Calgary Jeep Association
4x4 Related Groups => Tech Talk => Topic started by: Gearhead on January 28, 2006, 08:41:41 PM
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Diagnostic help would be appreciated , BUT ......... read the entire description of symptoms and diagnostic steps taken so far , first , before posting !
If you have nothing intelligent to add , don't bother posting !
Here's the deal ........
1994 YJ w/ 2.5 , it came home on the tow truck last night , has become hard to start recently , now it won't start at all .
I have a Factory service manual , a Haynes manual ( crap!) , an OTC enhanced 4000 scan tool w/ appropriate cartridges , an OTC digital multimeter , a good handle on electrical and vehicle diagnostics w/ several years experience.
Today I checked for spark @ #1 plug wire , had none , checked the coil , it's fine , I can enter a actuator diagnostics mode with my OTC scan tool and make a spark when a spark plug is connected to the lead on the coil , I can crank the engine with spark plug connected to the coil wire and make a spark that way too . I have back probed the coil connector , the cam position sensor ( dist.) and the crank position sensor , using my digital multimeter to test voltages and observe triggering events while cranking , as per the factory service manual . Additionally , using my scan tool , in the test actuator mode , I can hear the automatic shut down relay operate , the injectors , and can test the tach , the shift light as well as a few other relays . I have disconnected the PCM connector to visually check for bent pins or damage.
I should add , since I began driving this thing last August , it has , on occasion / intermittantly , at start up , exhibited rough running characteristics , sputtering , unable to take any throttle or build RPM . To correct the problem you simply shut the engine off , re-start and it runs fine ! . It would only behave this way once in a while.
Is this related to the no start ? described above ?
My scan tool is showing one trouble code only , ..... # 33 , this Jeep does not have air conditioning , so a code #33 can be ignored , you could erase it , but it would be back as soon as you started it again , ..... um ..... if it ran
Any ideas ?
A Further symptom , just brought forward by the co-driver , ...... the other day , on the way to work , she noticed the tachometer stayed at 1000 , then it started working properly.
I'm thinking the PCM ( powertrain control module ) is toast , probably a dealer item and not cheap , also , I'm not a parts changer type , ..... guessing what the problem might be , then throwing new parts at it till it runs , not fully understanding the problem or what fixed it , ........... so I'm not about to run out and buy a PCM , to see if that will fix it , Not my style !, however ..... if I could ..........BORROW ONE ? ......... I could dig that !
HELP ! this thing is a DD it needs to be running , every day ! , including this monday , to get to work .
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Careful on the pcm swap. If there is an external reason for the pcm to be toast, you could toast the borrowed one..... Which would be bad......
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Careful on the pcm swap. If there is an external reason for the pcm to be toast, you could toast the borrowed one..... Which would be bad......
Thats a risk I'm willing to accept , my theory is , this PCM had issues and was on the way out , when I bought this Jeep last year along with several thousand dollars worth of other problems , all of which have been corrected , many upgrades done too. There are a few more test's I'm doing first , before I get to changing the PCM .
Let's say , I went to a wrecker , bought a used PCM and it was NFG , but they un-knowingly sold it to me anyway , I took it home dicked around for another day or two , then tried to return it , .... not gonna happen . Many of these wreckers have no means of testing components such as this , have no problem selling it for a big price regardless of condition , I've been stung before.
So Im more inclined to borrow one that is known to work , at a small risk .
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So far ...... no good
Re - tested all that was checked yesterday
Did a full series of tests again with my OTC scan tool , relays , actuators , injectors , trouble codes ( none ) ..........
Went and bought a fuel pressure guage ............ has 38 psi fuel press.
Replaced cap and rotor , waste of time and money !! , and yes the rotor turns while cranking.
Using my digital multi meter , checked voltage @ cam position sensor and crank position sensor , also observed a pulsed signal while cranking , by backprobing the connectors.
Removed the crank sensor to clean and inspect , re-installed sensor.
