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[JamesH]

I'm pretty limited in my electrical skills but I have a multi-meter and can do the very basics. What I need is the help of those who have a bit more knowledge than me.

All of this relates to my XJ and trouble I've been having with sensors and rough running. I've done lots to it and improved the situation a lot. The only codes I have now are O2 sensors reading high, which I think is actually because the engine is running rich and not because of faults with the sensors (I've disconnected them and got no change and then replaced the upstream one and got no change). It is also running rough - when idling if you are leaning on it workig on stuff you can fee it cough and stumble more than it should.

Question is: I'm working my way through the wiring and testing grounds as this can cause problems according to the Yank bods on NAXJA and elswhere - what resistance in ohms is a good earth (zero I guess), acceptable earth for a 13 year old Jeep and what is a bad earth? Just to go back to basics I'm using the resistance (ohms) set to 20 with black lead on battering negative and using red lead to test wires and connecting bolts.

What should I be looking for and is there a particular one that gives up and causes problems on a facelift XJ?


It's driving me crazy, any help really appreciated 

[Bubba]

sorry dood ohms is not my area

[Dave69]

this is the basics,

you shouldn't need to connect to the negative terminal of the battery as the body shell of the car is an earth (return path).

0 ohms is excellant. put the two probes together and this is the reading your aiming for.

measure the resitance from the battery to the bodyshell earth point. this value might be in the 4 region which is ok.

now measure your wire and the reading should be between 0 and 4. the higher the number the greater the resistance which will indicate a poor wire or connection, or most likely the wire runs through an electrical component which in itself has a component whith a high resistance.

[bigjeepzj]

 
You want 4 ohms or less for any wire on your truck
the range on your meter you talk about is it 20 ohm or 20 Kohm

[JamesH]

Thanks, it's great to have some actual figures as reference points.

The meter is actually set at 200 ohm, that's the lowest on my meter.

Having read a bit more it might be worthwhile replacing all the major earths and some positive cables as an upgrade. If I also clean all the mounting point then I'll know the connections are all good. I have some spare winch power cable and heavy duty car stereo amp cable somewhere (a mis-spent youth).

Just picking up on Dave's point about components -  was measuring the 3 wires of the cam position sensor at the PCM and the earth gave a slightly high reading (about 13 ohm). Is this probably just because it is part of a loom which shares earths and powers between different circuit and when they all add up it gives this increase?

Can I test the O2 sensor wires - what voltages should I be looking for? Or do I need more complex kit?

[bigjeepzj]

To test an o2 sensor you need  more complex kit. an oscilloscope to check the switching  wave form the sensore

[willo]

You want 4 ohms or less for any wire on your truck
the range on your meter you talk about is it 20 ohm or 20 Kohm

i am surprised a bob standard multimeter is accurate enough, we at work, are only allowed to use a megger to test out heaters and compressor windings etc properly, then again, i don't know if the same theory here applies to dc

[Mike Pavelin]

Check for air leaks.

[bigjeepzj]

you normally only use the megger to do a 500v or 1000v insulation tests way to much for car wiring
High current earth testers (e.g PAT testers) are good for finding faulty connectors and joints.
But don't think James has ether so it's a mute point.

[supersparks]

electricity is not realy the mystic art people believe it to be . A cheap multimeter is perfectly acceptable to test the continuity of a cars earth path. to get slightly technical a multimeter pushes usually 3v via 2x1.5v aa batteries to get a reading as the car runs on 12v anyway it is perfectly upto the job.the whole idea of using a steel frame as an earth path is a ludicrous idea but it keeps weight down and reduces build costs .most of the sensors on an engine management run on 5v supplied from the ecu, a high resistance joint on either the +5v wire or the -0v earth can easily lose more than 1v giving a 20% loss which is really bad. so a joint from the body or at a sensor plug or even in the harness that looks perfectly good may actually be giving a bad joint, it is just a matter of being thorough and patient.

[Dave The Sparky]

electricity is not realy the mystic art people believe it to be .

shush you will have us out of a job! its very technical and specialised i think you will find

[willo]

electricity is not realy the mystic art people believe it to be . A cheap multimeter is perfectly acceptable to test the continuity of a cars earth path. to get slightly technical a multimeter pushes usually 3v via 2x1.5v aa batteries to get a reading as the car runs on 12v anyway it is perfectly upto the job.the whole idea of using a steel frame as an earth path is a ludicrous idea but it keeps weight down and reduces build costs .most of the sensors on an engine management run on 5v supplied from the ecu, a high resistance joint on either the +5v wire or the -0v earth can easily lose more than 1v giving a 20% loss which is really bad. so a joint from the body or at a sensor plug or even in the harness that looks perfectly good may actually be giving a bad joint, it is just a matter of being thorough and patient.

They say every day is a school day!

[Bubba]

i can sort most things given some time but newer sensor laiden stuff frys what bit of brain i have left

[JamesH]

Well I've had a bit of a play and unfortunately not found anything obviously causing the problem but I have ruled some things out and have some more questions

1. Mike - anywhere specifically to check for air leaks?
I assume you mean the vacuum system. I've been around from vac tank and brake booster and all line to/from intake manifold and charcoal canister. I can't find anything visually and have pulled most of the joints apart and back together and also done this for most while engine running (it changes in speed then almost stalls when re-connected but stabilises). There is good suction on the brake booster pipe which I read was one basic test to do.  Any further areas to look at or ways to test rather than just visual inspection?

2. Fuel injectors - when disconnecting the electric plug to each one the effect of disconnecting 5 and 6 'seems' to be slightly less than when disconnecting 1-4 while the engine is running. There does still seems to be a slight 'blip' when the plug is pulled and replaced but when disconnected the running of the engine is pretty much as bad as with them all connected. I have to make it clear that it's pretty hard to tell the effect as the idle is pretty rough and spluttery with the occasional cough.
Could 1 or 2 broken or half knackered injectors cause the rough and rich running and whats the best way to diagnose? (I haven't had any fault codes leading me to the injectors)

Finally, when I installed my LPG kit I remember being given specs for the 2 wires at each fuel injector. I can't find them now, does anyone know the voltages to look for so I can do a check with the meter?

I'm going to need the firelighters if this carries on 

[JamesH]

Since the above I'v listened to each fuel injector with a screwdriver and all are ticking away just fine. Fault codes appeared as soon as I disconnected the injectors so I think if there was an electrical fault there they either wouldn't be ticking or I'd hve a fault code (other than the codes I 'created' this afternoon)

Still no further into finding any air leaks

However, I've hooked up some OBD software that provides some sensor readouts - can anyone comment on the following as being right/wrong/suspicious:

Intake Manifold Pressure: 11.5 inHG (pretty constant)
Engine RPM: 753 rpm (pretty constant)
Absolute Throttle position: 15.3%
Calculated load value: 4.3%
Timing Advance (cyl #1): 9.0 - 16 degrees (fluctuating, mostly 10-13 degrees)

The last one worries me a bit, it seems a lot but I have no idea what it should be. Could I have buggered the timing somehow, stretched the chain maybe? something else??
 

[JamesH]

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Sorry for all the text and endless questions, any help really appreciated 

[supersparks]

i am very inclined to agree with mike , this sounds like an air leak, never really had to delve into the vac system on an xj but i believe they work on a PCV positive crankcase ventillation ,where all the vapours and oil mist is pulled from the crankcase into the inlet .the PCV consists of a one way valve and if it is  blocked will cause rough running or worse if it is stuck open it is allowing gasses to pass both ways so you are getting un metered air into the inlet chamber after the air flow meter which will give a high co2 fault code