Electrical: Engine Sensors, Etc.                                                                    FAQ Home
  Volvo Maintenance FAQ for 7xx/9xx/90 Cars                                                                                                                    Version 5.0
Engine Temperature Sensors

Diagnosing ECT Failures

Oxygen Sensor Life and Diagnostics

Crank Position Sensor Bad: No Re-Start after Hot Soak

Testing Flywheel RPM Sensor

Testing a Knock Sensor

Engine Temperature Sensors. The B230F of the vintage discussed, 1989-1990 have in effect three temp sensors: the Engine Coolant Temperature (ECT) and the temperature gauge sensor. Two of the sensing elements (NTC) are combined in one ECT sensor housing with two connectors and ground through the housing. One of the temp sensor "signals" goes to the LH unit, the other to the EZK unit. The third temp sensor is the one used by the temp gauge in the instrument cluster. It also has two connectors, one "signal", one "ground".

Removing the Sensors.  [Query]  How do I remove the gauge temperature sensor?  [Response: Mark Duval]  I bought the "stubby" 19mm combination wrench and a 19mm crow foot socket. First tried the open end side of the stubby and although I could get it squarely on the hex head, I could not put enough force on the wrench to turn the sensor.  Then used the crow foot socket with a long extension. This rotated off the hex and cracked the electrical connection at the top of the sensor. The answer all along was to remove the electrical connector (destroying the sensor) and gain access to the hex with an offset 19mm box wrench. Thereafter R & R in seconds.  Replacement sensor seems to be work and gauge
reads as expected so far.  [Editor]  It helps to first remove the knock sensor nearby, which is merely bolted into the block.  Removing the ECT is a little tougher, since it is buried under the intake manifold.

Diagnosing ECT Failures.  [Response: Don Foster, adapted to 700/900 series by Editor] Your engine has two temp sensors -- one for the gauge (it's mounted in the head about under intake header #2, and one for the FI (it's mounted in the head about under intake header #3).  The latter is the ECT sensor.
The sensor is an NTC thermistor -- negative temperature coefficient. As the temp drops, the resistance rises, and as the temperature rises, the temperature drops. Thus, if you have a broken wire, defective sensor, or bad connection (I've seen it happen) the resistance measured by the ecu will be very high or infinite. The ECU interprets this as minus a zillion degrees and pours in the gas.
The car I saw filled the oil with gas and certainly wouldn't run. And the problem was only a displaced spade lug in the plastic connector housing.
According to Chilton's (you may choose to disregard this):
"The coolant temperature signal to the control unit has a great influence on the computed injection period... For example, when the engine is being started and is cold, the amount of injected fuel must be relatively large.

"If the control unit receives a signal higher than 302F (150C) or lower than -40F (-40C), it will interpret the signal as a fault...the control unit will assume a substitute value corresponding to 32F (0C) on starting and 68F (20C) when the engine has started.

"With the control unit connected, connect a voltmeter across LH ECU terminals 13 and 5 (ground), turn the ignition switch ON.

At 68F (20C) the voltage should be 2.0 +/-.5v volts.
At 104F(40C)  the voltage should be 1.2 +/- .3volts
At 176F (80C) the voltage should be .5 +/- .2volts.
The resistance values between pins 13 and 5 or between pins on the sensor are (by eye from the chart):
                          32F (0C)-- about 6000 ohms within a range of +/- 10%
                          68F(20C) -- about 2300 ohms "
                          104F(40C) -- about 1300 ohms "
                          140F(60C) -- about 600 ohms "
                          176F(80C) -- about 300 ohms "
                          212F -- about 190 ohms "

[Response 2:  Steve Ringlee] ECT resistance "cold" should be around 6k ohms at 32 degrees F (0 deg C), 2300 at 68 degrees F (20 C), and 200 at 212 F (100 C). However, try checking your ECT wiring: Between pins 13 and 5 at the LH ECU (with sensor DISconnected) resistance should be infinite. Voltage with the ignition ON and sensor connected, measured between pins 13 and 5, should be:
                     0 C=around 3 volts +/-.5v
                     20C=around 2 volts +/- .5v
                     100C=around .3 volt +/- .1v
If these aren't correct, check the connections in the ECT wiring harness. Check engine ground connections at the intake manifold.  If the voltage is zero, your ECU is at fault.

