Renix Computer Question

I recently purchase an 89 XJ limited, I6, Automatic, Tow Package with rear Dana 44. Its my first Jeep.

I have been chasing down an issue where it would always sputter around 3000RPM (it actually wouldn't let me go past 3000rpm at all). It would still get up to 80-85mph fine as long as I was easy on the throttle. I've been testing sensors using the various Renix guides out there and everything checked out. Yesterday my local junkyard had had a sale going on engine computers for 25 bucks, so decided to try replacing that.

Sure enough as soon as I swapped in the replacement computer the engine is now running beautifully, however when I turn the ignition off the the engine remains running (all the accessories, radio, ect will turn off). Thinking maybe I just had another bad computer with a different fault I went ahead and got another computer. This 2nd one again runs great, but wont turn off the engine . My original computer will still turn off and on the engine but has the 3000rpm sputter.

The 3 computers have these model numbers:

53007842 = Original Computer with sputter
EF8953005428 = Junkyard computer that wont shut engine off
EF8953005026 = Junkyard computer that wont shut engine off

Am I missing something with the original computer? Is it the fact the original computer didn't have EF89 on its part number important?

I was under the impression that all I6 automatic transmission computers from 87-90 were interchangeable. Anyone have any idea's before I use one of the newly acquired computers and just wire in a kill switch?

Try plugging the original back in. Your whole problem could have been a bad connection between the ECU and it's harness.

Fascinating. 5-90 would love to hear this story. My spare is a EF8953005406

cruiser54 said:

Try plugging the original back in. Your whole problem could have been a bad connection between the ECU and it's harness.

Click to expand...

From what he said, he already did that, and the issue is repeatable. I wonder if he got a test ECU, experimental, from the numbering?

Also wondering if there is custom wiring in the XJ?

Except for it not happening with the original ECU, I'd be inclined to think it's a wiring fault - probably at the ignition switch down by the firewall atop the column.

I'd look there first anyhow, particularly since it's spitting distance from the ECU and you'll be down there checking for wiring troubles anyhow.

But, I'm thinking wiring fault first. Hang on to those other ECUs tho - I may want to try to talk you out of them later as part of my own ongoing research effort (I want to know more about RENIX. Much more...)

Well, I've cracked open 53007842 which is the original computer and confirmed at least one fried out transistor, I replaced it but still has the sputtering issue, so likely some other damage I cant visually see.

From what I figure there are a few possibilities.

1. Both computers I've gotten from the Junkyard have the same fault. For something as specific as the jeep engine not turning off I'm doubting that but not ruling it out.

2. There is indeed some difference beetween computers, I've opened all 3 and the 2 from the junkyard look identical, the original is slightly different but looks like its just a newer PCB revision as the core components and IC's match up. I think my XJ had an optional factory alarm system installed at some point, dont know if that made/makes a difference.

3. There is a wiring fault with the ignition or ignition switch that somehow the original broken computer was able to overcome or bypass. There is some lingering electrical problems I am working on, like the Dome lights are getting no power and the clock gets no power, but from the wiring diagrams I have those shouldn't be directly related to the ignition.


In the meantime, I've just slapped a simple engine cutoff switch in the dash.

5-90 said:

Except for it not happening with the original ECU, I'd be inclined to think it's a wiring fault - probably at the ignition switch down by the firewall atop the column.

I'd look there first anyhow, particularly since it's spitting distance from the ECU and you'll be down there checking for wiring troubles anyhow.

But, I'm thinking wiring fault first. Hang on to those other ECUs tho - I may want to try to talk you out of them later as part of my own ongoing research effort (I want to know more about RENIX. Much more...)

Click to expand...

I agree that it sounds like a bad ignition switch and or damaged wires at the ignition switch.

WE need pictures of the inside of those ECUs!!!!!

Since I already got 2 of the computer open:

This is the Original 53007842:




This is EF8953005026




And the 2 of them side by size, the one that appears yellow is the 53007842:




One thing I noticed on the 53007842 is that the PCB is stamped Siemens, guessing that means they manufacturer the PCB, not necessarily the entire computer.

Well its not the ignition switch itself, I have another one hanging under the dash that wont turn the engine off either.

I wonder what the bottom sides look like. Many of the parts look like different parts.

I wonder the Finex II and 2012209 on the boards mean, lower left corners? Is that F and R, or is it an F? If it is an F, it may be a prototype Renix board, with an early version name? Or an aftermarket board with a F where the R normally goes for Renix?

Both computers in the pictures actually say Fenix 2 on them. And while the computers do look different, they are actually almost identical as far as layout and traces are concerned. Some of the IC's are different, but I think thats got more to do with who made the IC's themselves, for example they may have sourced IC's from Texas Instruments instead of Motorola.

I think the 53007842 is just a late revision computer, they probably only made a small number of them just prior to Chrysler doing away with Renix systems. I actually think its a computer from an early 90's Renix. Theres actually a computer on ebay with number 56006078, which is also missing the EF89, and the poster claims it was from a 90.

Have you replaced your MAP sensor? I had sputtering after my engine swap and replacing it made it go away.

The sputter at this point is fixed, my original computer was damaged. I went through all the sensors before thinking of trying a replacement computer.

The problem now is both Junkyard computers I have wont allow the engine to turn off (yet the original broken computer will start and stop the engine fine).

So you have power on the yellow wire from the ign (pin 11 I think) with the key turned off? Is it still there with the switch unplugged? I wonder if you have a grounding issue and something is feeding power backwards. Did you actually install the replacement computers - meaning is the case of the computer properly grounded?

I have one of the new computers completely installed and screwed in at this point, and am using a temp cutoff switch till i can figure this out.

I can actually completely unplug the wires from the ignition switch and the engine will remain running.

I haven't done any electrical testing yet, (other than trying another ignition switch), and likely wont get to that till Saturday.

"FENIX" is an OEM - it's a relative to our RENIX (used, I believe, in Volvo) - and it's also an outfit that makes high-performance LED flashlights. (!)

I'm still trying to get a channel open with Actia Corp. - last known patent holders for RENIX - to see what I can finally get out of them...

The EF89, mostly just the 89, is a prefix Chrysler added to the part number in the first stages of assimilating AMC.

With the ign switch unplugged, I'd start pulling relays and fuses until the power goes away. That might help narrow down where the power is coming from.

Thats what I plan on doing. From what the wiring diagrams show the following all get power from the yellow ignition wire:

1.) Computer - B8
2.) Ignition Ctrl Module
3.) Resistor Pack
4.) Trans Ctrl Unit In-Line Fuses
5.) Fuel Pump Relay
6.) O2 HTR Relay

So I have somewhere to start. Ill probably start with the line going to the computer since thats whats been swapped out, basically see if once powered up if these replacement computers are somehow generating their own power.

5-90 said:

Except for it not happening with the original ECU, I'd be inclined to think it's a wiring fault - probably at the ignition switch down by the firewall atop the column.

I'd look there first anyhow, particularly since it's spitting distance from the ECU and you'll be down there checking for wiring troubles anyhow.

But, I'm thinking wiring fault first. Hang on to those other ECUs tho - I may want to try to talk you out of them later as part of my own ongoing research effort (I want to know more about RENIX. Much more...)

Click to expand...

remember that my MJ has always done this as well. We discussed it 5 years ago. Anything above 80% throttle and 3K RPMs and it pops the ECU fusible link.

You can wind it out to 6500 if you'd like at 70% throttle.

I never bothered to proerly diagnose it, and since it's a trailered trail queen now it's not really on my list of things to fix. Not having a full throttle option has probably kept me from breaking things. I look at it as my Jeeps own will to survive.