Technical 04 1.2 8v Punto - problem with RPM sensor?

[Devondan]

Hi I'm new here and like many only come when I have a problem, still beats hanging out on the Alfa forum!

My Punto broke down yesterday, I was travelling at about 70 up a hill on the A30 dual carriageway when I had a sudden and dramatic loss of power, I had enough momentum to get me to the crest of the hill and onto the french drain. The AA came out quickly and recovered me home (only a few miles) The car turns over with compression (cam belt is fine) but only fires every now and then and not enough to start (even when cold). He plugged in their diagnostic laptop and said the rpm was jumping around and the sensor has failed. I've ordered one from Europarts and will pick it up tomorrow (£24). My question is whether the sensor likely to fail completely and without warning, I thought they tended to fail when hot? And what resistance does a normal sensor have?

I should say the car has done 107k. mostly trouble free and is regularly serviced.

I've done a few searches but cant find anything relevant?

Many thanks

Dan

 

[a1ant]

Hi mate,

It's not uncommon, however as you say it is more common for them to originally start failing when hot and get progressively worse.

I assume the AA man checked other usual suspects?

Iirc the resistance should be ~ 1350

Alan

 

[Devondan]

He might of but didn't tell me! although he def checked the coil pack, cleared the errors and then repeated the exercise. I'm getting 1100, so its worth changing anyway, I'll post the results after I've fitted the new one tomorrow.

 

[Ziggy122]

If the Car isn't sparking consistantly, then its not injecting either

Which to me - would be the crank sensor, and when they fail then fail when hot or just stop

£24 isn't too bad for a sensor

Ziggy

 

[Devondan]

Oh well got up bright and early and was at Euro parts just after 8 - picked up new sensor and fitted it and turned the key - no difference! It still feels like its firing on one cylinder but wont actually run. I'm thinking coil pack but without cable and software to read codes is there any way to tell?

Cheers

Dan

 

[a1ant]

Oh no, I had a feeling it wasnt the crank sensor from your description but its hard to argue when you havent personally looked at the diagnostic data.

Have a look at THIS guide on how to test your ECU and Coils

I take it the code light isnt staying on on the dash?

Any idea of what the "other" codes were that the AA man cleared?

Also did he mention the Cam sensor?

Alan

 

[Devondan]

Unfortunately the AA man didn't divulge, and I didn't ask, choosing to believe his diagnosis!

The only lights on the dash are the usual oil pressure and engine man light which usually goes out when its running, (remember I cant start it) I think but cant be sure that it came on when the fault occurred but I was busy trying to get out of lane 2 and onto the verge!

I'll try a bulb test on the coil pack but as it doesn't start and there's only me, it might be a bit tricky! - long wire required!

Cheers Dan

 

[Devondan]

Ok I tested the power supply to the coil, the bulb was lit permanently (is that right?), then flashed as I turned over the engine, so I assumed it was the coil pack and bought one ..... big disappointment - it made no difference. After a bit more testing now it is not firing at all and neither coil is getting a pulse.

To make matters worse I thought I'd remove the cam sensor but the bolt has sheared, so now I have to drill that out!

This car owes me nothing but I'd like to be able get it running. I'm happy to get the ECU rebuilt but not before I'm certain that's what it is. Will a diagnosis help? What cable do I need to buy? Is it this one www.ebay.co.uk/itm/FIAT-500-BRAVO-P...iagnostic_Tools_Equipment&hash=item232e73e12a or do I need adaptors also?

Note - careful use of a mig welder got the sheared bolt out, so that's sorted at least.

Update, recharged battery and is now firing occasionally again, it seems to be on random cylinders?

Cheers Dan

 

[Ziggy122]

IIRC the Punto uses 12v Constant live feeds for Coils AND injectors

So yes - there should be 12v constant
its the earthing that is switched on and off (negative trigger)

If the bulb flashed and then isn't flashing, its possibly sounding like a coil pack driver error (basically ecu)
But i'd be checking the wiring to ensure its not a snapped wire

Ziggy

 

[Devondan]

Its all flashing like it should do now ... wierd? But I'm still thinking ECU. I rang the AA and said their fix didn't work, they are sending a van around, I'll make a note of the errors this time.

