Intermittent No-start condition

Hi All,

Have a problem here. 1994 Chrysler T&C. I think it has either a bad crank sensor or a bad cam sensor, but I can't figure out which one it is.

The car runs fine until whicher sensor it is decides to stop working. Then the entire engine just stops. Attempting to start gets no attempt to fire. I know it's getting fuel because a guage on the fuel rail connected when it's not starting shows around 41/42psi. I know it's not a jumped timing chain because when it is running it's smooth at all speed ranges. I'm pretty sure it's not the coil because all plugs are affected. When it won't start, it's not getting spark, I checked that.

I think it's heat related because if I let the engine sit for 10-20 minutes it will start up again.

It just started doing this today. Fortunately it didn't do it when driving down the highway or anything like that. This evening I let it idle after getting home and after about 15 minutes of idling it stopped. Trying to restart it, no-go. Temp guage for coolant does not show overheating. The CEL is -not- set. I haven't pulled engine codes yet to see if there are any.

There is a procedure in the FSM for determining which sensor it is, but that would mean letting it idle tomorrow probably for about a half hour, and hope that it dies. But I never know when it is going to start so the test results could be screwed. And I could let it idle for a lot longer but it could possibly not die.

Ted

Reply to
Ted Mittelstaedt
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Dont you have a scan tool?

Glenn Beasley Chrysler Tech

Reply to
maxpower

I do, and the problem was when I posted this last night it was late and I wasn't thinking straight enough to think of checking the sensors with it. Of course, today, cold, it starts up fine and both sensors are fine according to the scan tool. So I'll just have to let it idle for a while and cross my fingers and hope it dies again. I'll post a followup.

Ted

Reply to
Ted Mittelstaedt

Were you thinking straight enough to check for trouble codes?

Reply to
nospam

No, I hadn't pulled engine codes as I mentioned.

Today I spent some time with it. There were no engine codes. The scantool that I am using is an OTC Monitor 4000E with Pathfinder 96 and it has a sensor test that will test both the crank position sensor and the cam position sensor.

I idled the van in the driveway and ran the 2 sensor tests. The response is either

-yes- or -no- I got yes's on both while the engine was running.

I idled the engine until it got hot enough and died. I ran the scantool tests while cranking. The crank position sensor result was 'yes' the cam position sensor result was 'no' with an occasional 'yes' Ah ha, I chortled, bad cam sensor.

I replaced the cam sensor with a new one (that was a chore) and repeated the idling test. The engine died again! I then waited till it cooled and repeated the scantool test, with identical results (yes and no) However the second time I gave it a lot more time on the cam sensor and the result on the tool kept flipping between 'yes' and 'no' In short, the old and new cam sensors give identical test results with the scantool.

Poorer but wiser my thought now is either the coil pack or the pcm is the problem. I really doubt the pcm because why would it work fine while the engine is warming up? The pcm isn't even in the engine compartment and shouldn't be subject to heat buildup problems. By contrast, coils sometimes fail in temp-dependent ways.

I'm going to knock off on it today, tomorrow I'll swap the coil pack between my

95 T&C and the 94 T&C and see if the problem goes away.

Ted

Reply to
Ted Mittelstaedt

My bet would be the crank sensor. On my wife's car, a bad cam sensor (or disconnecting the cam sensor) will ALWAYS set a code, but the crank sensor doesn't. Apparently the engine managment software (its a 93 Vision) doesn't check the cam signal for pulses when its not getting crank signal pulses- it just ASSumes that the engine isn't rotating rather than saying, "Hmm, cam signal, but no crank- must have a bad crank sensor!"

That's just my observation based on... well, observation. If anyone knows a better explanation of why that model car doesn't set a crank sensor code, I'd love to hear it.

Ted Mittelstaedt wrote:

Reply to
Steve

Your problem is with the crank sensor, If im not mistaken we talked about this years ago at the Training center when these vehicles were new, it seems to me that the DRB/ scan tool would fail the opposite sensor that was causing the problem the problem, Replace it and make sure you get the proper air gap setting.

Glenn Beasley Chrysler Tech

Reply to
maxpower

Thanks! I had went ahead and bought both sensors when I bought the cam sensor. I should have done the put-test-light-on-B+-of-coil test in the FSM instead of depending on the scantool. Sometimes the simpler diagnostics are better. At least the crank sensor is easier to get at.

The aftermarket sensors come with a small piece of thick paper glued to the end of the sensor, you push in the sensor until the paper touches the flexplate (or cam), that sets the gap. Presumably, any scuffing on the piece of paper is of no consequence.

Ted

Reply to
Ted Mittelstaedt

Absolutely correct on all counts, it was the CRANK sensor. As soon as I replaced it I ran the CAM sensor test again, and this time the scan tool showed the cam sensor no longer flipping between yes/no, now it was steady on yes as soon as the crankshaft started spinning. I test drove the car a good long time and no problems.

