What sets a dtc 78

[ol9465er]

Hello...

I have done some searching on TDP but could not find the answer and my manuals do not say (complete Chev Shop manuals, set of 4).

What sets a dtc 78. The manual says it is a variation from a pre-set value; but what gives the indication that there is a deviation. Is it the boost sensor, the baro sensor or a combination.

I changed the WG Solenoid, checked the connector and leads as well as vacuum lines - all check out good. Got good vacuum also. Have also unplugged and cleaned PCM connectors as a last resort.

Any help appreciated.

ol9465er
1994 Chev 6.5 TD, 4x4 LWB

[moondoggie]

Good Day!

You got shafted - my 95 manual says DTC 78 is "Wastegate Solenoid Fault." I'm amazed at how much the factory shop manuals can change from one year to the next year or two, with virtually no vehicle changes. :eek:

I think I had this DTC on both my 95's. If memory serves, in both cases, it was indeed the Wastegate Solenoid. Again, if memory servers, it was never allowing enough vacuum to the Wastegate Actuator, meaning too little boost, meaning black smoke sometimes on acceleration (never was pulling anything when this happened).

This Solenoid is turned on & off several (10?) times a second, & the duty cycle is varied to provide the vacuum the PCM wants the Wastegate Actuator to have. At idle, it's on ~ 75% of the time, so ~ 75% of available vacuum is on the Wastegate Actuator. When it's on, vacuum is supplied to the Actuator; when it's off, it vents the Actuator to atmosphere. There's a little tiny foam air filter on the Solenoid, cleaning the air that is used to vent the Actuator. It gets real dirty after many years/miles of use, & I think the inside of the Solenoid gets plugged up. I cleaned one once with brake cleaner, & it worked a little better for a little while. Then I figured, at ~ $35, just change it out.

I have NO IDEA if this is your problem or not; I'd monitor your boost, take a look at the vacuum your vacuum pump is producing (should be > 20"), & see how much vacuum makes it to the Actuator. It's a pretty simple system, really.

Blessings!

[Marty Lau]

Originally posted by ol9465er:
Hello...

I have done some searching on TDP but could not find the answer and my manuals do not say (complete Chev Shop manuals, set of 4).

What sets a dtc 78. The manual says it is a variation from a pre-set value; but what gives the indication that there is a deviation. Is it the boost sensor, the baro sensor or a combination.

I changed the WG Solenoid, checked the connector and leads as well as vacuum lines - all check out good. Got good vacuum also. Have also unplugged and cleaned PCM connectors as a last resort.

Any help appreciated.

ol9465er
1994 Chev 6.5 TD, 4x4 LWB Did you try here: http://www.thedieselpage.com/members/trblcode.htm

[ol9465er]

Moondoggie...

Thanks for the reply and info.

My manual also says that it is a Wastegate Solenoid Fault. However, I have just changed the solenoid and checked all connections and I am pretty certain that the physical circuit is ok.

I was just wondering how the dtc 78 was set and from what comparative information. Does the PCM only see the WG circuit and determine if it only works or does not work. Or, does it compare the actual boost and that maybe this may be part of my problem. As I mentioned the manual says the WG measures against some preset values in the PCM - I was wondering if the preset values are part of other inputs from the other sensors like the boost sensor and the baro sensor.

Much appreciated.

ol9465er
1994 Chev 6.5 TD 4x4 LWB
Stock
172500 Miles

[JohnC]

I have been bloodied by DTC 78 issues. My experience is that the PCM only sets DTC 78 when boost is higher than it wants. The condition must persist for about 30 seconds and RPM must be over 1800 and (IIRC) fuel rate over 20 mm3. The PCM controls boost by varying the duty cycle of the solenoid. The solenoid alternately applies vacuum to the waste gate actuator and vents the actuator to the atmosphere.

The most common cause of high boost is a sticky solenoid. Other possible causes are plugged solenoid filter, reversed solenoid vacuum connections and sticky waste gate or actuator.

[ol9465er]

JohnC - Thanks...

Does the Boost Solenoid then feed the info to the PCM that the boost is too high. I am wondering if my BS may be faulty (I have the new WG Sol).

On the reverse of lines, can you tell me which way the lines should go. I notice that on the tips of the WG Sol, one side has a brass insert and the other side does not. Which side should be from the Vacumm pump and which side should goe to the WG valve...

Thanks

ol9465er
1994 Chev 6.5 TD 3/4Ton LWB 4X4

[moondoggie]

Good Day!

On my 95's, boost (& IAT) is monitored directly by a pressure sensor on the cast aluminum piece that turns the intake air from right-to-left to front-to-back to down the IM; likely your 94 is the same.

My wastegate solenoid, mounted on the factory bracket, has the vacuum pump connected to the front port, & the wastegate actuator to the rear port.

[i]

[JohnC]

I think Moondoggie covered it. The pump connects to the port with the restriction in it. There is a manifold pressure sensor on the intake pipe above and behind the injection pump It is rectangular and black and has 3 wires connected. If you can get a hold of a scanner, you can watch the manifold pressure and wastegate solenoid duty cycle as you drive.

[ol9465er]

moondoggie / JohnC

Many thanks. With your assistance I found that I had the lines reversed and changed them. I just went 500 miles on a fishing trip into the mountains pulling my trailer, boat on top, truck packed and dogs in the back and the truck ran great, with no codes...

Thanks again,

ol9465er
1994 Chev 6.5 TD LWB 4x4 Stock