Got the ecu out so I now can test its connector wires that was the worst installation I have seen, cursed the jeep gods the entire time. I want to meet the fool so called engineer that designed its mounting so I can bludgeon him, then spit on his cold hearted corpse. no room to work, self locking nuts fighting every one tenth a rotation, brake pedal wedges on my skull, feet up in the air out the door, really? and those crummy plastic cable retainers. worse job, I rather set up a ring and pinion. There is no reason to make this job as difficult as can be. I got cuts on my finger and my wrist is bleeding. I am so drained from this I need to walk away from this pile for a bit, that after dealing near all day trying to sort out the worst factory wire schematic which does not bother to tell you which of the ten pin connectors is which, literally six or seven pages of schematic to sort thru to figure out which wires go where on those connectors. . No wonder jeep chrysler has such a bad reputation. I now understand why.
I need to spend quality time with my 60 year old VW for detox therapy , a nice sweet logical, well designed, easy to work on car.

 

Did you take it out bracket and all?

 

Yes, that appeared the only way to get the connectors removed. The blood on my hands and wrist has now clotted I will live. So I went back to the 24 pin ecu connector, per the factory schematic B11 and B12 are pink wires (schematic sheet 111) but no, they are black ! Ugh! this is making trouble shooting so fun.

any way I ohmed each one to chassis ground near the under dash fuse panel, got 7 ohms, which is probably too much, right? Id hope for an ohm or less. of course maybe my ground point under the dash I am using for reference is not all that good? I need to get some longer leads made up for my ohm meter so I can go from the ecu connectors under the dash all the way to my engine block ground point (dip stick stud)
from the schematic it looks like B11 and B12 at the ecu go to the dip stick stud, via splice <99> [10] which is in the engine bay up high center of fire wall. per schematic page 8W - 60 looks like ten wires come together at that splice, six for the injectors, the two to the ecu, one to the dipstick stud, and one to diagnostic connector. However the diagnostic connector ground pin shows less than 1 ohm to battery negative.

 

The way I remove the ECU is using a ratcheting wrench on the nuts holding the bracket to the body. Going back together, only 2 nuts go back on.
7 ohms seems high. Not sure on the diag connector. I would want to measure from ECU to batt neg.

 

yes, I think I removed it just as you do, two nuts removed, both up high on the ecu. the ecu came out with a steel bracket attached to it. why didnt they put those nuts down lower?


Anyway, there is ten wires ganged on that harness splice if the schematic is correct, does that sound right? splice located in harness near top of fire wall in engine bay per the manual at <99> [10] see image

here is the typo, B11 and B12 are black, not pink, I crossed that out

Here is the splice

here is ecu pins B11 and B12 going to the ten wire splice

 

I'd have to go look at my 90 to be sure. 89 and 90 had no C101.
Not unusual for chafing to occur near the driver's side of the valve cover where the harness leaves the firewall to injectors.
Might be time to peel back some split loom and have a peek.

 

Ok, Today did a lot of ohms probing with the ecu removed. ohmed the mps sensor back to the ecu plug, the O2 sensor back to the ecu, the egr solenoid back to the ecu, also the ecu ground pins to the chassis, and they all were about one ohm. also ohmed one ohm or less to battery post negative to the dipstick ground point

I think this gives my harness and its splices a clean bill of health for what we tested. Correct?

rehooked up the ecu and ran the motor. the egr solenoid green/black wire which is supposed to be an ecu switched ground was again at 3.7 volts to ground, and thus is wrong, my O2 sensor per my scan tool was not working, stayed at 2.3 volts this time, engine ran very rich smelling then would not smell rich, back and forth as before
and the idle hunts. same problems as before
we also tested fuel pressure 35 psi with vacuum on the regulator, 40 psi with no vacuum. (but we found pressure rapidly drops after shutdown, so the check valve is leaking, we can deal with that later)

So It looks like the ecu has gone south, not my harness.

Does this seem correct, the ecu is the problem?

 

Or one or more of the injectors is leaking.....

 

Oh yeah, brain freeze here, spending too much time on the ecu issue to think straight. I will pull the injector rail and check, I guess I ought to have new injector to head seals handy also. Thanks!

I got an used ecu ordered, exact same part number, EF8953005428

Thanks Cruiser54, you are a shining star!

 

Hope to have the new ecu soon. First replacement ecu from a ebay seller was a bait and switch scam the ebay ad showed a nice clean ecu, close up photo of the connector, looked nice, What arrived was an ecu with busted connectors, plastic bits broken and missing ( i.e. not shipping damage) lots of corrosion, and mud caked on it. peering inside, found the interior had water damage too, corrosion. It went right back for full refund, ugh!

another seller had a nice clean unit, and confirmed Id be getting the one pictured in the ebay ad. It should arrive this week. I will let you know how it works out,

So hold tight, results will be posted!

 

I'm still willing to bet it's the ECU. Not that this analogy means anything, but... is every AM radio, cassette and 8-track player, Beta video recorder, reel-to-reel tape recorder, Apple IIE personal computer, Atari video game console and everything else that was of the electronics era from over 30 years old still working? As an electrician, I have seen a lot of circuit board-related failures over 21 years, though not related to vehicles of course.

The ground provided by the ECU isn't via a relay inside; it's done through a semiconductor (SCR or MOSFET perhaps). These never give a perfect continuity of 0 ohms. A power rectifier, generally, has a full volt, or a fraction thereover, of drop across it. The transistor conductive-state resistance depends on the type/style/manufacturer, etc. When circuit components start to fail, they send the wrong voltages to the wrong places and ICs and other more sensitive/less tolerant silicone components on the same board become negatively affected. So, I wouldn't be surprised that you're having issues.

 

That leads me to wonder what it will be like in 20 more years. there are no new Renix ecus made, then what? i have an early 1972 injected car, D jet by Bosch, but its ecu is all discrete components, no i.c.'s, so it has held up well.
Golly I read Tesla has a big recall cause the touch screens are failing in under five years, and it is a safety issue cause the touch screen is needed for things like window defogger and wiper operation. Tesla is said to say that those electronics can NOT be expected to last the life of the car! Yeah a touch screen subjected to the sun in Arizona all summer long, that can be a problem.

So what do us XJ owners do in 20 years, when all the ecu start to have component failure? I guess we start replacing components at the board level, but what of specialized IC's, which may have no replacements available?

 

I have wondered that before myself considering the one I have; i doubt I will ever find another one.

There are aftermarket replacements still available. And, they require core charges which leads me to believe the housings are still sought after for rebuild/refurbish purposes by a company specializing in vehicle control modules. Quite a few 4.0L XJs were sold between '87-'90. Additionally, there is a good portion of the US where vehicles last longer than they do in places like where I live. So, for a while at least, the rebuilt/refurbished and salvaged computers will still be available. Eventually, replacements will start to fade. Cardone finally discontinued rebuilding the rear wiper motors for pre-'91 XJs to the best of my knowledge, for example.. The '91 is identical in shape and mounting, but the number of wires increased by one to incorporate the rear delay feature-- so, without adding a constant 12V+, it won't work in a pre-'91 as is.

There used to be a Yahoo group of guys that kept a database of pictures of the internals, component numbers and a lot of other information specific to the Renix ECU. Last I knew, Yahoo was going to close its group accounts. Cruiser54, weren't you a part of that group? It may one day come down to getting out your soldering iron and de-soldering tool, and ordering electrolytic capacitors and rectifiers from Digi-Key or Mouser.

 

Did the ECU solve your problem?

 

yes