4.0 89 Renix ECU Computer Help

I tried searching but so far this is what I've come up with...

So far I believe the 4.0 87-90 Renix ECU is interchangable. Is this true. Does it matter what trans the engine comes with? I have an auto and my CPS is not getting any voltage from the C1/D1 pins on the ECU (key in ACC position) so I figure that means my computer is dead. So any help in finding a new one cheap??? Thanks.

Someone else should pipe in here, but I don't think it should have voltage from the ECU. Rather, it makes a wavefrom from the rotation of the flexplate due to the magnetism inside it.
If you are getting a crank but no start symptom, you should replace it.
They are very often the reason for that...

Key in ACC is not Key in RUN - plan your diagnosis accordingly. ACC simply powers up the radio, lamps, and creature features that are electrical in nature.

You're going to want to check TPS supply voltage with Key in RUN or Key in START.

As far as I recall (I'd have to check,) the RENIX ECU units are fully interchangeable for the AMC242, 1987-1990. Since the TCU is separate (and the TPS supply for the TCU is also a separate circuit,) transmission likely makes no odds.

Yeah, its the common no-start problem. Here is the back story, I had a buddy weld up my floor (common rusting taking the whole driver foot well) and when we were finished, the jeep wouldnt start (negative cable was disconnected before we welded). We troubled shooted it down to the injectors (have spark, fuel pressure, motor turns over) (didnt use a noid light cause I dont have one) That lead us to the CPS, MAP, and ECU. Given that the CPS is the most common failure mode (this site is just amazing at what yall are able to figure out and post for others use), I started checking that before I spent any money. Which brings us to where we are now.

I was trying to measure the CPS, I remember from some thread there that there should be some constant voltage across the cps during the ACC of 4-5 volts. I got nothing. Just got done trying it while the motor was cranking and got nothing.

I measured the resistance across the CPS and got 235 ohms, according to fsm I found online it should be fine. I just wanted to be sure it wasnt the sensor before I try to change it and drop a bolt in the bellhousing.

Given that I am not measuring any voltage over the sensor while ACC or running, I dont think it is getting the required power. Accroding to the wiring diagram from the online fsm, the power is from the ECU. So now I am on the hunt for a ECU cause I am out of ideas.

A CPS is $35 from a local parts store, but dropping those bolts is a lot worse than changing out the ecu...

Look up my posts on "notching" the CPS - I haven't done a writeup yet. However, doing so will make it easier to replace it the next time, and can help you with the first time. Be sure to pack the socket with grease to help you hold onto the screws - bubblegum can also be used in a pinch (freeze the socket to remove the stuff.)

You're not likely to get any voltage supplied to the sensors with the key in ACC, for the reason I mentioned. Accessory is just to power up "creature features," and doesn't have anything at all to do with engine management. Period.

I'm inclined to think that the CPS actually generates a variation on a sinusoidal waveform (AC voltage of varying frequency,) and you'd have some difficulty measuring that with a DMM - an o-scope would be better. I really should check this out myself one of these days...

Anyhow, repeat your checks with the key in RUN (if you haven't already...) and report back. Once again, engine management should not be powered up with the key in ACC...

the ecu's are different between auto and manual. i'm sitting here at work with nothing better to do, so i ran the o.e numbers. i'm guessing it involves ignition curves? i'd stick with an auto ecu just to be safe.

If the engine cranks over, and the CPS wires aren't shorted to ground about 6-8 inches above sensor, (burnt from exhaust manifold) , and the CPS wasn't removed and reinstalled, I guess I would look elsewhere also.
I dunno but another shot in the dark is to do the reset procedure. You know, remove positive cable from battery, short that cable to negative for a bit, turn on lights, then turn key to run. (not sure of EXACT procedure) Maybe even without a ground during the welding it did mess up something. That would/does make your problem a bit unique. You could always just spend 40-50 bucks and get an OEM CPS...

slightly off topic but, I am wondering about this.
Why would removing the negative lead from the battery aid in averting any welding electrical problems? For mistakes (arcing thru starter feed wires etc..) I understand, but wouldn't it be better to remove positive lead. To avoid any potential difference in voltage affecting the electronics? Or no difference? In theory with the battery connected wouldn't it act as a huge spike filter?
apologies for going dweeb...

Well here's a lesson for those welding their jeeps, take both cables off the battery just to be sure, I found another ecu at the local junkyard and it now runs. I just wanted to thank everyone for their help. and just for those future searching. If the ecu is from a 4.0 Aw4 87-90 will work cause I got two computers, one from a 88 Pioneer auto and one from a 87 Laredo auto and they both work .

Now I can work on those damn water leaks:rattle: and do tha5-90's CPS bracket mod for when it craps out to make life easier.

Are you refering to the Renix year ECUs, 87-90????

What years did you check? If they are truely different it may have to do with an ECU interface (comunication) with the TCU on the AW4, versus the manual transmission, and the NSS gear selection signals to them.

Ten High said:

the ecu's are different between auto and manual. i'm sitting here at work with nothing better to do, so i ran the o.e numbers. i'm guessing it involves ignition curves? i'd stick with an auto ecu just to be safe.

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I wonder if a manual renix cherokee could use an ECU from an automatic renix cherokee..

If the flywheel and CPS are the same it would probably start, but it might run in open mode all the time killing mileage, because it would probably be looking for missing data of some sort from the NSS (gear possition data), or a TPS feedback signal. Don't know for sure, but there are plenty of AW4 Renix ECUs sitting around becuase they almost never go bad. For that reason few us have needed to replace the Renix ECU and therefore determine if they are interchangable.

By the way I found the paper work on the teamcherokee.com order for the TPS, Shipped May 28 (ordered saturday May 26th) and received June 5th. Merchant was listed as Kenosha County Classics (a DBA) in WI., on the statement (not teamgrandwagoneer.com), their email was listed as [email protected]

mattbred said:

I wonder if a manual renix cherokee could use an ECU from an automatic renix cherokee..

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Yes. In the Renix setup there is zero connection between the ECU and TCU. I'm using a manual ECM with the AW4 just fine so there is no different between the teeth on the manual flywheel and the slots on the auto flexplate.

There is a difference for OBD-I and OBD-II though as ECU gets its TPS signal from the ECU and there is a diagnostic signal hookup.

Interesting... Might check this myself later. I have a bunch of the auto ECUs' and only a couple of manual. I am nearly finished with a "full" restoration on an MJ that is 4.0, AW-4. Seems like a worthy candidate for tinkering with.