Code 78 even after replaced WG solenoid, twice.

[carl6405]

1994 GMC K2500 6.5L F engine

Hey guys. Looking for help again. Just finished replacing the engine, now have some small things to take care of.

SES is flashing a 78 code when I reach speeds over 50mph.

I am getting virtually no vacuum off the WG solenoid at idle. I have replaced the WG solenoid twice now. I took the part back, thinking it was bad and got another new WG solenoid.

Vacuum lines are not leaking. I have almost 20 PSI of suction going into the WG solenoid, according to the vac tester. Nothing coming out.
I have checked my voltage on the connector and I am getting 13.8 volts.
I really think it has something to do with the signal from the computer. Is there a way to check and see if it is getting the signal from the computer?

 

[Turbine Doc]

Dirty/intermittent gnds ? what kind of resistance reading are you getting from plug to block on the gnd leg ? should be pretty close to 0 ohm, possibly also some issues with the plugs wire crimps to pins

 

[carl6405]

You might be correct about the grounds. We just got finished replacing the engine and the ground wiring are all over the place. I will check that and see. Thank you.

 

[chefmike]

I was setting a 78 code on hills at 65 plus mph... Turned out the sensor on the intake was cracked and not contacting. Fit the wires into it and ziptied the remains of the casing t o hold it all snug and the code dissapeared. Apperantly it was either overboosting or sensing overboost. Any how... That was my fix: checking wiring connections. I also cleaned the atmospheric sensor on the firewall.

Not sure if this will help you, but maybe somebody searching these codes.

 

[carl6405]

Chef.. thanks I'll look at that too. On the ground issue. Could I splice a ground wire in to the negative side of the connector going in to the WG solenoid and ground it to an intake manifold bolt?

 

[JiFaire]

Carl, the problem with grounds in these beasties is that the engine itself may not be well-grounded; or not well-grounded to the same plane as the PCM, or as the WG solenoid. Very often, the biggest culprit is the woven-wire strap at the back side of the passenger head, grounding (or not) to the body. This can cause lots of hiccups in the computer.

There are a couple great posts on finding and fixing your grounds, in the 6.5 Tech Library, under (what else?) Grounds.

Check the whole circuit; as was mentioned, it can also be a problem with the baro sensor on the firewall or with the manifold pressure sensor. Your PCM won't call for boost if it thinks it already has some.

Good luck finding the little bugger. Lots of us just went the TurboMaster route and fixed the darn thing for good.

 

[L98TPI]

I just fixed a no/low vacuum at the the waste gate problem. Those hard plastic lines to the waste gate had collapsed or was crushed at some time in the past. I did the usual checks at the pump at idle, then teed into the line at the waste gate and ran the hose inside the cab to the test gauge. A quick drive showed what was going on, the vacuum was slooooowly coming up as the ECU was holding the solenoid wide open wanting full boost. The thing would bog with black smoke then clear, if you could hold the pedal down eventually the power would come up. I replaced the vac lines, all of them with quality vac hose.

Sharing, it might be of some help.

 

[carl6405]

Jifaire & L98.. Thanks for the tips. I spent most of my time this weekend replacing the passenger side cv axle and stabilizer bolt. While I was under there I found the ground strap that is supposed to be bolted to the body, just hanging down flapping in the breeze. I drilled a hole in the frame and put a new connector on that and bolted that strap to the frame. I couldn't figure out where the other end goes. But also fixed a couple other ground issues with the battery ground cables not bolted to the engine block. I found that write up on grounding, but have not got to all of them yet. Will be working on that through the week.

I would think you could simulate the computer signal to the wg solenoid?

 

[L98TPI]

I would think you could simulate the computer signal to the wg solenoid?

Its a pulse width modulated drive. But, just for testing, You could put 12V on it with a connector pigtail... or you can also make the waste gate go shut by pulling the hoses off the valve and connecting the vacuum directly to the waste gate. I did this to make sure all the rest of the hose, servo and the turbo were OK. Just keep the rpm down to say 2500 so you wont over speed the turbo. You might not be able to actually over speed it but to stay on the safe side. If you get a MIL light you can bet it was over boost so get off of it.

 

[carl6405]

L98..I took a short piece of small diameter fuel line I had laying around, poked a bunch of holes in it and pushed it in to the vacuum harness that would be connected to the WG solenoid. So the WG has suction going to it to close. I poked hole's in that short piece so it wasnt getting the full 20 lbs from the vac pump.

Hey.. I see you are in Lynchburg, Va. I am in Chesterfield, Va. Cool.

I tried removing the 12v connector from the wg solenoid to see if it made any difference, and no. I may try an external 12v source tomorrow, like a cordless drill battery and make a connector to the wg solenoid and see if that wakes it up. If so, may be a faulty connector.

 

[carl6405]

Actually, while writing this, I remembered that my old wg solenoid was in the garage. I grabbed that one and connected a 18v from a cordless drill battery and I can hear a clicking sound coming from the inside of the wg solenoid when i apply voltage and remove voltage. So, if I try this on the one in my truck, and it allows air to pass through, then I know it is a faulty connector. Even though, I can measure the voltage from it, with my digital multi meter. I have high hopes! The things I do at 1am,on a work night..

 

[carl6405]

Went out this morning and checked the resistance from the negative side of the connector to ground cable from battery. It was pretty much an open. Read like 400k ohms. So, that is definitely an issue.

