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Old 03-18-2020, 01:14 AM   #1
tommyt
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Default Car wonít start -PCM Bad?

I have a 96 Saturn SC2. It always starts up immediately. The other day it didnít start, just kept turning over. I checked the fuses, crank sensor, fuel injectors, and fuel pump. All appear to be working. Even replaced the CPS because it was old. The problem I found was there is no spark. I replaced the ignition module even though it was only a year old. No dice. I checked that the wires from the CPS to the computer have continuity. All good. Resistance is around 760 ohms so thatís good. Tried to check the bolts at the CPS while cranking. Only got around 130 mV but up to 170 for a moment. Should be 200 or more but I donít think Iím getting a good connection. I did get over 200 mV when I manually waved a piece of metal over the sensor. The old one and new one give me the same results so I think that is not the problem (plus Iím getting voltage at the injectors). I am running out of ideas other than the computer. As an important side note, I did some welding on the car recently but I disconnected the battery ground. Could the computer have gotten kinda fried from welding but decide not to quit until a week of driving?

Thanks for your help!
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Old 03-18-2020, 08:04 AM   #2
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Default Re: Car wonít start -PCM Bad?

How many miles on your set of copper-conductor spark plugs and the wire set itself?
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Old 03-18-2020, 08:35 AM   #3
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Default Re: Car wonít start -PCM Bad?

While you may have gone over many points, there's a procedure to diagnose a no start problem with some specific steps that can reveal which way to proceed.

1-Pull spark plugs after several failed starting attempts. In normal starting, cold engines are automatically fed extra fuel. Either spark or fuel failed when cranking the engine doesn't result in starting up. Pulling plugs should reveal wet smelling ones. Once pulled, a spark test can be performed.

2-A fuel pressure gauge can help determine if the pump is running and pressurizing. Without it, remove the black plastic test valve cap covering the fuel test valve. Depress the valve stem when someone turns ignition on. When ignition is turned on the pcm cycles the fuel pump for a few seconds then shuts off if the engine isn't started. Pressure should build up immediately and spray fuel out the test valve.

Test for fuel and spark. Both have to occur for engines to run.
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Old 03-18-2020, 01:02 PM   #4
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Default Re: Car wonít start -PCM Bad?

Use a 'noid light or voltmeter to check for injector pulses while cranking. if you get pulses while cranking, then you must be getting a good CKP signal and you can quit pursuing that. Checking for wet plugs is not definitive for this purpose, they can be wet from the priming pulse alone.

If you pull the plugs, for any reason, then check compression; on one cylinder at least.

Check fuel pressure with a gauge. Again, looking for a spurt from the test port or for wet plugs is not definitive for ensuring pressure is OK.

Use live-data to check that all sensors (like ECT, MAP, TP) are reading reasonable. ELM327-type devices are very inexpensive for doing this, but live-data can be had using just a voltmeter; but it is a tedious and error-prone process.

Disable fueling, pull the injector fuse(s), and try firing it with starting-fluid. Make sure the plugs aren't already wet when you do this so, we know the engine isn't flooded. Just letting the engine sit for several hours is good enough if you don't want to pull the plugs for inspection and airing out the cylinders.

The PCM? Where did you attach the grounding clip, relative to the weld zone? If the clip was attached to the same piece you were welding, then I doubt that could cause a problem. However, check the 5V supply from the PCM and see if that comes up instantly at key-on. That 5V supply can be easily checked at the harness connector for the MAP sensor or TP sensor or EGR valve; whichever is easiest for you. I suggest that because I had the PCM (in a '94) go bad, and that was the main clue that it was bad; the 5V would come up slow (1-2 seconds) and not always to the full 5V.

Those suggestions are in no particular order, do them according to your skills and the equipment that you need to acquire.
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Old 03-18-2020, 01:11 PM   #5
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Default Re: Car wonít start -PCM Bad?

I'm not aware of any priming pulse. Can you point to service manual explanations of this? Injectors operate from pcm pulses as long as the pcm is turned on by crank sensor signals. Fuel is injected only from injectors getting signals from the pcm. One injector fuse supplies power to injectors.
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Old 03-18-2020, 01:44 PM   #6
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Default Re: Car wonít start -PCM Bad?

