the gauge wobbles all over, the pump is shot.

With the system hooked up, you should have good vacuum to the solenoid and to the waste gate actuator on the turbo.

With the engine running the waste gate arm on the turbo should be stable and you should not be able to move the actuator rod back and forth.

There should be at least 20" hg vaccum at the waste gate actuator.

If not there is a leak somewhere.

When the solenoid is activated the vaccum is bled off to allow the wastegate to open and reduce or maintain the boost level

Wastegate solenoid failures are common on these. If the solenoid fails electrically it can set a code, if the failure is mechanical and the thing leaks off the vacuum when it should not, there may be nothing more than NO BOOST.

Check the vacuum readings first and be sure you have vacuum all the way from the pump to the waste gate actuator. If not trace it back one point at a time to find out where its failing.


Keep us posted.

Robyn

[JohnC]

The PCM controls the ground. It is pulsed (duty cycle) so you can't measure it with a VOM. If you have a dwell meter you can get a reading that way. At idle it should be about 70%.

What is the symptom you are trying to correct? The connections can fail, usually right at the plug. The most common failure is a sluggish solenoid.

[JohnC]

There should be at least 20" hg vaccum at the waste gate actuator.

Maybe. The spec is minimum 15".



When the solenoid is activated the vaccum is bled off to allow the wastegate to open and reduce or maintain the boost level

I think she meant to say "deactivated". When power is to the solenoid vacuum is routed to the actuator. When it is deactivated the actuator is vented to atmospheric pressure.

[DmaxMaverick]

The vacuum pump output should be 20"+. The wastegate is modulated to a max of 15", and it goes down from there, depending on PCM boost control. If you have more than 15" at the WG actuator, something is wrong, or the system has been modified.

[brherrmann]

I have 21"hg at the pump. When I bypass the soleniod I get 21"hg at the wastegate(15"hg if guage is tee tapped at wastegate).I test drove with gauge tee tapped after the solenoid and I get no vacuum reading at any speed/ RPM. I replaced the soleniod with no change. I bench tested both new and old soleniod and both activate at about 8 volts.This is why I'm thinking an electrical issue.
I am still not ruling out a weak vacuum pump as its not making the familiar noise I'm acustomed to. Also the turbo does not whine as noticably when I rev the engine(maybe the turbo is seizing?It still spins.) I am acustomed to 12lbs boost under load (Kennedy chip). Now I'm lucky to get 2lbs.If I bypass the soleniod, I get about 6lbs boost. I put the factory chip back in - no change. Perhaps its time to proceed with my HX35 - I would still like to know whats wrong.
BERT

[DmaxMaverick]

Doesn't sound like the turbo. You may have an issue with fuel delivery, either physical (obstruction or low lift pressure), or a sensor (baro or MAP) is causing the PCM to pull back the fuel. No fuel, no boost, no matter what you do. Without the turbine driving the compressor (either by the WG bypassing, or lack of fuel), the turbo won't whine like you've become accustomed to hearing.

Your vacuum system sounds healthy, with 21/15" indicated. Can't ask for more than that.

[JohnC]

The turbo won't do much with no vacuum on the wastegate. Sounds like the wiring to the solenoid. Look right where the wires enter the plug. They often break there.

You said 15" teed into the line after the solenoid; this implies the solenoid and its associated circuitry is working. If it is, you should have vacuum on the wastegate at least up to about 1800RPM. The PCM will not disable it at lower RPM's. If you're getting zero at idle that points to an intermittent wiring fault or loss of vacuum source.

Any DTC codes stored? I'd expect DTC-78.

Also, make sure the vacuum pump is connected to the port on the solenoid with the restrictor in it.

[brherrmann]

I have checked the wiring back to the computer (I read 10K ohms resistance across the wastegate soleniod leads until I pulled the computer). Since the vacuum pump works and the soleniod works and the wastegate actuator works and the wiring is good, than either the computer is not telling the soleniod to operate or I have a blown port on the computer. All other engine operations seem to be normal - no codes.
Am I missing something?
I now have an excuse to install a Holset HX35 - new project!
Thanks for everybody's input.

BERT

[Robyn]

You can try swapping out the MAP sensor. (little rectangle plastic box on top of the intake plenum)

The Barometric sensor (firewall slightly to the passenger side fo the hydro boost) can also cause issues.

These two sensors feed info on whats going on with air pressure inside and outside the engine and if faulty can make a huge difference in the way things work.

With 15" at idle then near zero when running means that the ECM is pulling the vacuum off way too soon.

The last one of these I had an issue with, I replaced the chip with a Heath Chip (vacuum system overide programming) plus I installed the turbo master.

End of issue.

Just be sure to have gauges (Pyro and boost) and drive the thing accordingly not exceeding 900F Post turbo or 1000-1100F preturbo

Keep the boost set no higher than 12 PSI .

This works fine and then you can stop chasing the gremlin.

The vacuum system was a good idea, but the application hardware and software were a tad unreliable.

If you are not getting any codes (78) for wastegate solenoid then there is another issue.


With the solenoid bypassed, you should see more than 6 PSI of boost under a hard pull.

Was the vacuum reading on the pump steady at 20" ?? if the needle wiggles it means the pump is shot.

The last several pumps I have checked, all ran 26" at the pump.

I agree on the 15" being minimum, but 20" is not uncommon.

What do you get for vacuum at the Turbo if you just rev the engine ?? it should not change much, should be near 15-20" HG

Keep us posted


Missy

[Kennedy]

I have 21"hg at the pump. When I bypass the soleniod I get 21"hg at the wastegate(15"hg if guage is tee tapped at wastegate).I test drove with gauge tee tapped after the solenoid and I get no vacuum reading at any speed/ RPM. I replaced the soleniod with no change. I bench tested both new and old soleniod and both activate at about 8 volts.This is why I'm thinking an electrical issue.
I am still not ruling out a weak vacuum pump as its not making the familiar noise I'm acustomed to. Also the turbo does not whine as noticably when I rev the engine(maybe the turbo is seizing?It still spins.) I am acustomed to 12lbs boost under load (Kennedy chip). Now I'm lucky to get 2lbs.If I bypass the soleniod, I get about 6lbs boost. I put the factory chip back in - no change. Perhaps its time to proceed with my HX35 - I would still like to know whats wrong.
BERT

Without reading the entire thread, if you have good source vacuum, good voltage, but no vacuum at the actuator it is likely the ECM itself.

The way I read this was that you had 21" source and 15" when the solenoid is bypassed but 0 when the solenoid is inline? Bypassed there should be no vacuum drop. I may have this confused so if I am maybe you could lay out the test results in a more simple and blunt manner.


I would start by testing source vacuum under driving conditions as well as verify that the actuator is not leaking.

I also like to shoot some WD-40 in the vacuum lines at idle to help lube the system.


P.S. The HX 35 is too small on the exhaust side.

[JohnC]

You said 15" vacuum after the solenoid. This implies the solenoid is working correctly.

If that doesn't convince you, put a dwell meter on the solenoid wires. You should see about 70%. Read the wastegate duty cycle with a scanner. They should be about the same.

[Kennedy]

JC

Is this the line that you were referring to?


When I bypass the soleniod I get 21"hg at the wastegate(15"hg if guage is tee tapped at wastegate).

This is why I asked for more specifics on tests and terms. The only reference I saw to 15"hg also contained the phrase "bypass the solenoid"

[JohnC]

Doesn't make much sense, does it? :confused: