'90 Xj Renix - No injector pulse. Faulty ECM Confirmation ?

Hi everyone,

I'm stumped on starting a 1990 renix 4.0. I bought it like that, previous owner said it was harder and harder to start it until it wouldn't start anymore.
The injector are not firing, there is spark, it runs perfectly when sprayed on ether.

What have been checked :
Fuel Pressure is ok
Injectors are new (none of the are firing
CPS is new and OEM (works fine since spark is good when running on ether)
TPS is new and OEM (tested the old one as well), 0.85v w/ closed throttle, 4.3v feed)
Ground to the injector are all perfect (0.2ohm or so)
Common grounds refreshed
The ECM receive the dedicated 12v supply for the injector, at key on for a few second and constantly during cranking (two orange wires)
The fuel pump is priming and feeding during crank
Ignition is good (running on ether is smooth on all cylinder)

I'm on the edge of ordering a new ECM. Is there anything else to try/test before or anything to confirm that it is for sure the ECM ?

Has anyone seen ECM causing no injector pulse only on these engines ?

As a reminder renix is supplying 12v to fire injectors that are constantly grounded (and not the opposite...)

Thanks in advance

-johan

How does the wiring harness look? Pull the loom off and inspect, especially where it transitions from the valve cover to the firewall. Have you probed the injector wiring at the engine computer itself?

I don't recall if the anti-theft was an option for the 1990.

Otherwise, it looks like you've covered all the bases and swapping the computer would be my next test.

lawsoncl said:

How does the wiring harness look? Pull the loom off and inspect, especially where it transitions from the valve cover to the firewall. Have you probed the injector wiring at the engine computer itself?

Click to expand...

The wiring harness is not that bad. I unplugged everything one by one to check for corrosion, oil or any liquid. Everything was sprayed with contact cleaner. If you are referring to the C101 connector neaer the valve, this model has the revised harness and does not have it. If not what are you talking about (to be sure) ?


What do you mean by probed the ECU ? I measured 12v at the ECU pin for the 2 orange wires (that supposedly feed for the injector signal). I also measured the continuity between each injector pin and their respective injectors, all is good.

I was asking about the harness where it goes from the engine to the firewall where it can chaff inside and short or break wires. Usually, I bad motor mounts which added to the movement and flexing. A short there can blow the injector driver in the ECU.

On the probing, looks like you've already verified the wiring between the ECU and the injectors. I agree, the next step is a computer swap.

I get skeptical when a Renix ECU has supposedly failed. I always look for something else. How did you test to see if the injectors are or aren't firing? Is your MAP plugged in?

The injectors each have an individual driver, the pulse is millisecond. Most meters aren't quick enough to sense the pulse.

8Mud said:

I get skeptical when a Renix ECU has supposedly failed. I always look for something else. How did you test to see if the injectors are or aren't firing? Is your MAP plugged in?

Click to expand...

I am too except when there's an obvious short or lack of output, hence the post to look for other option.

I noticed at first by putting my finger and not feeling the clicking, then by removing the fuel ramp and looking for spray while cranking. I havn't tested with an oscilloscope since all the injectors are new and it's working with the old ones.

The MAP has been tested with a functionning one (if I remember correctly) i'll test again with a functionning one, I don't have access to a vacuum pump.

I forgot to mention that the muffler exploded when it was in a 'repair shop' (it never backfired or popped on me), but again work fine on ether.

I read somewhere a long time ago that if the MAP fails to find a baseline reference the injectors won't fire. Red w/tracer wire should be 5 volts, the violet wire 5 volts, and dropping voltage while you crank with TB closed.
I've seen two bad MAPS one had a bent pin that I only spotted with a mirror. The other had a battery voltage input that obviously indicated a short someplace, likely in the wiring.

Renix ECU has 6 injector drivers.

Some news.

TPS test and continuity to ECM = ok (same voltage than on sensor)
MAP test and continuity to ecm = 4.8v engine off and voltage drop when cranking (same voltage on sensor and on ECM pin)

I tested continuity between each injector driver pin on ECM and ground pin on ecm (disconnected) and I got 10.12k ohm. Is it normal ? Are there any test I can do with the ECM to test if it's faulty ?

Also I noticed something strange and tested on all cars I had in the garage at the time and only the renix does that :

When battery is hooked up no continuity between positive and negative terminal. When battery is not hooked up at all or only the positive, I get continuity (8ohm between the positive terminal and negative termimal or any other ground). Is that normal ? I've never seen that before...