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1993 CL713 E7 VMAC1 idle rolling


JTesta1966
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This truck was towed to me as a no start. Tow driver had it running on the hook. It was loping or rolling at idle. It wasn't smoking or puffing anything out of the exhaust. I don't know what he did. I get to it. No 1939 communication. Look at wiring diagram, only 1708 on the dlc, so I scan it "generic" using my nexiq etech software, techtools only does 1939 and 1584, which this truck doesn't have on the data connector (6 pin deutch). It has a mid 143, pid 190 fmi 02 (engine speed signal erratic or missing). Its running so must be getting signal. I scope it. It looks like crap, so I condemn the engine speed sensor. I replace it. No change in signal. Same code. I do note, if I clear codes, and key on. The code doesn't return until the engine hits about 600rpm. If I monitor data, at about 500-600rpm, the rpm data freezes and the malfunction light comes on. I note, theres a bunch of hacked wiring, so I get a diagram, and find ect and iat wires are switched, I repair, and those read correctly now. the engine speed sensor wiring is correct and during crank I have my expected signal (ac generating sensor), once it hits the signal goes wacky. If I bump the rpm up and unplug the crank sensor, the signal remains whacky as the engine stalls. once just about stalled, the signal looks right and as the engine bounces back and forth at the last revolution just before it finally stops, signal looks great. I took the sensor out, and inspected the flywheel. There were a few rusty looking spots so I glued a piece of scotchbrite to the old crank sensor and screwed it in to just touch the flywheel, and cranked it over a few times. flywheel is sparkly clean now. I have no change in the signal. One thing of note, is the tach reads steady, it surges with the engine, but its not erratic or "spikey" like the signal to the vmac computer is. Looking at the wiring diagram, tdc and engine speed signals enter the vmac ecu, then come out and are passed to the cluster and the bosch ecu. since the cluster isn't wigging, I have to think the signal coming out of the vmac ecu is acceptable. Now my question after all this is, does the vmac ecu condition the signal for the cluster and bosch ecu (theyre spliced to the same wire 31-z-0.8) and maybe my bosch ecu doesn't like it, or, since the cluster isn't wigging, could the bosch ecu be wonky? Is the fact I have no mid 128 codes and only mid 143 codes mean my fault is isolated to the fuel ecu (the bosch ms3 module correct?) Customer states his mack buddy said the econovance solenoid is bad. He gets it for me, I change it, no difference. Not surprised. I didn't think so because I could watch the computer pulsing the unit like it was trying to control it, via my scope. Anyone have any input? Im trying to find some good theory on the system, but man theres no info out there. I see a few real sharp people on here and wonder if one  or all will chime in and give me some info here.

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Bro ! just a few checks and experiences for you !  Check along the top of the fuel pump wear the wires clamp to the the pump ! look for any rubbed thru wiring there very common spot for issues ! Econovance to check the econovance you can run 12  volts  and a ground to that there will be a noticeable  change in timing and the running of the engine! 

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