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Amedee
Amedee, ASE Master Tech
Category: Car
Satisfied Customers: 21981
Experience:  ASE Master Tech advanced level specialist. Wisconsin certified emissions state inspector
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1993 oldsmobile cutlass supreme: stalling..scan tool..rpm..the tach

Customer Question

I have a 1993 oldsmobile cutlass supreme 3.1 t that has a stalling problem.The Modis scan tool is reading erratic rpm(NNN) NNN-NNNNat idle but the tach on the dash is reading correct.
Submitted: 4 years ago.
Category: Car
Expert:  Amedee replied 4 years ago.
Hello!

WHy does this question sound familiar?

Did you ask this question earlier tonight?
Customer: replied 4 years ago.
I didn't get into all the basic checks but fuel pressure and vacuum leaks are all ruled out.The I.a.c. is also operational.The problem I am seeing is the computer is seeing qiuck changing erratic rpm readings even though the car is idling fine.The rpm readings are scrolling like a slot machine after you pull the lever.I ran a new wire on ckt. 430 from the module to ecm to rule out a possible break in harness.I am lead to believe if the computer is seeing that type of random rpm readings it would be impossible to correct the idle with the IAC or by any other means because it doesn't see the correct Rpm.The wiring diagrams show the dash tach has a different wire (121) and again that is reading correct with the actual rpm off the motor.What I have read is the crank sensor feeds the Ignition module which in turn sends a signal on ckt.430 to the ecm to give engine speed. I was hoping some one may have encounterd this before but that may not be the case.
Customer: replied 4 years ago.
I didn't get into all the basic checks but fuel pressure and vacuum leaks are all ruled out.The I.a.c. is also operational.The problem I am seeing is the computer is seeing qiuck changing erratic rpm readings even though the car is idling fine.The rpm readings are scrolling like a slot machine after you pull the lever.I ran a new wire on ckt. 430 from the module to ecm to rule out a possible break in harness.I am lead to believe if the computer is seeing that type of random rpm readings it would be impossible to correct the idle with the IAC or by any other means because it doesn't see the correct Rpm.The wiring diagrams show the dash tach has a different wire (121) and again that is reading correct with the actual rpm off the motor.What I have read is the crank sensor feeds the Ignition module which in turn sends a signal on ckt.430 to the ecm to give engine speed. I was hoping some one may have encounterd this before but that may not be the case.    
Customer: replied 4 years ago.
I sent you more info did you get it?
Expert:  Amedee replied 4 years ago.

Hello!

 

YES... I just got it... THanks for the update!

 

Give me a minute so I can go through all of this info

Expert:  Amedee replied 4 years ago.

Ok...

 

Sorry for the delay!

 

I just had a chance to read what you have wrote.

 

WHat is the o2 sensor doing when this happens?

MAybe you just have a bad oxygen sensor?

 

WHat is the block learn at when the engine is at idle and off idle?

 

HAve you smoke tested the intake?

 

WHat about the fuel pressure regulator?

 

If you were to pull the IAC motor out and plug up the passages, does the engine idle better?

 

Customer: replied 4 years ago.
All of that seems in line I may not be explaining myself correctly to you.The car does have a stalling condition but It will idle most of the time.When idling the RPm reading in the scanner is cycling like an o2 reading all over the place ranging from(NNN) NNN-NNNNvery quickly even though the car has a steady idle and the tach on the dash is reading correctly.The rpm to the scanner is like it is getting interfearence or something.My thinking this is the cause of the stalling because how could the computer possibly adjust conditions with a rpm reading like that.I have to believe it needs to see correct rpm.This is not something i have seen and I have been in the business for over 25 years.
Expert:  Amedee replied 4 years ago.

Sorry for the delay!

 

Are you saying that the engine idle speed on the scanner goes up and down but not the engine rpm itself?

Customer: replied 4 years ago.
That is correct.The scanner is showing a very rapidly changing Rpm from300-5000 rpm all while the engine is steady at about 900 rpm.I have also tried another scanner to verify that wasn't the problem.
Expert:  Amedee replied 4 years ago.

WOW! THis is definitely an issue... maybe you have a bad computer??

 

I would scope the crank sensor to see if in fact the signal is erratic. If it is, then maybe you have a crnak sensor or a wiring issue. Also..just a thought... maybe the reluctor the engine speed sensor reads is a problem?? A quick check with a lab scope will be able to verify this for you.

Customer: replied 4 years ago.
I had written before I have replace the crank sensor,Ignition module,pcm and prom.I also cut the signal ckt 430 at the ignition module and ran it direct to the pcm.I am confused by the way the tach in the car is reading correctly but I know it comes of a different leg in which would indicate the reluctor and NEW sensor is Ok.
Expert:  Amedee replied 4 years ago.

But yet you have not scoped checked it yet have you?