Ted

Reply to
Ted Mittelstaedt

I agree completely. My guess is that the software in the engine computer is written to key everything off the crank sensor. When it starts getting crank pulses as I recall it gets 3 pulses in rapid succession, then there is a gap then the crank is 1/2 way round and it gets another set of 2, then more gap and repeat. The pulses are generated by the holes in the flywheel flexplate.

The program uses the first 3 to ascertain the RPM then times all injectors and airflow sensing and such to take place at specific times in that sequence. The pulses from the cam sensor are handled in that sequence and if they aren't there the software sets a code.

If the crank sensor isn't pulsing the computer assumes the engine is at rest and isn't executing the instruction sequence that fires injectors and such and among other things checks the operation of the cam sensor.

What they would have had to have done to check the crank sensor is to have a probe wire going to the pcm that came from the starter. If starter is spinning and crank sensor isn't pulsing, then set a code. Otherwise the only other sensor the pcm can check that looks at engine rotation is the cam sensor, and it's only being checked in the loop that isn't triggered yet because of the cam sensor

Ted

Reply to
Ted Mittelstaedt

Our 92 Caravan 3.3L had a similar problem the dealers shop had a tough time to trace down. It would not start after sitting for a cpl of days. I could start it with a small shot of eather. It was a poor wire from the crank sesor. Seems that with the shot of eather it gave it enough boost to lift the voltage from the alternator to register the sensor as closed. Replaced the wire and all has been fine since.

cheers Marv

Reply to
Islander

I dont think so!!!! Let us know when you put the fuel pump in it after it wont start again.

Glenn Beasley Chrysler Tech

Reply to
maxpower

Hmmmm very interesting.. what your saying actually makes more sense. However the above is the story the service shop at the Dodge dealer I bought the car from gave me. This was over 2 yearsand over 50K ago now. I should go look for the seperate wire they say they put in.

cheers Marv

Reply to
Islander

It had nothing to do with a boost lift from the alternator that this started, if you had to spray eather to get it started.........that would mean there was no fuel at the injectors, nothing to do with a crank sensor.

Reply to
maxpower

I remember reading YEARS ago, I suppose it was for a MPI 4 cyl, that for added reliability the computer can pulse all injectors to emulate TBI injection if the cam sensor fails, keeping it running in limp-in mode with the light on. In a similar way, maybe it can approximate the crank signal using the cam sensor, if the crank sensor quits.

I am thinking this may well apply to newer vehicles also.

?
Reply to
clemslay

you are thinking of the 4 cyl but it isn't the crank sensor that can quit and still run. Its the cam sensor, if it goes out the vehicle will still run but not to efficient.

Glenn Beasley Chrysler Tech

Reply to
maxpower

But if the crank sensor wasn't working then how did it generate the spark to ignite the ether to get the crank sensor working? ;-)

Here's my $0.02 on this, and an explanation on your crank sensor problem.

I think that when the engine is hot that the transmission bellhousing, being aluminum, expands some. I suspect it's expansion increases the air gap slightly between the sensor and the flexplate.

This is a hall-effect sensor (magnetic) and distance is important - too far away and the sensor won't pick up the sense holes on the flexplate.

In my case I think my crank sensor was weak. Probably from age and being hot - heat is no friend to electronics. When my engine was cold the gap was narrower and the sensor was just barely triggering enough to allow the pcm to sense it. When my engine was warm the sensor was moved just a hair further away, just enough to make it no longer register to the pcm.

In your case I think your sensor may be weak/borderline as well. So that sometimes it would start, sometimes not. Perhaps your fuel pump is weak also, and the times you were testing, your crank sensor was working all the time so the ether got it going.

In your case I think when the dealership pulled the sensor and 'rewired it' and reinstalled it, they put it in slightly further than before. It is rather difficult to gap these things. When I installed mine, even with the paper spacer that is supposed to set the gap properly, when I started the engine there was a 'tick tick tick' that wasn't there before, coming from the sensor, quite obviously the sensor was in too far. I shut down and repositioned the sensor just a hair out further, and it was OK then. In your case your sensor was too far out, so that when the car was new it was sensitive enough so that this didn't matter, but when your van got older the extra gap did matter as the sensor lost sensitivity. Probably the dealership didn't think that the gap might have been too wide before, so when they got weak readings on the scope (or however they arrived at the conclusion) they didn't think about sensor gap and just invented an explanation about a weak wire to try to explain it.

Ted

Reply to
Ted Mittelstaedt

Acually that is the most common failure of the crank sensor, when it gets warm due to expansion it shuts down, you can see that with a lab scope

Reply to
maxpower

Thanks!

Reply to
clemslay

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