 

[L98TPI]

Hold on, the drive to the solenoid is likely from an open collector transistor. I don't have my books here so I can look it up for sure but IIRC one side of the connector should have 12V on it and the other side is pulled to ground by a bipolar transistor (or a FET) to make the current flow. So what you are reading is normal. Can you rig a way to back probe that connector and drive the truck? That would be informative.

 

[THEFERMANATOR]

You need to know what it is seeing for boost to figure out if it is a vacuum issue or sensor issue. If the ECM sees boost from the boost pressure sensor or if it doesn't agree with the baro sensor, the ECM will not trigger the wastegate solonoid to send vacuum to the wastegate. When I first bought my BURB it threw no codes, but would roll black like a freight train. Hooked up a scanner and it was showing 16.7 pounds of pressure at idle while the baro was showing 14.8. Swapped out the boost pressure sensor and had boost again with no more smoke. You need to diagnose it before you throw parts at it, otherwise you will just end up cursing it.

 

[L98TPI]

Indeed...
IIRC those are 2 bar sensors so yours Ferm likely failed to the 5v rail. it was putting out up near 5v all the time. But not enough to set a over boost code?

Back probe the boost and baro sensors (if you don't have a scanner) and see if the output makes sense. They have 3 wires gnd, 5v and signal. I don't know off the top of my head what the switch on, but not running signal V is. It should be 1 bar of voltage likely around 1/2 of the 5 volt reference, for a 2 bar sensor. I can look it up later.

 

[millsroger]

Ferm, I just went thru this, got code 78 pulling out to pass, touched 90 at Wot. First, the waste gate actuator gets full vacuum to start out with, the control solenoid REDUCES vacuum to allow the waste gate to open.

My problem was not that the vacuum was not dropping when it was told to--the actuator was not releasing all the way. Kind of sticking in the middle. Result, waste gate not opening as much as needed to drop the boost. ECM saw more boost than expected, dropped the vacuum some more with no change and tossed the fault. Replaced the actuator--no more faults.

Waste gate actuator must be free to move easily end to end with engine off!...hard pull closed at idle and on up for max boost to when the ECM starts backing off the vacuum. When that happens and how much is up to the baro sensor(s), etc.

From a GM boost map max boost is about 11.5 at between 1500 and 1800 RPM tapering down to about 8 at 3400. Now, that being said, it's a paper chart and your results will vary. The chart does not say anything about altitude or throttle postion/load so assumed to be wot at sea level.

Appears to me that a simple spring holding the actuator closed against exhaust pressure would up the boost across the chart, trick is where does the fault code kick in, ie, how strong a spring.

 

[carl6405]

So, are we saying that at idle, the wg solenoid should be allowing air through it? By the way, is there a way to rev the engine while under the hood? This is the first vehicle that I have owned that you can't rev the motor with a throttle linkage or something.

I do not understand this WG solenoid at all. There is only 2 wires going to it. + & - 12 volts. How is it getting a 5 volt signal from the pcm?

 

[THEFERMANATOR]

So, are we saying that at idle, the wg solenoid should be allowing air through it? By the way, is there a way to rev the engine while under the hood? This is the first vehicle that I have owned that you can't rev the motor with a throttle linkage or something.

I do not understand this WG solenoid at all. There is only 2 wires going to it. + & - 12 volts. How is it getting a 5 volt signal from the pcm?

The wastegate solonoid does not get a 5V reference, the boost pressure sensor and baro sensors do. The wastegate solonoid is a simple on off pulse width modulation style solonoid controlled by the ECM. It gets a key on 12 V+ when the key is on, and the ECM controls the ground to it to control how much vacuum makes it to the actuator. At an idle the actuator should be held closed pretty tightly, if not you have a problem either with low vacuum to the wastegate solonoid, bad wastegate solonoid, cracked vacuum line going fro mthe solonoid to the actuator, or a bad boost pressure sensor. Throwing poarts at it will most likely eventaully fix the problem, but cost you ALOT more in the end than to sit down and get some tools and diagnose it. If you can get access to a scanner it would greatly simplify the process of narrowing it down.

 

[carl6405]

I have no vacuum coming out of my WG solenoid. WG is good and lines are good.

I believe I just figured out something. I think the problem is a low current issue. I made a test jig and can apply 12volts and no voltage while I have it out of the vehicle in my hand. I can hear the gizmo(relay? inside opening and closing when I apply voltage. With the test jig connected directly to the trucks wiring harness, I do the same test, with the wg solenoid in my hand, nothing happens. Although, the voltage is there, nothing happens. So, that is telling me that there is not enough current to trigger the WG solenoid device to work.

So.. barring other possible issues, the WG solenoid should at least open and close if the connector has the correct amount of volt and current being applied. Is this correct?
I am really thinking ground issue or does the ECM control that current?
I am thinking about splicing a ground wire in to the negative(yellow wire) of the wg solenoid connector and tapping in straight to the negative side of the battery for a test to see if the WG solenoid opens and closes then.

What do you guys think?

 

[carl6405]

Hip hip..... Huraayy!!!

If I wouldn't have tried it, I would have been laying in bed all night thinking about it. I fixed it. Simply spliced in a wire to the negative side. Crimped a round connector on the other end of the wire and bolted it to the ground wire that is bolted in to the intake manifold. Turned the key to on position and I could hear the WG solenoid click, finally. The way it was outside of the truck. Had the wife start the engine. Bingo! WG is now pulled closed, or close to close on idle. Suction to the Waste Gate.

When I peeled away the wire loom from the that wiring harness, I could see that someone had been there before. There was electrical tape around the two wires going to that connector.

Thanks to everyone for your in put. Hopefully this thread will help someone in the future.