 

In addition, there are two crank sensors and even one cam sensor. I would recommend to check them all. The computer will use all of these inputs to factor in engine rpm.

Customer: replied 4 years ago.
I know there is a cam sensor but do not see or know about the 2nd crank sensor.the cam to my knowledge helps tim the injectors correct?
Expert:  Amedee replied 4 years ago.
Sorry for the delay!

YES... there is the 3X crankshaft position sensor and there is the 24X crankshaft position sensor. Here is some information about the 24X crankshaft position sensor from alldata.

24X Crankshaft Sensor Installation

Attachments are only available to registered users.

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PURPOSE
The 24X crankshaft sensor is used to improve idle spark control at engine speeds up to approximately 2000 rpm.

OPERATION
The 24X crank sensor contains a Hall-effect switch. The magnet and hall-effect switch are separated by an air gap. The Hall-Effect switch reacts like a solid-state switch, grounding a low current signal voltage when a magnetic field is present. When the magnetic field is shielded from the switch by a piece of steel placed in the air gap between the switch and the magnet, the signal voltage is not grounded. If the piece of steel is repeatedly moved in and out of the air gap, the signal voltage will appear to cycle. The piece of steel is the interrupter ring mounted to the rear of the crankshaft balancer.

Each interrupter ring has blades and windows that, with crankshaft rotation, either block the magnetic field or let it pass to one of the Hall-effect switches. The Hall-Effect switch is called the 24X crank sensor because this interrupt ring has 24 evenly spaced blades and windows which are all the same width. The 24X crank sensor produces 24 "ON-OFF" pulses per crankshaft revolution. The hall-effect switch closest to the crankshaft, the 3X crankshaft sensor, is so called because the interrupter ring has a special wheel cast on the crankshaft that has seven machined slots, six of which are equally spaced 60°apart. the seventh slot is spaced 10°from one of the other slots. As the interrupter ring rotates with the crankshaft, the slots change the magnetic field. This will cause the 3X hall-effect switch to ground the 3X signal voltage supplied from the ignition module. The 24X interrupter ring and hall effect switch react similarly. The ignition module interprets the 24X and 3X "ON-OFF" signals as an indication of crankshaft position, and must have both signals to "fire" the correct ignition coil. The ignition coil determines crank position for correct ignition coil sequencing by counting how many 24X signal transitions occur during a 3X pulse.

LOCATION
The dual crankshaft sensor, secured in an aluminum mounting bracket and bolted to the front left side of the engine timing cover, is partially behind the crankshaft harmonic balancer. A three wire harness connector plugs into the sensor and connects it to the module.
Customer: replied 4 years ago.
This engine doesn't have 2 crank sensors.I wish it did
Expert:  Amedee replied 4 years ago.
Hello!

According to the factory manual it does have two crank sensors.

Which one doesn't it have?
Customer: replied 4 years ago.
It doesn't have the one on the balancer in front of the motor.The only one is behind the motor in the center of the block,which was replaced.
Expert:  Amedee replied 4 years ago.
WHy does this motor not have this crank sensor? Was it removed?

Does the balancer have the teeth marks for the trigger for the sensor?
Customer: replied 4 years ago.
no there is nothing on the balancer or any wiring or bracket to indicate it ever had one.This is my elderly neighbors car that has been well taken care of.She informed me yesterday this has been an on going problem and even brought the car to trhe dealer in the last 12 months but they missed diagnosed the problem also.I'm sure everyone thought the same basic things we talked about earlier (egr. IAC Map clean throttle plates Check vacuum leaks ect...)
Expert:  Amedee replied 4 years ago.
Right... because 9 times out of 10, weird things like this just turn out to be something extremely simple and basic that was just overlooked.

WHat is the block learn at?

Did you smoke test the intake?

Did we scope either cam or crank sensor yet?

WHat is the ignition timing doing on your scanner when this happens?

Did we pull the IAC motor out yet and plug up the passages to see if this gets any better?
Customer: replied 4 years ago.
I did not forget you I just have another business I needed to be at today.I will let you know Friday about these questions.I am focusing on the false rpm readings only because it will idle most of the time and vacuum leaks and iac shouldn't have anything to do with those.I appreciate your help and look forward to finding the problem.I will let you know more friday
Expert:  Amedee replied 4 years ago.
Hello!

Sounds good!

I will be looking forward to your reply!
Amedee, ASE Master Tech
Category: Car
Satisfied Customers: 21981
Experience: ASE Master Tech advanced level specialist. Wisconsin certified emissions state inspector
Amedee and 11 other Car Specialists are ready to help you
Customer: replied 4 years ago.
This car only has a crank sensor in the back of the block,There isn't a cam sensor either
Expert:  Amedee replied 4 years ago.
Hello!

Thanks for the accept!

Let me know if you have any further questions!

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