Viper 1996 Gen2 Key On BARO: Anyone know how this is then used?

[MrCreosote]

I cannot believe (but suspect strongly) that they DO NOT adjust WOT Open Loop which means PCM is about a dumb as a carburetor or a bucket of nice-looking rocks...

[hexdummy]

I'm not sure what you mean by "they DO NOT adjust WOT Open Loop", could you elaborate ?

The base fueling in a JTEC works like this: 9x17 fuel lookup surface, which outputs a direct pulse width value, is modified by various fuel multipliers and adders, but typically, if the engine is warmed up, the ones you are concerned with for steady state tuning are:

Part throttle enrichment multiplier and the WOT enrichment fuel multiplier: part throttle enrichment is actuated by a map threshold. The wot enrichment multiplier takes over from the part throttle enrichment table above a specific throttle voltage delta from closed throttle, on all GEN2-3 Vipers, that is 2.45 volts stock. Now, there are some other qualifiers for part throttle and wot enrichment, but these are not used on GEN2 Vipers. There is a fuel multiplier that works off of the amount of spark retard that is being given from the air and coolant based spark retard tables. And regarding altitude, there is an altitude enrichment table, that adds fuel based on baro and rpm. Besides that baro influences various routines for :

Idle control (many)
Deceleration idle valve positioning
Ignition retard
Prime and cranking fuel
Accel fuel
OBD2 monitors
etc.

[MrCreosote]

hexdummy, Thank you so much for spilling some beans

I meant the cannot adjust WOT according to BARO during engine operation.

If BARO modifies the WOT fueling, then there is a problem: You Key On at 8000 ft, get a WOT lean out due to altitude, then you drive down the mountain to sea level and now you are running WOT way lean since BARO has not been updated with a Key Off / Key On.

Are there any references or standards for JTEC fueling, etc? I've been looking to no avail...

Roe Racing sells SCT tunes where you specify what altitude you way the tune for. I have a mildly modified 96 Viper GTS that gets very weird at my max altitude of 8000 to 9000 ft in the Sierras - in particular exaggerated idle hangs. I shudder to think about driving Pike's Peak or touring roads around 13k in the Rockys.

[hexdummy]

I understand that it may seem counter intuitive, but normally the altitude compensation only adds fuel, it does not remove fuel. On a 1996-97 cal you have a single 2d lookup for altitude compensation:

96-97 JTEC BARO fuel mult.JPG

Since the map sensor reads lower pressure at high altitudes, and the ecu uses MAP for it's lookup index for the fuel tables, you get less fuel as you go up in altitude. At the same time, as you go up in altitude, because the exhaust back pressure reduces, the pumping efficiency increases, this is why the table typically adds fuel and does not remove fuel.

The BARO does learn at key on, but it also will re-learn as you are driving the car under certain throttle/map combinations.

Regarding a reference, the best way is disassemble the binary, and learn all there is about the various routines for fueling. Another thing you may consider, if you are really interested in learning, is to get a spare ecu, setup a bench harness with a sensor simulator and experiment on the bench with a scan tool attached, the JimStim works well and is cheap. I think some of the points you are questioning can be answered in the factory technician student workbook for Chrysler speed density engine management and in the factory service manual, I would recommend that you start there. JTEC base fueling is straight forward, and I think some of the main points are covered in my first post on this tread. Where it gets quite a bit more convoluted is with transient fueling and closed loop fueling, but these are both modifiers to base fueling.

Another challenge with these ecu's is that there is no readily available logging tool that has a decent refresh rate. For example, all of the fuel modifiers are stored as ram variables, and can be logged. If you had the ability to log those values as can be done on other more well supported vehicles, you could see exactly what is influencing the base pulse width.

Not sure about the Roe Racing tunes, it might make sense for a forced induction cal since you effectively have to disable the baro fueling adjustment, but for normally aspirated don't know. Chrysler sold many trucks, Jeeps and Vipers with these ecu's and certainly many of them operate fine at high altitudes with no problems. The 1996 JTEC cal is somewhat of a special case. Chrysler came out with a service bulletin, that instructed you to let the car idle for ten minutes with the A/C on and then ten minutes with the A/C off so that several idle control adaptive could update. I would recommend that you do that first, and ensure that you maintain good cranking voltage at the ecu. These ecu's especially the 1996-98 versions, are susceptible to volatile memory reset while cranking. If that happens you lose you adaptive fueling, and well as other adaptives, such as those for idle control and learned stepper position.

[MrCreosote]

...posted a reply... and it disappeared

Found the training manual I believe:https://techauthorityonlinedemo.extr...8169900066.pdf

Let me see if I've got BARO right: MAP is all the engine needs to know on the intake side. BARO is used to adjust for reduced backpressure at altitude. Note that while BARO increases fueling, the net result at altitude is a leanout.

EDIT: Regarding Boost: If BARO is a back pressure/pumping loses adjustment, it would have to be further adjusted for the density of the charge which would require code. Nevertheless, wouldn't BARO still make a partial correction?

Nevertheless, If you Key-On and then drive up 4000ft in altitude, you will not get the BARO adjustment. So going up you end up a little lean, and coming down you end up a little rich. I do +4000 ft all the time in NV. Pike's peak and Rocky Mountains will be worse.
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QUESTION: That 10 min idle relearn TSB for the 1996 - is that the same as the dealer idle re-learn that has to be done after flashing a tune?

QUESTION: What is a "good" cranking voltage for the ECU? 10v?