Its at times like this that I like distributors and points!

Dan

 

[a1ant]

I'm thinking possibly cam sensor. But we shall see what the AA diagnostic says as could be several things ATM.

Alan

 

[Devondan]

Ok AA man been and gone, a very nice man! ... spent a good deal of time and checked everything I think, we have spark and power to the injectors, it fires on random cylinders, cam belt looks good but you cant check timing without some dismantling. Original error codes were

P0355 crankshaft position sensor A circuit, and

P0500 Vehicle speed sensor A.

He cleared the faults and connect up his laptop, when cranking over the engine speed showed 0rpm with an occasional fleeting 2-300. So I'm thinking either the new RMP sensor I fitted is broken (unlikely) there is broken continuity to the ECU or the ECU is knackered? The ECU did not record any faults after he cleared it?

So any thoughts?!

Cheers Dan

 

[Ziggy122]

Right Check the New Sensors and OLD sensor resistance and compare

Next honestly check the connectors
Do they fully mate? are they both clean with shiny pins? Dull pins work, but offer greater resistance

My thinking is the wiring has somehow become damaged, i'd be tempted to find which pin it goes to for the ECU and checking the resistance on both wires
Both should be low and balanced

If 1 wire has lots of resistance, i bet there is a chaff/break/snap thats allowing then signal to temporarily pass

Even the ECU thinks there is a problem with crank, but that has to have been failing for a while for that

Ziggy

 

[Devondan]

Will have a look at the wiring tomorrow, guess I'd better buy a manual unless the wiring diags are available on line?

PS how is he ECU sending a coil and injector pulse if it doesn't know the engine is spinning?

 

[Ziggy122]

Will have a look at the wiring tomorrow, guess I'd better buy a manual unless the wiring diags are available on line?

Follow the cam sensor back to the ECU - it'll go to the big multiplugs on it

Ziggy

 

[Devondan]

Ok recap:

My punto died while driving at about 70, I had serious power loss but limped to the french drain on one cylinder. Refused to restart but fired occasionally.

AA called - checked faults and said it needed a new crank position sensor.

New crank sensor bought and fitted still refused to start but fired occasionally as before.

AA called again, they checked that it has spark and injector pulse but ECU is still not saying it can detect RPM it its at 0 then occasionally spikes at 2-300.

Compresion tested by current draw, looked perfect. Cam belt intact.

Went out tonight to recheck, wiring looks fine and undamaged, engine still firing occasionally exactly as before. Disconnected crank sensor and it stopped firing completely, put it back on and its back to firing occasionally but not enough to start.

So if the wiring was broken it would not fire at all.

I understood that the ecu wont send a fuel and coil pulse unless the crank sensor is working - so what else could be stopping it from working and showing the crank sensor fault? I'm thinking ECU is there anyone who rebuilds them?

Any help appreciated

 

[a1ant]

Hi mate,

Bit of a long shot, but have you tried resetting the fuel cut off switch? Didnt go over a bump or anything just before the engine cut?

Have you checked if theres fuel getting to the injectors?


Alan

 

[Devondan]

Didn't know I had one! but I'll give it a try, but it is firing occasionally but consistently.

 

[a1ant]

Did you say the AA man had checked the coil pack and your getting consistent spark?
If thats the case then it could well be a fuel issue. And as above, the fuel cut of switch will cut fuel to the injectors from the lift pump. It could be that as your spinning it over, the HP fuel pump is just sucking through a small amount of fuel (enough to slightly fire on one cylinder).

Do you hear the lift pump priming when you switch the ignition to MAR (the one under the rear seat)?

Alan

 

[Devondan]

Yes the pump primes when switched to MAR. Would a fuel issue trigger a crank sensor error?