1994 6.5 3500 code 78/cruise won't work.

[lmdangerous]

I have a 1994 6.5 3500 dually. It's our shop/ tow truck. 92,318 actual miles. Runs great. Only problem that we've ever had was the PMD/ and eventually the injector pump which I replaced and relocated and that instant fit throwing session came to an end.Replaced a vaccum /waste gate solinoid or two along the way. It has coded a 78 this week and my cruise won't hold. Yep they are connected I know.
I have checked vaccum at pump, sol.,wast gate all at 26 pounds of vaccum. It only codes ses light when it goes past 1700rpm or I stick my foot in it. It has good power. The waste gate arm is moving free when not running and it's not rolling black smoke.Never has as compared to others I've seen Mileage is not any worse /same as has been. It runs,drives , and starts as good as ever. It is bone stock other than the PMD and the remote wiring harness from Pensacola diesel. i know the code 78 is a solinoid /waste gate code and I changed it out with a known good one from another truck ( dr 227 delco ). Same thing happened. Same code. Won't hold cruise past 1700 rpm. And the light still comes on when I push it past 1700. As good as it cranks runs and drives considering all the stories I have heard it has me stumped. Is there any way to program the Ecu so that my cruise etc will work and not flash codes and may be go manual on the waste gate. I would really like to fix it and not have to change anymore than needed. I like cruise etc. I know I am spoiled. But this truck is only used to pull a race car /trailer and as a shop truck. Any suggestions or info is appreciated.
Lmd

[More Power]

Is it possible the vacuum lines at the solenoid have been reversed?

[lmdangerous]

There have been a couple of waste gate solinoids put on so I would say it's possible. Which line goes to the front nipple with the little hole in it?
I also checked the solinoids with a pair of lavatory clips clicking and opening and closing. I even sucked on them to make sure the vaccum flow was being cut off or allowed to flow.The connections looked good. Is there anything I have over looked other than possible hose reversal.

[More Power]

Try reversing the lines temporarily to see if that cures the code.

[lmdangerous]

Ok here's the latest stage. I reversed the vaccum lines. Instant black smoke and hesitation / stumble. I also watched when I put the vaccum lines back and watched the waste gate arm. It actually moved when I hooked the vaccum line up. It actually reads 22 at the waste gate vaccum pod at idle. I used a little wd 40 inside the solenoid. Seems to be a little better. I get the impression that this thing may be over boosting. It actually showed a little and I mean a little smoke today we I put my foot in it. I've seen 7.3 s ,Cummings , and duramax roll smoke and they were running great. I ordered another waste gate solonoid reference switch - it will be in tomorrow morning. I figured that maybe the electric part is giving a faulty signal or connection. For 32$ at Napa I'll try it. Any way to flash the computer/ reprogram it, or what does a boost fooler or what ever it is called do? It's running very well just keeps coding 78 and kicking the cruise off. I welcome any and all ideas on this. Lmd

[DmaxMaverick]

22" Hg vacuum at the WG actuator only means your vacuum pump is good, and working as advertised (22-26" is operational normal at idle, from the pump boss). You should have ONLY ~15" at the actuator at idle. The vacuum should be modulated by the WG solenoid (via PCM signal voltage). The solenoid is likely bad (causing the DTC 78), or you have an electrical fault at some point between the solenoid and PCM, or a ground issue, all assuming it is plumbed correctly.

[lmdangerous]

When you say ground. I assume you mean the switch itself to the mounting bracket? Thanks you at least made me feel like I am going at this logically.
Considering putting a lift pump as well while we are here at this point. It's never been replaced. Again I am pleased with the trucks performance and pulling etc. I would just like to keep it dependable. It's still my favorite to drive.
Lmd.

[Aftermath]

First, check the grounds at the rear of the head (passenger side) Then take the vacuum pump and solenoid, find a dumpster to put them in, call Bill Heath Diesel to program the chip, make your own manual wastegate controller ($6 in parts at HomeDepot) Many of your future problems solved as well. More Power and better MPG, Worth the cost for the gain.

[lmdangerous]

Update. Switch came in. Installed the same thing. 22 at the waste gate pod. How should I go about checking for electrical signal at the switch? Will a test light do or ??? I don't want to burn the ecm or anything. I check the grounds pretty often due to normal vibrations. Do I need to sand the paint off the bottom of the switch where it mounts on the little bracket also ? Still runs good just would like this fixed. Lmd

[lmdangerous]

Checked all connections and even grounds that I can see. None of the wires look broken or even worn. Any chance that the ecm is having issues. Would it even code if it was? I wouldn't think it would run as good as it does if that were the case. Lmd.

[JohnC]

It's been a while since I had my '95, but, my recollection is that DTC 78 doesn't have any effect on the cruise control, although I could be wrong there. DTC 78 sets when the PCM cannot get the manifold pressure it's looking for. I think it only sets when it's too high, too, not too low. Do you know what the manifold pressure actually is when the code sets? It could be that the manifold pressure sensor on the firewall isn't working correctly. Can you compare your measured manifold pressure to the pressure the PCM is reporting?

