How do Jeep ECUs work?

[ATL ZJ]

Since I don't have a speedometer (205 transfer case), I'm wondering if it's preventing my computer from "relearning" its programs to optimize itself. What exactly does the computer relearn? Shift points are not a concern because I run a manual transmission. As far as I can tell, the ECU thinks the vehicle is running in neutral all the time because I have the NSS grounded out. I'm more interested in fuel delivery- could tricking the ECU actually be hurting my performance? Engine and ECU are 1994 4.0 liter.

Not looking for answers like "who cares, it's a 4.0" or "it's not enough to worry about." Let's get some real tech in here for a change... I have plans to swap in a real v8 down the road, but the old trusty 4.0 has at least another season left on it. Would reprogramming my ECU with a "manual program" help? Were ECUs in 1993 ax-15 ZJs different than ECUs in 1993 AW4 ZJs?

[Spr-T-23]

if its a manual then you have nothing to worry about in the lines of the ecu freeking out not having trans info. the fuel delivery system is based off the engine, not having an auto trans for shift points, so i would think you would be good there as well, as long a your ECUs KAM (keep alive memory) is getting power. if it fires up fine everytime you are in the green.

[DJJordache]

from the 94ZJ FSM section 14-22
VEHICLE SPEED SENSOR—PCM INPUT The speed sensor (Fig.11) is located in the extension housing of the transmission (2WD) or on the transfer case extension housing (4WD). The sensor input is used by the power train control module (PCM) to determine vehicle speed and distance traveled. The speed sensor generates 8 pulses per sensor revolution. These signals, in conjunction with a closed throttle signal from the throttle position sensor, indicate a closed throttle deceleration to the PCM. When the vehicle is stopped at idle, a closed throttle signal is received by the PCM (but a speed sensor signal is not received). Under deceleration conditions, the PCM adjusts the idle air control (IAC) motor to maintain a desired MAP value. Under idle conditions, the PCM adjusts the IAC motor to maintain a desired engine speed.

I can't quantify the change by not using a speedo but it sounds like that signal input should/could be used by the computer even though you have a manual, to determine correct fuel maps, IAC and PCM useage

[zjrog]

Hmmm, maybe I need to look for the elusive 6 speed TJ trans, and the far mire elusive clutch/brake pedal assembly for the ZJ...

[ATL ZJ]

Interesting. Seems like not having the signal from the speedo may be affecting me after all. A friend told me that guys running TBI setups tend to notice a difference with a speedo hooked up.

I found an electric speedometer sensor to convert the 205's signal (Ford 205s use a cable style speedo from the factory) to the Jeep style... http://store.summitracing.com/partde...0&autoview=sku But I'm not sure if it will work because it generates 16 pulses/rev.

The speed sensor generates 8 pulses per sensor revolution.

Any more ideas?

[DJJordache]

I have a spare Jeep speedo gear assembly (electronic part that the gear goes into) from a 2000 AW4 231 XJ or a 95 42RE 249 ZJ will have to check for sure.... what if you adapted the 205 cable output into the speedo gear input of the Jeep electric speedo pickup to get your correct signal

[paulkeith]

I found an electric speedometer sensor to convert the 205's signal (Ford 205s use a cable style speedo from the factory) to the Jeep style... http://store.summitracing.com/partde...0&autoview=sku But I'm not sure if it will work because it generates 16 pulses/rev.

Any more ideas?

wouldn't that just result in the ECU thinking you're going twice as fast? doesn't seem like a big deal...

[ATL ZJ]

Not sure about adapting/mating the 205 cable output to the Jeep speedo assembly. Here is a pic of the Ford cable assembly (which I do not have):



Here is the hole in the 205:



So I guess the conclusion we've reached is that the PCM is constantly thinking the vehicle is in a "deceleration" condition and is adjusting the IAC motor accordingly... which may not be ideal for acceleration or power? Exactly how is the "fuel delivery system based off the engine" SPR-T-23?

[Spr-T-23]

i was saying that having a manual trans means that the trans has nothing to due with the fuel managment, 12 hour midnight shift man, just got home.....

[Army_Monkey]

I think you are misinterpreting it a little. The engine "knows" when it isn't decelerating based on all of the other inputs, mostly the MAP sensor and TPS. The fuel and timing maps in the PCM are based almost entirely on engine speed, MAP value, and TPS reading, it uses the oxygen sensor to make minor corrections, and doesn't care what the speed sensor thinks.
Obviously, there are other factors, such as engine temperature, that also play a role, but that is mostly so the computer knows when to go from relying strictly on its built-in maps to using the O2 sensor for input. If you are coasting down from speed with the transmission in gear, the PCM sees the loss in engine speed and checks for the speed sensor signal to verify deceleration. When it doesn't see it, it will ignore that fact and use strictly the engine RPM, MAP, and TPS to adjust the fuel and IAC.
It won't be ideal for the conditions the engine is seeing, but hardly noteworthy. Under acceleration and cruise, it will rely on the other readings and perform almost identically.
To sum it up: There will be a minute change in performance, but it would likely not even show up on a dyno

In reference to the NSS, the PCM really doesn't care about that, it is used only when starting the engine and otherwise it is ignored.

From what I could find, the '93 ZJ PCMs with the 4.0 are exactly the same for the manual and with the AW4, and different for the 42RE. My reading into it is that the situation is the same as with TJs, which have all the same wiring for autos versus manuals other than transmission-related. It's likely not worth the hassle of doing the swap, however. Especially if you plan on doing a V8 swap, since you'll just be doing it again.

[jborushko]

i dont know how involved they can do it but cant you have your PCM custom programed for whatever? to include "deleting" sensors? sorry i dont have any links to send you, ill look into it alittle more...

or a custom ecu/pcm/whatever else you call it like from http://www.perfectpower.com/default.asp i dont know if those will work on a amc motor though
a stand alone fuel management system might be a good option if you can find one to work on our motors... but $$$pricey$$$

ha ha you could say fuck it and toss a carburetor on there...

[DTrinh]

as stated before, typically the PCM uses map, tps, and rmp, to calculate fuel. Its on a fuel density algorithm. Then the PCM will use IAT, coolant temp to adjust the fuel for startup and ambient air. The O2 sensors are used to adjust the base map programed in the PCM. All these additional sensors are just used to maximize fuel economy and reduce emissions. It will not effect performance. When you get on the throttle quick, the PCM will check for rate of change in TPS and give accel enrichments for extra fuel for extra power. Same when you let off, it will provide the reverse with decel and cut fuel to be more fuel efficient. You may notice a bit of fuel saving with the speed sensor but not enough unless you're daily driving this thing.