747spokes

Hi. I have a right hand drive UK registered 1998 Cherokee XJ that refuses to start. Fuel pressure good in the rail, big spark at the plugs, 5 volts feed to the cam position sensor (inside the distributor) but no electrical pulse to the injectors. Engines fires and tries to run only with easystart ether into the inlet manifold. Seems obvious that the injectors are not pulsing to allow fuel into the intake ports. Some people with similar problem report finding the fuel injector electrical circuit fuse blown. On replacing this fuse and repairing damage to the feed wire insulation resulted in the engine starting with no further trouble. However this injector circuit fuse does not exist in my Jeep - it simply is not installed and is not mentioned in the fuses list detailed in the owners manual. There is 5volts present in the cables leading to the camshaft sensor but not sure about the serviceability of the actual sensor - probably it is ok because the spark plugs are functioning. Definitely no electrical pulse to the injectors. Crankshaft sensor must be ok because very strong ignition spark for the plugs – it will jump about ½ inch.Any ideas would be very welcome. As an engineer I have to solve this problem myself! This jeep is in need of refurbishment which I intend to do but at least must get the engine running. Thankyou. 747spokes.
Should have mentioned that this is the 4 ltr 6 cylinder inline petrol engine.

4WD4EVER

Check for codes, even if there is no "Check Engine" light illuminated and let us know what you find, if anything.

XJ fuel injectors get their power from the ASD relay, which your RHD should have. It is in the PDC (power distribution center) under the hood (bonnet) near the battery. The PCM operates the injectors by grounding them. Sounds like your issue is in one of these circuits.

BlueRidgeMark

Nor should there be. As mentioned, constant voltage is supplied to the injectors, and the ECM fires them by providing ground. You should check for a pulsating ground. It should float at 12v when it's not being grounded by the ECM. The supply should be a constant 12v. If not, look into that ASD relay.

On a left hand drive, the PCM grounds to the same plate as the battery, near the distributor on the side opposite the intake/exhaust manifolds. It wouldn't hurt to check that ground.

747spokes

Dear 4WD4EVER and BlueRidgeMark,
Thankyou for your quick response. I will check the codes tomorrow. So if the injectors fire when the 12 volts is grounded for each injector in sequence does that mean that there should be 12 volts continuously at each injector all the time except when it pulses to zero momentarily causeing the fuel spray to be released? I should be able to read 12 volts on the supply side of the connector plugs for each injector (dropping to zero during fuel release) all the time the ignition key switch is in the on position? Would a test led light be illuminated continuously at each injector supply connector only blinking off for the pulse? At the moment there is no voltage at the injectors circuit connectors that is with the engine being motored over on the starter.
Also I will follow your advice and check the ASD relay. I did not know about the ability to check for fault codes but do now as I googled the method - very easy and clever.
Thankyou again.

4WD4EVER

You're quite welcome. See this, particularly posts #4 & #12:

https://www.cherokeeforum.com/f2/how...njector-39236/

BlueRidgeMark

Correct.

No pulsing on the supply side.

Correct.

No, it should stay at 12v even when the injector is firing.


Yes.

No, it should be constantly illuminated. No blinking. The supply side should remain at 12v whenever the key is in the Run position.

The blinking is on the ground side. If you check the ground side of the injector, it will pulse when the injector is fired. When the PCM is not supplying ground, it will be at 12v. When the PCM is grounding it, it will be at ground.


There's your problem. That's actually good news. It's probably a cheaper fix than a problem with the computer.

747spokes

Thanks again for your responses. Regarding below with your comment:"There's your problem. That's actually good news. It's probably a cheaper fix than a problem with the computer."
What is the likely fix to provide voltage to the injector circuits? Could the ASD relay be faulty? With the relay removed and the ignition on I do get 12 volts at two of the ASD sockets, ground on one other and nothing on the fourth. I've tried to get fault codes today by turning ign switch On-Off-On-Off-On but no codes appear, perhaps I'm doing it too slowly? The other test of holding in the trip mileage reset button whilst turning on the ignition does cycle through the gauges etc but no fault codes appear. I'm waiting to take receipt of a Snapon Vantage Pro tester which I'm hoping, when I learn how to use it, may provide me with some answers.
Determined to solve this as I want to renovate the whole jeep eventually but have to at least get the engine running again. Thanks.