[lmdangerous]

Ok. We have an answer and success. And had a very weird disheartening experience to boot.
First. It was the PMD. I located / and went to probobly the most knowledgeable / kind / and patient diesel tech guy I've ever met. Period. Gary Harper. He was employed for years by Tupelo Diesel. He decided to go solo built a shop at his home and now even the dealers send their stuff to him. Go figure.
He actually likes the 6.5 and didn't do the normal I don't work on that type of diesel etc. pulled out a obd1 code box and went to work. About 30 min later he walked in his shop and made one change and one change only. Unplugged the Pensacola Diesel PMD and plugged in an old greasy Standidyne ( nope at this time I didn't know he had swapped it) and told me to take it for a spin. I left like a bat out of well you know and wow no codes. Cruise worked.It pulled hard and hit 75 pretty quick. So he finally showed me and I asked what I owed him. He asked for my spare remote mount harness only. So I plugged my pennsacola box back in and headed home.
Now I was and am very happy about the fact that I finally know. So I had purchased two of those PMD units form Pensacola diesel about two years ago. One for this truck and another for another friend who never finished it so I had a spare. Got home and swapped the PMD. Ran great for about 4 miles and started stumbling and coded and went dead. Took the other box out and put it back on. It at least ran decent. Called Gary back and he said he was not suprised. I paid a lot for those two units and neither was any good. I am going to call them I. The morning but I am not expecting much. Maybe I'll be wrong and they will honor there pieces. That was kinda disheartening.
Now the question what PMD box do I get. Standidyne ( grey) or ? ???? I have made up my mind that I am going to put both of the heat sink plates on and mount two PMDs. One as primary and seal the plug in area and bolt on a spare.
What PMD do you guys recommend. Lmd

[Robyn]

I have run nothing but DTECH pmd units for years without issues.

Mount outside the engine bay on a good cooler.
Majke certain the ground wire on the PMD harness at the IP is on the IP and has not been moved elsewhere.

Should fix ya problem.

Good luck

[gvig]

Have a Flight Systems installed and a black Stanadyne and two Dteck in reserve, just don't have PMD problems.

[More Power]

Someone will have to refresh my memory concerning how a PMD can cause a code 78....

[DmaxMaverick]

Someone will have to refresh my memory concerning how a PMD can cause a code 78....

I was thinking the same. Either we'll be revisiting this soon, or all the messing around under the hood accidentally "fixed" something (like a poor ground or chafed wire).

If it is still showing 22" vacuum at the WG actuator, the DTC 78 and WG solenoid issue isn't fixed, and will return. I don't recall if the code sets immediately, or after so many start/warm cycles after clearing.

[Robyn]

The 78 sets if there is a failure in the solenoid.

PMD should not set a 78

The 35 and 36 pulse width errors can occur with a bad PMD

The 78 codes was a common issue due to failed solenoids.

As was mentioned.
Toss the vacuum system, get a chip or reflash and a mechanical waste gate control (AKA turbo master)

End of the BS.

I tossed the waste gate solenoid on the Dahooooley a long timer ago after I installed a chip and a spring thingy on the waste gate.

Steady 15 psi at max throttle and no SES light.

[lmdangerous]

True and true. When Gary ran the obd-1 on it. It coded both 78 and 35 / 36 both. Short / and long injector impulse etc. I know the ground on my remote harness is on the ip where it is suppose to be.
Pensacola diesel told Me had they been Dorman it had a life time warranty. Nope they weren't. They have no markings what so ever. So they are mine. And nope they are not going to do anything.
Now who handles a real Standidyne / or D tech. Anybody run a Dorman ?
I also have an idea and a tig/ and milling machine so here it goes. In the racing world we make what we want or to improve a piece. So A can I weld my two cooling plates together as long as the mounting surface stays flat and the weld is not in the way( I could always mill it off if it did) and mount two units so if one fails I can plug and play? Any reason that won't work? Also thought about taking my Bridgeport and making a very large as in about 4 times the size of the standard remote cooling plates and mount two at opposite ends. In theory wouldn't it increase the cooling exponentially and increase life of the unit ? Also I have read and seen Every location under under the sun. Where is the best overall place. I want to fix this once and for all. And if I am going to do a back up I want one that will work anytime or place. Keep me straight here guys. I am used to hilborn injected sbc's and Donovan 417s. You will not offend me and I'll admit very quickly that this thing was kicking my rear. So fire away.

[lmdangerous]

Also the vaccum issue turned out to be wrong line wrong place. A line had been replaced and spliced. What appeared to be the yellow line was only yellow for about 2 feet and the other one was black so I assumed. When swapped around walla. Shows about 18 now. Have no idea when that happened other than when the other switches had been replaced. Written record I keep in the glove box says it has had 4 total including the one I just put on it. When it was under warranty and when it was newer i let someone else do the maintanance cause I could write it off. Now no one wants to / and most run from it so I can understand this happening also. But to make it short it's behaving now in the area also. I purchased a replacement AC delco plug in to be soldered in if I needed it for the vaccum switch. I figure if I have it I'll.never need it. That what has usually been my experience. Again keep me straight. Many eyes / minds are better than one. 😎

[More Power]

Try reversing the (vacuum) lines temporarily (at the solenoid) to see if that cures the code (78).


Also the vaccum issue turned out to be wrong line wrong place. A line had been replaced and spliced.


........

[lmdangerous]

Yes. My screw up here - overlooked the splice. Now I have to make a decision on the Pmd. I get the feeling that the over counter Dorman etc is not the deal. So the D tech or the Standidyne black is the way to go. I am going to mount both so I can plug and play on the road. But I also think I am going to find a place and mill out the biggest cooling plate that will fit. I have a lot of scrap 3/4 and 1 inch and even some thicker aluminum plate. My old Bridgeport may be getting a work out. Like a radiator bigger has got to be better.
Thanks for the information. I HOPE this has everything covered. You all have been very helpful with information and tech. In the racing world everything is a secret for obvious reasons - yep Ive been guilty myself. In this case most people simply do not know and the information is not there. Again we will see soon enough.
In small town USA as in tiny, the one small Napa parts house we have doesn't give many options. And so many have jumped on the cummins path and I understand that also. Again thanks and I will let you know how this all turns out.