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'99 MACK RD688S, E7 ENGINE W/ VMAC III LOW POWER PROBLEMS

Monkeywithawrench

Senior Member
Joined
Nov 8, 2019
Messages
281
Location
New Hampshire
Ok.....hoping somebody on here has experienced this before or is familar with how this system works.
I have a customer with the above truck. He was driving it last week, loaded, when the rpm gauge and speedo stopped working and the engine lost power. This truck has never been a problem before. I plugged my laptop into it using Texa Truck software to diagnose it. It showed a problem with OIL PRESSURE SENSOR. I replaced it with a new one. That code disappeared.
New code showed up showing INTAKE AIR TEMP SENSOR was no good. Back to Mack I went. Replaced that sensor, rescanned, now the speedo sensor showed up as being bad (data invalid).

So this was Friday...I gave up around 8pm. I went home and bought the VMAC III manual online and read it. It said that if the speedo sensor was bad that it could/ would derate the power. It also talked about blink codes if you hit the cruise control resume switch so many times to display the code. I am not sure where the blink code is supposed to show up. This is an old truck. So there are a bunch of things not working and held together with baling wire and duct tape.

SATURDAY
So I ohmed out the speedo sensor and it was reading 1.4 M ohm.......no good. A good sensor is supposed to read between 150-170 ohms. Off to Mack again, new speedo sensor, checked on meter and it read 164 ohms. Plugged it in, rescanned, still showing bad speedo sensor. Pulled VECU from passenger side dash panel and ohmed out J2 PLUG J2-11 and J2-12 for the speed sensor. From the pins at the VECU, I got 164 ohms. Checked power to VECU at J3 plug. Using J3-16 (12V from battery) to J3-18 (VECU ground) I got 13V (battery voltage). J3-17 (12V from Accessory Relay) to J3-18 (VECU ground) I measured 0 volts. I tried jumping from J3-16 to J3-17 in case that should have been powered as well.........it didn't change anything. I ordered a new VECU from Mack.

WEDS.
The NEW VECU showed up yesterday and I put it in today. Code for the speedo sensor disappeared. Now it said the Accelerator Position sensor was out of calibration. I am assuming ............I don't have anything confirming this.........that the throttle position sensor calibration was in the old VECU. Tried calibrating it with Texa software. It didn't work. Tried 4 times........no goes all the way. Called Mack...........new throttle sensor is $300 and is 5 days out.

So we are up to $1200 and the truck has been down for a week.
Replace one sensor and another shows up bad. Always 1 sensor at a time, which is frustrating. Right now the Engine ECM is good, no codes. The tach, oil pressure, and speedo all work as they should now. With replacing the VECU, the speedo came back to life. Still have lack of power in the truck.

Other tests of note.........I checked fuel pressure first. It reads 110 psi at idle. On test drives, fuel pressure never dropped below 100 psi. Turbo spins free, no damage, no bearing play. No black exhaust while driving from lack of boost showing unburned fuel.

I'm at a loss here. AND WHY THE HECK DOES THE LAPTOP ONLY SHOW 1 BAD SENSOR OR CODE AT A TIME?? Fix one and the next shows up.........like its daisy chained or something. Runs into 1 bad sensor and then it stops and just displays 1 code. Is that normal from this generation system??
Or maybe its the air pressure in the LR tire is low?? :confused:
 

Jonas302

Senior Member
Joined
Jan 4, 2015
Messages
1,197
Location
mn
Blink codes flash the check engine light cruise off hold the set switch down
 

Monkeywithawrench

Senior Member
Joined
Nov 8, 2019
Messages
281
Location
New Hampshire
I don't see a 'Check Engine' light in there. Cycling the key on and having it go through the bulb check, no check engine light is shown. Taking a flashlight and shining it into the indicator bezels for the different warning I don't see the outline for the 'check engine' wording. I do see 'STOP ENGINE', ABS, etc. There is one bezel thats misssing from the dash. The lamp I assume goes to the missing bezel is inside behind the dash. That is on constantly and doesn't go into a blink code when I cycle the 'SET/ RESUME' switch for the cruise control.

I was just wondering if anybody had run into this before and why only one bad sensor at a time shows up when running diagnostics.
Its an old truck and is relegated to standby duty so it doesn't get the attention and love it should get.
 

mekanik

Senior Member
Joined
Aug 20, 2015
Messages
944
Location
Canada's Northwest
I have very little experience with Mack electronics but I'm sure
the same troubleshooting applies. The first thing I do when I run
into electronic problems that are illogical is start by cleaning all
power and ground connections. Start in the battery box and then
to the starter. Check all grounds to the block and frame and cab
exterior. Most trucks power the electronics with a direct power
and ground that connects to the batteries. If Mack uses a
designated power supply load test it. I use a pair of high/low
beam headlights with all four filaments wired to light up about
15 amps draw.
Corrosion and connectivity problems seem to be worse in trucks
that sit a lot.
 

Monkeywithawrench

Senior Member
Joined
Nov 8, 2019
Messages
281
Location
New Hampshire
Thank you Mekanik!! You are absolutely right!! I never ran a load test on the power and ground. I just trusted my multimeter. And I have played this game before!! I have a round headlight in my truck that I use specifically for this purpose!! Such a humbling experience...........overlooking the obvious!!
And the award for "Way to go, sh!thead!!" goes to....................(drum roll)............Monkeywithawrench!!
Hats off to you, sir!! Thank you again Mekanik!!
I'll try and get over there today and load test it and report back.
Fun and games with electrical.........where do you think the problem is!!
 