BlueRidgeMark

Right there. That's the most likely culprit.

I haven't looked at a schematic, but that sounds right. Let's think about what that relay is doing and what it needs.

4 terminals.

1. One has to be supply from the battery. (power input to the relay)
2. One has to supply the 12 from the battery to the injectors, when the coil is energized. (this is the output of the relay)
3. One has to be 12v from the ignition to supply one side of the coil.
4. One has to be ground for the other side of the coil.

So, with the ignition on, you are seeing exactly what I would expect.

BUT, is the relay good? Swap it with a known good relay and find out. There are several there that are exactly the same. It's just a standard automotive relay. Nothing special about it. Just swap them.

If the relay is good, time for some continuity tests. We need to know if 12v is getting from the relay output to the injectors. An easy way to do that is to just make a test jumper. Get a section of wire, put a male .250 quick disconnect on each end, an inline fuse in the middle, and plug one end into the 12v you found in the relay socket (#1 in my list above) and one side into the "nothing on the fourth" that you found. That should supply voltage to the injectors. Make sure you plug into the constant 12v from the battery. Remove your key and see which one still has 12v. One of your two 12v terminals should still have 12v.



Ooooh, nice! I'm jealous!

747spokes

Hi BlueRidgeMark! Thankyou for your suggestions. I have now fitted a brand new ASD relay and still the same result. So now I'll follow your advice and check all the wiring for chaffed through insulation, earthing/grounding problems etc and hope fully will find something. Got the Vantage Pro now but still got to learn how to use it!! Sorry for the slow response but have had other distractions. Regards Simon.

BlueRidgeMark

I'm posting a few screenshots of schematic info related to the ASD relay. That relay provides power to the coil, the injectors, and the fuel pump. With all three, it supplies constant 12v, and the PCM supplies ground as needed to the coil (to regulate spark) and the injectors (to regulate fuel).


Use this to figure out where to put jumper to supply 12v to the ASD relay output (as a test).


----------------------------------------------------

Those numbers indicate which cavity corresponds to the table above. There should be constant 12 at #2. Jumper from 2 to 8 to test if there is continuity to the injector (and coil) supply side. 4 should see 12v when the key is on. 6 should see ground when the PCM supplies it. (should be key on, I think, but there may be other necessary conditions)



----------------------------------------------------
Check the fuse!

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All injectors are wired the same, of course, so I didn't take a screenshot of every one of them.

----------------------------------------------------

djgrayxj

Note from DJ
I hope this diagram helps.

747spokes

Thankyou everybody for your help, especially thanks to DJ for the circuit diagram. Have been distracted with other things but will continue to get my jeep back and running! I'm sure I will have some more questions but thanks for your help so far. SW.

freegdr

You state it starts with ether ? Are injectors not clicking ? Did you confim no pulse with noid lights?

batchplow

This is a great thread guys. I am having an identical problem with my 98 Cherokee Sport 4.0L inline 6. Fuel pressure, relays, fuses and spark are all good. My injectors are not pulsing though. I say this based on no fuel on spark plugs and listening with a screwdriver while turning over. I realize now this is not the best method of troubleshooting but I am learning a lot from this forum. I am away at work for a couple of weeks but a friend has lent me some great troubleshooting tools and I'm hoping between that and a bit more knowledge I'll be able to get the old girl running.

I was just wondering if 747spokes' issue was ever resolved. There is talk of issues with the ASD relay or injector fuses being the issue however when I look at the wiring diagram (big help) it looks like if either of these fail and you are not getting 12V at injectors then you shouldn't be getting spark either since the ignition coil is powered off of the same circuit. So I'm thinking he must have a wire break somewhere if he is seeing spark but no voltage at injectors.

Based on some research it sounds like if you are getting 12V at injectors but they aren't pulsing it could either be a bad ground, no signal from cam position sensor or a bad PCM or of course a wire break.

Does this sound right? Let me know if I'm missing something.

Fernando C. Diaz

I have an injector that is shorted on the negative cable. Can I bypass the short by installing a new cable from the PCM cable to the injector???