I have a 94 C3500 with about 64,000 on a complete running Jasper rebuild. The service engine light is coming on intermittently. The engine appears to be running fine. I'm getting a code 78, which is blank in the code list. Any idea of what code 78 means?

The vac-WG control is failing. I say buy a turbo master or put your own spring on it and remove the vac pump, and replace the belt with the 2" shorter one.

It's a better solution than fixing the vac pump. The vac pump and solenoid and lines are constantly failing due to the nature of the vac system, it sucks(literally) and has to be dragged along by the crank. The vac system is factory programmed to dump boost in the upper rpm and it never performs as well as a simple spring which is as good as it's tension and not really falable.

I moved this from the FAQs post, I'll link the pertinent threads from main forum to there if you have questions the main forum is the best place to get answers.

What spring would you need, and where would you get it? Are there any downsides to installing a spring and removing the vacuum system? Wouldn't you get a constant SES light? EGR codes and such? Thanks.

With the LD egr controlled pcm, yes it will give an ses light unless the baro get 20" vac. I used a heavy return spring and bent it to fit.

The code is a wategate/low vacuum code. before condemning vac system for the spring system (check Heath Diesel web site for Turbo master). Vacuum is supplied by a belt driven vacuum pump, this vacuum is used on L65 to control waste gate, and L56 also EGR valve.

With engine shut down check WG actuator for freedom of travel and integrity of vac lines and connector fittings for swolen rubber/cracks. Vac pump is lower left when facing the engine, trace line from back of pump up to solenoid/s drivers side rear of engine connect vac gage on main line from pump into solenoid or bank of solenoids for L56 engines, vac at idle should be 22+" with healthy pump, if that is good then check outlet of solenoid should be 15-17" vac. waste gate actuator should be fully retracted for closed WG.

If those check good put tee in line and drive with gage in outlet of solenoid with hose long enough to reach into the cab as you drive, monitor for 17-22" vac, if vac is low possible solenoid problem or power to solenoid, WD-40 sprayed into fitting has resurrected failing solenoids in some cases, if solenoid is cause not very expensive to replace.

Springs are viable alternative for completely failed vac pumps, and work well to give more boost for towing power, they do have potential for setting code as they will give boost higher than a non reflash PCM will accept as a valid level. Boost foolers will allow a working vac system to operate at higher boost, and also keep spring systems from setting code as well. Foolers can be built or bought from Kennedy Diesel (simplest neatest version, plug and set) Spring or vac with fooler keeps WG closed longer, too much boost is a bad thing without post turbo cooling see FAQs for more info on IC & WMI for cooling, also check this thread linked there for more on WG operation http://dieselplace.com/forum/showthread.php?t=16888.

Symptoms sound like over-Boost, where wastegate is not wasting - solenoid failing, 'gate is stuck, over-sprung, wired shut, etc

Check that the 'gate moves freely before cold-starting engine

Check for ~15" vacuum at wastegate at idle, ~22" at pump at idle.

Check wiring\connections at Baro sensor, on firewall, and Boost sensor, on intake manifold.
Could be failing sensor.

My understanding is that the source of the FC 78 is the MAP sensor on the intake manifold. It is basically telling the computer it is seeing wrong amount of boost for a given load condition. Mine was working when I got a FC 78 with turbo-master installed that's when I got the chip.

From what I've observed If you have:

Correct boost the MAP on intake is bad = Normal power until code tripped and engine goes into limp mode

High boost the Soleniod is bad or the actuator is sticking = Higher power until code tripped and engine goes into limp mode.

Low boost is a loss of vacuum or the actuator is sticking or wastegate is bent/broken = Low power more black smoke( Not sure if low boost will send FC 78)

Not sure how 78 works in OBDI but in OBDII I think this is same as P0236 turbo/boost performance, so any of above scenarios will lead to this code, this is where a boost gage comes in very handy "Is it real or imagined problem" trouble shoot as above could very well be sensor related, make sure though you are asking for right sensor, MAP and Boost are different sensors.