I was able to fire the coil by a third method , ..... using a few jumper wires and a condensor , ....... not that this is important , it has been long ago established , the coil is working.
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You checked the fuel pressure but how about volume? Should be able to fill a litre bottle about half in 20-30 seconds. Also if you can, borrow a lab scope so you can actually see the waveform of the crank and cam sensor.
Also, but I'm not sure on a Jeep, the engine will still run with a bad cam sensor. The cam sensor in alot of vehicals with SFI just times the injector pulses just as the intake valve opens. When sensor is bad all injectors just fire at the same time.
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Monday
I tried another used PCM from the same model and year , still no spark. Ran another series of tests , had spark at the coil while cranking , tested actuators and relays with my OTC scan tool ...................... became tired , disgusted and frustrated .............. went to bed
Tuesday
Tried a used crank position sensor , .............. resulting in ....... no spark grrrrrrr
Tried my other distributor , it's an earlier one but has the correct connector , again ......... no spark
Now I'm back to the distributor that was in there and was an autoline rebuild , purchased last summer , the cap , rotor and wires that were new at that time too. the crank sensor it was running with is back in there too.
So far all I have invested is , what seems like an excessive amount of time to find what I thought in the beginning to be , a fairly simple problem , ................
Next step ........................ check every wire for continuity from the 60 pin PCM connector back to each and every sensor.
This is a no spark issue
I'm starting to think it's within the wiring harness , that's why I'm going to start checking for continuity.
Lab scope ? don't have one , don't know anyone who has one , check wave forms ? on an engine that won't run .............. excuse my ignorance but ?? , having not used a lab scope before ................. please enlighten me
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A lab scope is like an occiloscope sp?. When hooked to the output wire of say a crank sensor it will show you the wave form pattern it is outputing, even when cranking over. Most crank patterns look like this, a square wave: Crap pattern didn't transfer right
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As to your problem hmmm. You changed the three things needed to give it spark. The PCM is basically your ignition module, crank sensor to output an rpm signal to PCM, and distributor/cam sensor to output timing information to PCM. One thing to check would be hooking your test light to the neg bat post and neg on the coil with the connector connected and turn it over. Does the test light flash? If not and you are sure the parts you tried were in fact operational, then possibly a shorted or open wire somewhere.
Hang on, I just re-read your posts. So you have spark at the coil but not the plug wires? Then the problem is dist cap, rotor, or wires. Look closely at the rotor. I have seen them burn a tiny pinhole thru the center thus causing the spark to go to ground via the dist shaft. Sorry just saw you changed cap and rotor. hmm this is very odd. does the engine do anything while turning over ie kinda fire, backfire etc??
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No backfiring ..................
This evenings head scratching session revealed a bad camshaft position sensor , I'm not equipped with the best diagnostic tools , the OTC scanner I have allows a variety of test's to be done , some that I was not aware of. I had a buddy over that is more up to speed with this stuff. We discussed it , checked a few things , he has seen many failures with this sensor.
I'll be trying another one tomorrow
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Generally speaking, a crank position sensor is responsible for proper spark, while a cam sensor is responsible for fuel issues. If the crank sensor is bad, engine will crank and crank and no sparky. If cam sensor is bad, engine will try and sputter and fart around. So the lesson here kiddies:
Crankshaft sensor = spark
Camshaft sensor = fuel injection
At least thats what they said in 3rd year school last June.
DISCLAIMER :!: it says "Jeep" on the front of this vehicle, so anything is possible.
Darren 8)
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Yay ! it's fixed !!
Thank's Crash , for helping when my patience was beginning to get very thin .
The cam sensor was fine
The problem was a combination of two things , a poor quality / defective distributor cap , ..... and a weak coil , it worked , but was weak and could not jump the air gap in the cap as well as the plug gap . The cap that was on there had far more resistance than normal between the coil terminal and the button inside where the rotor makes contact. those two things were enough to interupt the current in the secondary ignition circuit.
Thank's to anyone here who offered suggestions , your help has been appreciated !