Renix FI firing voltage?

Got a 88 XJ with a 4.0l engine with 260K on it. The engine recently started to "not fire" on #2 cylinder. I "think" Ive chased it down to this cylinder as the engine developed a "stumble" at all speeds (idle included).

I used my induction timing light on each of the plug wires (which are brand new) and the light "fired" on all the wires. This led me to believe that the plugs are firing. Next I pulled the injector connectors one at a time off the injectors and when I did I noticed that the rpm of the engine would drop....until I pulled the #2 FI connector....no rpm drop. WHen I took the #1 connector off the #1 cylinder the rpm dropped BUT when I installed the #1 connector onto the #2 FI the rpm picked up. THis leads me to believe that the #2 FI is working properly although it may not be injecting fuel at the correct time

I check the voltage to the #2 connector with a digital meter and was getting around .8 VDC to the contact this was the same voltage as the other FI connector were getting. BUT when I used me analog meter (cheapy) it showed around 12ish volts.

What is the voltage to the FIs?

I wiggled the wiring harness and the voltage at the #2 FI connector did not change (Im putting the meters' + to the connectors + and the meters' - to the connectors -).

I reconnected the wiring to the FI and took the XJ for a spin. The "misifiring" is gone although I suspect it will be back.

Im thinking that the wiring connector to the FI is bad.

Any ideas?

I checked compression and all the cylinders are 124 or better. I put my vacuum gage on and the vacuum although low ( the engine has 260K) at all speeds is steady as a rock.

The drivers for the injectors are 12 volt (to the best of my knowledge) but at the injector, the pulse so quick it is hard to get an accurate reading. Ohm test the injector, should be around 16 ohms, though there can be a pretty wide error margin in the ohm reading and they are still considered good. As long as they aren't open or below around 4 ohms they are likely OK electrically. Check the injector ground for ohms, should be very low. Check the harness where it makes the bend at the firewall. I had one injector that periodically acted up, finally figured out the wire under the insulation was broken. I couldn't see the break, I kind of felt it with my thumbnail. It was just luck I found it, I kind of felt the break with my fingers, the insulation got real soft in one spot.
I've heard they sell noid lights to test the injector pulse, an LED that lights when the injector is pulsed.

Hallo. This is the NOID light that I was receiving from McQue. LOL
You can also make your own from a LED with a resistor of 1.5 K Ohm in series for 12 Volts.
But with a led , you have to deal with polarity.

p.s. 8Mud, the noid light of Bosch has a little bulb inside. Strange isnít?



'92 XJ

As I recall, firing voltage will be 12VDC - but you're going to have a hard time reading it with a conventional meter, as the pulse width isn't very wide (even at idle.) You're going to need an O-scope to get an accurate picture of what's going on, since it's +12VDC for a several milliseconds.

Debugging my 89 tonight, and it seems the FSM shows 12 V battery voltage to the injectors, and the ECU firing the injector via a solid state millisecond ground path. Tonights problem with mine, the 12 is not getting from working relays to the injector, so I have a hot wire problem it seems.

8Mud said:

The drivers for the injectors are 12 volt (to the best of my knowledge) but at the injector, the pulse so quick it is hard to get an accurate reading. Ohm test the injector, should be around 16 ohms, though there can be a pretty wide error margin in the ohm reading and they are still considered good. As long as they aren't open or below around 4 ohms they are likely OK electrically. Check the injector ground for ohms, should be very low. Check the harness where it makes the bend at the firewall. I had one injector that periodically acted up, finally figured out the wire under the insulation was broken. I couldn't see the break, I kind of felt it with my thumbnail. It was just luck I found it, I kind of felt the break with my fingers, the insulation got real soft in one spot.
I've heard they sell noid lights to test the injector pulse, an LED that lights when the injector is pulsed.

Click to expand...

Injector wiring harnesses chafing at the back of the head was a common issue. Good place to look. Don't forget on the old 88, the C101 connector carried many engine signals between the eCU, sensors and actuators.


Renix Jeep C101 Connector Refreshing
　
The C101 connector on 1987 and 1988 Renix Jeeps was a source of electrical resistance when the vehicles were new. So much so that the factory eliminated this connector in the 1989 and 1990 models. The factory recommended cleaning this connector to insure the proper voltage and ground signals between the ECU and the fuel injection sensors. We can only imagine how this connector has become a larger source of voltage loss and increased resistance over a period of almost 25 years. The C101 connector needs to be cleaned at least once in the lifetime of your vehicle. Chances are it’s never been done before.

Almost every critical signal between the engine sensors, injectors, ECU, and some to the TCU, travel this path through the C101 connector.
The C101 connector is located on the driver’s side firewall above and behind the brake booster. It is held together with a single bolt in it’s center. To get the connectors apart, simply remove the ¼" bolt and pull the halves apart. You will find the connector is packed with a black tar like substance which has hardened over time.
Take a pocket screwdriver or the like and scrape out all the tar crap you can. Follow up by spraying out both connector halves with brake cleaner and then swabbing out the remainder of the tar. Repeat this procedure until the tar is totally removed. This may require 3 or more repetitions. Wipe out the connectors after spraying with a soft cloth.
If you have a small pick or dental tool, tweak the female connectors on the one side so they grab the pins on the opposite side a bit tighter before bolting both halves back together.
　
Revised 03-02-2013

Ecomike said:

Debugging my 89 tonight, and it seems the FSM shows 12 V battery voltage to the injectors, and the ECU firing the injector via a solid state millisecond ground path. Tonights problem with mine, the 12 is not getting from working relays to the injector, so I have a hot wire problem it seems.

Click to expand...

Unless there is a major difference between the 88 and 89 (besides the C 101 connector) I know absolutely nothing about it. My harness forays into my 87 and 88 and the FSM wiring diagram (88 XJ and MJ Renix) show the injectors all sharing a common ground.

8Mud said:

Unless there is a major difference between the 88 and 89 (besides the C 101 connector) I know absolutely nothing about it. My harness forays into my 87 and 88 and the FSM wiring diagram (88 XJ and MJ Renix) show the injectors all sharing a common ground.

Click to expand...

Yes. You are correct in your assumption.

Also know that the "rail" splices for the injectors use "open" splices wrapped in what is essentially duct tape - my 87 had an injector issue that I finally traced to shonky injector connections on the "rail" - and I had to clip, clean, and reterminate everything (rearranged the "rail" to a "tree" setup, and cleaned everything right up!)

5-90 said:

(rearranged the "rail" to a "tree" setup, and cleaned everything right up!)

Click to expand...

Id Love To See A Pic Of This If You Have one

I have a photo of the harness opened up. It's also why I did a sensor ground test write-up and a sensor ground upgrade write-up.

Go to post #6 in the following link to see the procedure and photos.
Post #5 has the test procedure.
http://www.cherokeeforum.com/f51/cruiser54s-mostly-renix-tips-153657/

And sands, if I were you, I would just do the upgrade. Easy and worth it.

sandsgg said:

Id Love To See A Pic Of This If You Have one

Click to expand...

Wish I could - but I don't have the vehicle anymore (damn & blast Municipal Codes!) and I didn't have a digital camera when I did it.

I do recall it being a fairly simple exercise in creativity and done with parts from the local parts house - nothing exotic. So, it should be quite doable for you.