Monkeywithawrench

Senior Member
Joined
Nov 8, 2019
Messages
281
Location
New Hampshire
Soooo..........in my defense, and its still not right that I didn't load check the power supply to the VECU..........when I replaced the VECU the speedo started working correctly. And I'm still baffled as to WHY only 1 sensor at a time shows up as being bad.
 

mekanik

Senior Member
Joined
Aug 20, 2015
Messages
944
Location
Canada's Northwest
I'm pretty sure its a connectivity issue. I recently was dealing with a Cummins
that was turning on the engine light and having a variety of issues. turned out
there was 20 ohms resistance between the battery negative post and the ground
post on the starter. There were a few corroded connections but most of the
resistance was in the ground side battery disconnect switch.
It cranked over fast and fired up without a problem. I cleaned the
connections and replaced the switch and problem solved.
From my experience more than half of electronic I deal with problems are the
result of connectivity issues.
 

mekanik

Senior Member
Joined
Aug 20, 2015
Messages
944
Location
Canada's Northwest
There should be an engine ECU and a vehicle ECU and maybe an ABS ECU
These modules talk to each other on a data-link there could be a communication
issue with the data-link. I'm not familiar with Mack electronics.
 

Monkeywithawrench

Senior Member
Joined
Nov 8, 2019
Messages
281
Location
New Hampshire
MEKANIK
YOU
ROCK!!

Thank you for taking the time to post a reply and share your knowledge and experience.......and reminding me to check for the obvious!!
(Lawn mower won't start......must be the spark plug...nope.....coil??.....nope......compression??........nope.....carb??.......nope........fuel pump??.......huh, no gas out of the fuel pump!! DAT MUST BE IT!! (ONE new fuel pump later, and 6 inches of grass growth) "Is there any gas in the tank??"

Disclaimer: I haven't gone through this with my own equipment. Other people.......yes. Just using this as an example of overlooking the obvious.
 

Monkeywithawrench

Senior Member
Joined
Nov 8, 2019
Messages
281
Location
New Hampshire
Yes........Engine ECU is on right side of engine, VECU AND ABS are under kick panel on passenger side. VECU and ABS sit right next to each other......they are actually very easy to get to and have plenty of room to work and alot of slack on the wiring harness. That was a refreshing find.
 

Monkeywithawrench

Senior Member
Joined
Nov 8, 2019
Messages
281
Location
New Hampshire
No.......no data link faults being identified by laptop. Just individual sensors being identified as being bad one at a time. Maybe the data link faults will show up after all the bad sensors are replaced??LOL
 

Monkeywithawrench

Senior Member
Joined
Nov 8, 2019
Messages
281
Location
New Hampshire
No programming was necessary, according to McDevitt Mack in Manchester NH. I made sure to inquire. They said it was plug and play.
Based on the oil sensor being bad......replacing that and having the code go away, then intake temp sensor.......replacing that and having code go away, speed sensor showing up bad..........testing and verifying that, replacing it, and having that code go away. I think I kinda believe that maybe possibly the accel sensor is bad too. I think something catastrophic occurred to cause all this. Still need to verify voltage to VECU.........I'll probably attack that tomorrow.
 

Monkeywithawrench

Senior Member
Joined
Nov 8, 2019
Messages
281
Location
New Hampshire
MEKANIK..........2ND OPINION SOUGHT!! YOU'SE IZ RIGHT!! NEEDS TO BE PROGRAMMED........Well, the truck characteristics need to be pulled off the old VECM and transferred to the new one. And it needs to be in the truck when its done.......and since the truck only goes 10-15mph at present, guess I'll be low boying it up there.

So I had a chance to check the throttle position sensor today. Its getting 5.2V open voltage through the sensor harness ground. Plugged it into pedal and was getting 4.85V plugged in. Voltage on variable side was 4.85-1.2V when I pressed on the pedal.....so that's working correctly. Pulled panel for VECM, unplugged J2 and connected incandescant headlight up to it, using harness ground as well. Open voltage was 12.25 w/ no load. Headlight on it was 11.87V.....so no power problem. I then called a Mack dealer in MA, who were super friendly and helpful; and talked to parts........yes, it needs to be programmed.........they then sent me to service to verify.......again, super helpful (not like the Mack dealer in NH we have an account with). They verified it needs to be programmed in the truck (actually that was the first time I heard that). I called our local Mack dealer to try and talk to service......everyone was at lunch and they'd have someone CALL ME RIGHT BACK!! 3 hours later, I called them. They verified it needed to be programmed with the truck characteristics and that I'd need the old VECM. They said they could get to it on Tuesday....maybe........today is Weds. If the other Mack dealer in MA wasn't so far away, I'd be trucking it there.
Thank you for everybodies input. I appreciate. Hopefully this post can help someone else out.
 

Monkeywithawrench

Senior Member
Joined
Nov 8, 2019
Messages
281
Location
New Hampshire
UPDATE: So I had one of the drivers low boy the truck up to the Mack dealer on Thursday 9/9/21 and they dropped it off. I called on Monday 9/13/21 around 4pm to see if Mack had gotten around to reprogramming it/ transferring the truck characteristics from the old VECM to the new one. Mack didn't realize it was there. So Tuesday morning they set to work on it and got it squared away. Runs just fine now.
 
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