'94 850 no spark

socket_wrench · Post by **socket_wrench** » 19 Dec 2011, 14:45

My 1994 850 non-turbo will intermittently crank but not start. Sometimes it starts on the second or third try, other times it takes up to ten minutes. Once it starts, it runs perfectly.

I've checked the primary and secondary ignition systems. There is no spark - the ignition module (on the driver's side strut - what Volvo refers to as the DI power stage) is not being pulsed by the ignition control module. (Checked this by disconnecting the DI power stage and inserting a test light between the yellow/white wire from the ICM and battery ground while a friend cranked the engine - no flashing.)

I initially assumed the problem was a faulty RPM or camshaft position sensor. However, no DTC's have been triggered. Disconnecting either sensor during cranking triggers the corresponding DTC, and both sensors generate the correct output, so I believe the problem isn't either of the sensors.

Out of curiosity, I attached a test light in parallel across a fuel injector, and noticed it was not being pulsed either. This renewed my suspicion of a faulty RPM sensor circuit, as I understand the injectors are pulsed once per crankshaft revolution by the RPM sensor.

Tomorrow morning, I'm going to further investigate the fuel injection circuit, and check for high resistance between the ignition control module and DI power stage.

Does anyone have any other theories to look into?

Post by **Ozark Lee** » 19 Dec 2011, 16:57

The problem with the DTCs for cam and crank sensors is that the car has to be running for them to set a code. If it gets no signal the car won't run. It really only throws those DTCs if the signal gets intermittent.

Check for battery voltage on the green fuel injector wires to ground with the key in position II, you might have a problem with the master fuel system relay or the wiring to its connector.

...Lee

socket_wrench · Post by **socket_wrench** » 19 Dec 2011, 18:01

Hmm. Is it possibly that an intermittent signal might prevent a start but not throw a DTC?

I verified that the fuel injectors are getting battery voltage - the ground just isn't getting switched on by the ECU. I'm going to double check this tomorrow, it was the last test I did, and it was a little dark out. May not have had a good connection.

Thanks for your response.

socket_wrench · Post by **socket_wrench** » 20 Dec 2011, 12:27

I've verified that the cam position sensor and RPM (crank) sensor both work. The fuel injectors are not pulsing, but I'm under the impression that they are disabled when the computer detects a no-spark condition, so this is probably a red herring. Other than the cam and crank sensors, what could prevent spark during starting, but allow the car to run normally once started?

Post by **taxi** » 20 Dec 2011, 14:15

A bad battery or a bad starter. Both can cause the voltage to drop under 10 v during cranking and disable ignition. I dont think this would generate codes either. Please correct me if needed?

socket_wrench · Post by **socket_wrench** » 20 Dec 2011, 15:26

I checked the battery with a voltmeter while cranking, voltage stayed above 10 volts for 15 seconds (which should indicate a good battery). So it's probably not the battery or the starter motor. Nice theory, though.

I've observed that the tachometer doesn't move while cranking - might this indicate a faulty RPM sensor/circuit? However, the tachometer operates correctly after the car has started. I've verified that the RPM sensor generates the specified 0.3 volts AC while cranking, and has a resistance of 300 ohms, but without an oscilloscope, I feel like I can't rule it out. Thoughts?

Post by **taxi** » 21 Dec 2011, 01:13

How about the main relay, the grey one, on top of the radiator?

socket_wrench · Post by **socket_wrench** » 21 Dec 2011, 11:56

The main relay's good.

I borrowed my father's 850 (same year, but with a turbo) to compare. I noticed my Volvo's cranking very slowly. I initially assumed the battery was run down from so much cranking, but after attaching jumper cables to my father's running Volvo, it still sounds sluggish.

This made me think back to taxi's earlier comment - that my problem may be caused by a bad starter. If the engine isn't spinning fast enough, I believe the ICM may not deliver an ignition pulse.

On my way to buy an inductive ammeter to test the starter draw, will post results.

socket_wrench · Post by **socket_wrench** » 30 Dec 2011, 10:16

Update: Still no progress

To summarize: My '94 850 will crank, but not start. For a while, this problem was intermittent, and once it started it would run perfectly. However, now it's in my driveway and won't start.

The DI power stage/ignition coil module is not receiving a pulse from the ICM. The fuel injectors have power, but aren't getting pulsed (grounded) by the MFI.

There are no DTC's. Disconnecting either the cam sensor or RPM sensor while cranking trips a code, and I've tested the output of both sensors, effectively ruling them out.

I noticed the engine was cranking slowly, so I replaced the starter and fully charged the battery. The engine cranks normally now, but still no spark.

I've ruled out the cam and RPM sensors, and main relay. What could cause an intermittent no spark/no fuel condition while cranking, but allow the car to run normally once started, without tripping any codes?

Thanks for everyone's input.

Grahik · Post by **Grahik** » 09 Jan 2012, 22:41

Hi,

Does the ignition ECU acknowledge the engine is cranking?

To perform diagnosis, turn ignition on. Ensure function
selector cable is in diagnostic unit "A", socket No. 6.
Diagnostic Test Mode No. 2 (Function Testing)
Press test button 2 times to activate test mode No. 2. LED
should flash rapidly. ECU should deliver an acknowledgment code when a
signal is received from the following:
* 1-4-1 - VSS (as flywheel rotates).
* 3-4-2 - CMP (as camshaft rotates).

I think i might have the same condition on a '92, B5254S & LH3.2+EZK.
The ignition ecu is not seeing the engine turning while cranking with the starter. However, if i turn the engine manually ie. from the crankshaft pulley via a wrench, the ecu gives out the above codes.

I verified the sensors' signals are fine even when starting, using an oscilloscope at the ignition ecu connector.

In other words, the engine starts right away when it's towed or by doing a hill start, but not while cranking the starter. The problem began as intermittent and got worse.

Comments?