from what i've seen on my bench(no scope), even with no MEMCAL attached(just a PROM) and setting a LOT of error codes, it still seemed to fire based on the setting in the BIN. i'm pretty certain it's only used in limp-home mode for fueling, since limp-home spark is done via the EST module without bypass voltage applied.
EDIT: easiest way to determine limp-home: ALDL will not function, since the PROM is entirely bypassed.
1995 Chevrolet Monte Carlo LS 3100 + 4T60E
Interesting, I did not know it would function at all without a memcal (prom only)Originally Posted by RobertISaar
I also assume that the check engine light could possiably be a way to tell you if its in limp home mode? Only time I have seen it do the super fast flicker spas attack is when it was in limp home mode. I would assume if it does its normal slow once flash, its a reasonable bet its working and not in limp home mode?
79 Jeep Cherokee, AMC 401, T-18 manual trans, hydroboost, 16197427 MPFI system---the toy
93 Jeep YJ Wrangler, 4.0L, 5 speed, 8.8 rear, homebrew hub conversion and big brakes, hydroboost, 2.5in OME lift, 31x10.50's---the daily driver
99 Jeep WJ Grand Cherokee limited, 4.0L, auto, 2wd, leather and power everything, 99% stock---the long distance highway ride.
well, if it does the expected blink at key-on, that means the PROM is being read and that the light is being commanded on off on by the program, so it's a safe bet that if the light acts normally, at the very least, the ECM started in a normal mode.
for those of us with factory electronic fans, usually fan 1 will be stuck on at all times during limp-home mode. and if you have a speedometer that is ECM driven, that will fail to work as well.
1995 Chevrolet Monte Carlo LS 3100 + 4T60E
Limp mode is just that, a way to limp the vehicle to a safe place. The check engine light illuminates and stays on because the code to turn it off isn't running. The injectors fire once per reference pulse as the REF in signal is routed to the injector driver(s). Ignition timing is limited to whatever advance is built into the module. Electric cooling fan (if equipped) is energized but usually no other outputs work.
The super fast flicker happens when the code is messed up. Yes, the ecm is in limp mode but the flicker is due to the code looping through the shutoff steps over and over, quickly. When I built my ecm bench I used LED's to show me when the REF and EST lines were active. With those in place it's easy to tell if REF is working and EST is not, which is a good indication the ecm may be in limp mode.
The real name for the limp chips is "Redundant Fuel Device" which means that what it does is already done by something else, namely the code in the eprom. It's redundant. I have custom memcals here which have no limp chips or resistors. The ecm will function just fine but if the code or the eprom fails there's no backup and the engine will die.
It appears that code 41 is just a code. There doesn't appear to be any change in code operation. In the days of the C3 the RFD was a separate, replaceable chip which had to be moved to a new ecm and I'll bet that one was forgotten more than once.
Why do I keep thinking you'll see alternating injector pulses from a single driver ecm??
Limp mode is just that, a way to limp the vehicle to a safe place. The check engine light illuminates and stays on because the code to turn it off isn't running. The injectors fire once per reference pulse as the REF in signal is routed to the injector driver(s). Ignition timing is limited to whatever advance is built into the module. Electric cooling fan (if equipped) is energized but usually no other outputs work.
The super fast flicker happens when the code or chip itself is messed up. Yes, the ecm is in limp mode but the flicker is due to the code looping through the shutoff steps over and over, quickly. When I built my ecm bench I used LED's to show me when the REF and EST lines were active. With those in place it's easy to tell if REF is working and EST is not, which is a good indication the ecm may be in limp mode.
The real name for the limp chips is "Redundant Fuel Device" which means that what it does is already done by something else, namely the code in the eprom. It's redundant. I have custom memcals here which have no limp chips or resistors. The ecm will function just fine but if the code or the eprom fails there's no backup and the engine will die.
Funny story... talked to a friend / tech at a Ford dealership who had a TPI F car from the '90s in his bay. He was stumped over a strange problem. Car would not crank. He could jumper the starter relay and make it crank but no start / due to no fuel. He thought it might be the chip and grabbed another memcal that he had in his toolbox. The car would start and run but really badly. So he'd start the car on the bad chip then pull it really quickly while the car was running and replace it with the original which wouldn't crank and wouldn't start. As long as the car kept running during the swap he was fine, could run all day. But first time he shut off the engine the problem reappeared. I told him to fix the wire to the VATS ignition cylinder and put the original chip in. I also told him that he'd found the simplest method I'd heard of to bypass VATS. He was starting the car on a memcal from a 165 car, so it was starting on the limp module. Then he'd change it really quick before the engine could stall, putting the correct memcal back in. Apparently GM's code was written so that once the car was running, a failure to recieve a good VATS signal didn't kill the engine.
wow..... i actually never thought of even attempting that.
and yes, the PCM only looks for the VATS signal for a short amount of time after it powers up due to the ignition switch. usually, there is a timeout constant to play with in the DTCs section for the VATS diagnostic.
1995 Chevrolet Monte Carlo LS 3100 + 4T60E
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