Still no Start - Injectors not pulsing - Please Help!

Hey All:

My 90 Limited has been a planter in my driveway for about 2 weeks now.

Finally discovered that the injectors are not firing.
Replaced the:

CPS - reading 240 Ohms (key off) and 3.5 VDC (engine cranking)
ECU
Distributor

Still nothing.

Reading through a whole bunch of posts here, I had thought that the CPS is responible for generating a pulse signal to the the ECU, which in turn uses this signal to fire the injectors at the appropriate times. Is that it? Or is there more sensor input(s) involved?

Or should I concentrate on the wiring between the CPS-ECU-Injectors?

I am out of time, patience, and money here - Please Help!

TIA,
Chris

Actually the injectors´ timing is controlled by the ECU (PCM) based on the pulses from the CMP (Camshaft position sensor) that's in the distributor. There's also a relay in the PDC (Power Distribution Center, a black box on the passenger side of the engine)

CMP? Also known as the Sync Signal Generator, right?

I thought that CPS finds the signal to fire the injector at the right time, and the CMP "matches up" the CPS signal to each injector - it finds #1 cylinder for the ECU. Also, I read in my FSM that ECU will randomly pick an injector to be "first" and the Jeep will still start and run (really crappy) without the CMP signal.

Am I reading this wrong?

Chris

Also which relay should I check - the power latch or the starter motor relay?

TIA,
Chris

Check the ASD relay (Auto Shutdown). Any spark? ASD also controls power to the coil.

Spark is fine.

Where is the ASD relay?

Chris

First off the 90 XJ has the Renix engine control, not the Chrysler control used on 91 and later 4.0 engines. As far as I remember this is not auto shutdown relay on the renix engine.

There are two relays which control fuel. I believe it is the latch relay and the fuel pump relay - will have to check my book when I get home to make sure. I replaced both relays as a preventive measure a few months ago. Figured 16 years of service was long enough. You will find them by the start relay on the passenger fender. The start relay has the 10 ga POS wire from the battery connected to it. I have a 88 XJ Pioneer 4.0 NP231 Dana 35.

There is a sensor in the distributor, it is called the sync sensor, also called the camshaft position sensor. It is a hall effect sensor which tells the computer if #1 cylinder is on the compression stroke or the exhaust stroke so the correct cyldiners are feed fuel.

The crankshaft position sensor is used to determine when the spark plug fires.

If I was closer I'd hook up my Snap-on MT2500 test set and see what going on. There are no error codes stored on a Renix engine control. The scanner would tell you a lot of things. That why I searched Ebay and found one. If you buy it off of Ebay make sure you get the Jeep-1 adaptor and the data cartrages have JEEP on them. Do not buy it if the cartrage only says GM-Ford-Chrysler.

Have you done a fuel pressure test to see if the fuel pump is working? It 31 and 39 psi, one reading with 14 in Hg vacuum, one at atmospheric pressure.

You could have a bad connection. Have you unplugged and replugged the computer, along with the other connectors?

You may also need to invest in the factory manuals to have the souce data vs "I was told..." to fix it.

Getting good fuel pressure.
Getting good spark.
Replaced ECU

Everything appears to be in good working order, except for the injectors - they are not getting their signals to open.

Chris

Thanks, Martin. My point of reference is my 2001XJ.
SInce no injector is working, it must be something they all have in common. Are you getting a 12V continuous signal there?

Then I'd focus on the wiring from the injectors back to the computer, you must have some damaged wires. I'd also find a buddy with that snap on scan tool I mentioned and have them hook it up. The only scanners which will work are the Dealer scanner and the Snap-on MT2500. All other aftermarket scanners leave out the 87-90 Jeep 4.0 EFI engines.

You may end up towing it to the shop so they can use the scan tool on it. Before you take it anywhere insist on talking to the mechanic who will work on it, ask him if he understands the RENIX engine control. If he says "what is that" them find another guy. I found an old guy at the local Jeep dealer who understands the RENIX system; in fact, he told me he likes the Renix XJs. He'd like to have one to fix up. Quite a compliment considering the mechanic considers himself a Ford guy who works at a Chrysler/Jeep dealership.

Your XJ can be fixed, keep the faith. It just may require you to pay for some professional help. With it dead this long you might consider taking it to a shop now.

I have a stupid question. How did you determine that the injectors were not working? Did you use a set of noid lights? The drive for the injectors comes directly from the ECU on the Renix engine. If all injectors are not receiving drive, you can rule out a bad wire since it would effect only one injector.

If there really is no power pulsing to the injectors, either you have a bad sensor (CPS or Camshaft sensor in distributor) or the ECU is bad or isn't receiving the info from the sensors.

Actually all the injectors are fed from one single power feed, but each injector has its own ground wire going back to the ECU. The ECU completes the ground to fire the injector.

Or so I've read.

I've already replaced the CMS, CPS, and ECU.

Guess I'm going to have to start running continuity tests on the wiring between (sigh).

Chris

My wiring diagrams show the common side on the injectors to be ground. Here is a the Renix injection system manual if you want.

http://www.4x4getaways.com/89JeepFI.pdf arty:

OK - I got that backwards.

I have that FSM - thanks, though!

Chris

Since all injectors are giving you the same problem and the common is ground, Just take a jumper wire and tie the common point to the battery negative terminal and see if the problem goes away. If not and you have replaced the ECM, you need to think about wiring to the cam position sensor in the distributor.

The ground for the injectors is the ground strap at the back of the head rear bolt and goes up to the bulkhead above. If the ground strap is lose or broken = no start and no run. Been there done that. If broken at head bolt don't losen bolt just tuck strap under one of the rear fuel rail bolts. Mine has been there for 6 yrs now. Greg

are the injectors not firing or do you have no fuel pressure in the fuel rail. do you know if you have any power to the fuel pump, and is it running if so?

Thanks everyone for the responses.

I am almost positive that I found the problem.
Last night, I pulled the ECU back down from under the dash and ran continuity tests on the harness for all the major players (CPS, CMS, Injectors, etc) - didn't find a problem. Just for the hell of it, I plugged back in the ECU harness, and the Jeep fired right up!

So I'm thinking to myself - what the $%$#!
Started the Jeep, and while it was running, wiggled the ECU harness connectors - Jeep died instantly.

So I have a loose wire(s) right at the ECU connectors.
Didn't have time to find the wire last night, but shouldn't be too hard to isolate it tonight.

It would almost be funny except for all the time and money I threw under the hood of this thing over the past two weeks - and the problem wasn't even under the hood!

Thanks Again,
Chris

sounds like you got it figured out chris.. i was going to tell you to inpsect the entire length of the wire harness from the ECU to the engine .. ESPECIALLY the C101 connection (if they still had that in 90.. not sure)

Hey Mike!

No C101 connection in the 90.
Still have the Laredo?

Chris