Oxygen Sensor Life and Diagnostics. Oxygen sensors should last over 100k miles under ideal conditions; various contaminants will shorten that considerably. For testing procedures and other info see:   http://www.f-body.org/oldfaq/html/tech/sect2.html#oxysensor

[Further Notes on Sensor Tests:].  [Query:] Anybody know what the resistance of the O2 sensor heater should be? mine says 2.2 ohms. With only the green wire connected to the ECU I get a pretty stable .52 volts, which I have read in the archives to be the ECU reference voltage. Hopefully all is well at that end. If I connect the heater (there is ~13.5v on the harness side, btw) the sensor voltage decays over about 30 seconds to .01 volts. At no time does voltage sweep from .3 to .7 v.   Also, can you replace the heated one with non-heated?  A Bosch Ford 5 litre sensor is $97.80 (cdn) at a local import autoparts store, the Volvo sensor is $280 something.  [Response 1: Abe Crombie] That is the right resistance on the heater.  Before you replace sensor warm up engine attach voltmeter to O2 sensor lead and then momentarily pinch rubber return line behind fuel pressure regulator. If the voltage of sensor goes to .7-.9 V then look for something else making it lean. A faulty sensor can give you the low V reading but an air leak or defective MAF sensor can do the same.  With regard to heated (three wire), versus non-heated (two wire),  yes and no. The heated sensors are used to keep the signal active while idling. Idling in cold weather primarily, the sensor will fall below its 600F temp at which the reaction that makes it output voltage begins to work. If it were to stay above this temp the non-heated would be okay.  There are different sensors out there also. Most all the Bosch up to 92 have the same output voltage at the same O2 gradient. After 92 there are differences in some of the applications in this respect.
The Bendix/Siemens fuel sytem models use a different type sensor and it is not interchangeable with the Bosch units. This sensor does not work with a .5 V reference out of the ECM as does the Bosch versions. This type uses the heater power supply to feed a sensor circuit responds to O2 level variations. The Bosch type employs an electrochemical reaction and the heater is for stability of the temp (and therefore the signal) and not for the supply of current to be modified and used as output signal.

Crank Position Sensor Bad: No Re-Start after Hot Soak. Suppose the car already has the upgraded silver-terminal fuel pump relay but won't restart promptly after a hot soak. Take an educated guess on a weak crank position (RPM) sensor. Replacing this permanent-magnet sensor, which is located in the top of the bellhousing, fixes many hot restart complaints on 700s. See also
[Note from Steve Seekins:] Note that if your car is a turbo, you do not have the crank position sensor, but you do have  a Hall effect sensor in the distributor that can also be the problem (cars with the crank  position sensor do not have the Hall sensor in the distributor). [Editor's Note: Not true in post '89 B230FT cars: these have rpm sensors.)  See also Intermittent No Start Problems

Testing Flywheel RPM Sensor.  [Query:] I decided to find and remove my flywheel sensor. Externally it's in perfect condition. The cable and ends are pristine-looking.  Is there a way to test it with a multimeter?  [Response: Ivan K] The problem with these rpm sensors is that they will only fail intermittently. Usually, when they get hot. This makes it hard to test.  They are cheap enough. Just replace it.  With high mileage cars, it's only a matter of time before  they fail.

Testing a Knock Sensor.  [Query:] How do I test the knock sensor? [Response: Abe Crombie]  If there is not a fault code for it then it is okay. The ignition control unit tests it every time you exceed some thing around 3000 rpm with moderate to high throttle. If it is not torqued properly it might not be as sensitive as it should or be overly sensitive. Torque is 8-11 ft-lbs.  It produces a high voltage pulse when it senses the shock wave in the range of combustion knock.  The voltage level is proportional to the severity of the knock. It will read 1.5 or greater megaohms but this is not really an effective test, only the control unit detecting a sensor signal when the conditions would absolutely assure a signal would be made is the real check. The knock sensor test by whacking on block (better than on the intake) next to knock sensor (see Duane's note below) is only effective on Chrysler systems with the throttle opened as the closed throttle signal is supplied to Chrysler ignition  ECU and it cancels knock sensor retard activity.   If you have Bosch ignition systems you can't do the whack it and watch it test for knock sensor operation. The Bosch systems are cylinder specific and you would have to knock within a few crank degrees of a given cylinder firing and be monitoring the timing light attached to that cylinder's plug wire. I don't think any of us are that good. [Tip for Chrysler Ignition Systems from Duane Hoberg] On the right side of the engine, in the area above and to the rear of the oil filter, tap the  block with a metal faced hammer while observing the timing with a timing light.  The timing should retard 6 degrees then return. If not then the sensor or the circuit in the control box is bad.  When installing the new knock sensor, use thread lock.  Apply too much torque and the sensor becomes sensitive as it is prestressed and reacts to normal engine vibration.  When this happens you will idle fine but above idle run at retarded timing all the time. Acts like poor fuel delivery and doesn't like hills.

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