I need to clarify: No, I don't know for sure that there is a priming pulse for the injectors at key-on, I assumed there is, since there is a 2-sec run of the pump and there is little reason to do that if the injectors are not also pulsed for priming. Running the pump to pressurize the rail is not necessary if the fuel system is in good condition; why bother to do that and not prime???

Let's not debate the usefulness of that "pump run", it won't help the OP here.

My point, I think still valid, is that the plugs can get wet from reasons other than the injectors being pulsed; like leaking injectors or wiring shorts or faulty PCM. We want to be sure there are pulses indicating the CKP sensor circuit is probably working; and "wet plugs" does not prove that. A 'noid light might even show if the pulses are consistent or sporadic, so would be a better choice than a voltmeter for checking.
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Old 03-18-2020, 02:31 PM   #7
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Default Re: Car wonít start -PCM Bad?

Injectors are the only way to get fuel into cylinders. Fuel pump priming is to pressurize the fuel rail whether the engine is started or not. Safety was considered in every EFI system since operating pressures exceed 30 psi with some operating at 60 psi. Imagine a fuel pump continually running if ignition is turned on but the engine never started and a rupture occurs anywhere in the fuel line? Priming the fuel rail for 2-3 seconds then shutting off if the engine isn't started ensures against fire hazards. As a rule, there have been little to zero reports of EFI system fires from this safety design. Technically, the EFI system doesn't operate injectors until the crank sensor generates the timing signals the pcm needs to operate injectors and spark. The pcm turns on the fuel pump too.

By removing spark plugs after repeated starting attempts, all plugs should be wet with fuel. This verifies whether or not injectors and fuel pump are operating. Fuel on plugs also verify crank sensor operation since injectors cannot operate unless the crank sensor is outputting signals to the pcm. Crank sensor timing signals are the electronic clock allowing the pcm to operate the entire EFI system. Spark is generated from ignition control module but its controlled by the pcm.
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Old 03-18-2020, 02:44 PM   #8
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Default Re: Car wonít start -PCM Bad?

Injectors are the only way to get fuel into cylinders. Fuel pump priming is to pressurize the fuel rail whether the engine is started or not. Safety was considered in every EFI system since operating pressures exceed 30 psi with some operating at 60 psi. Imagine a fuel pump continually running if ignition is turned on but the engine never started and a rupture occurs anywhere in the fuel line? Priming the fuel rail for 2-3 seconds then shutting off if the engine isn't started ensures against fire hazards. As a rule, there have been little to zero reports of EFI system fires from this safety design. Technically, the EFI system doesn't operate injectors until the crank sensor generates the timing signals the pcm needs to operate injectors and spark. The pcm turns on the fuel pump too.

If you are thinking of fuel priming, the EFI system knows it must inject more fuel, a rich mixture for cold engines based on the engine coolant sensor. A cold engine needs a rich mixture so the coolant sensor sending cold engine temps informs the pcm. The pcm uses its fuel map to increase injector pulse time to keep an injector open a little longer resulting in a richer mixture during each injection pulse. The rich starting mixture is the EFI system equivalent of carburetors with a thermostatically controlled choke and cam mechanism. With EFI systems, less fuel is wasted as fine tuning of fuel mixtures occurs in the wide engine operating temperature range. No choke mechanism, springs, fuel accelerator pump, vapor locking, dashpots, etc. Comparing carburetors to EFI systems, EFI is far more reliable and trouble free since everyone is accustomed to instant starting without touching the gas pedal. More complicated with electronics and sensors but emissions controls requires EFI systems and catcons for less polluting vehicles.

By removing spark plugs after repeated starting attempts, all plugs should be wet with fuel. This verifies whether or not injectors and fuel pump are operating. Fuel on plugs also verify crank sensor operation since injectors cannot operate unless the crank sensor is outputting signals to the pcm. Crank sensor timing signals are the electronic clock allowing the pcm to operate the entire EFI system. Spark is generated from ignition control module but its controlled by the pcm.
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Old 03-18-2020, 10:18 PM   #9
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Heart Re: Car wonít start -PCM Bad?

Hey...Read the OP again Guys... "The problem I found was there is no spark." NO SPARK...
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Old 03-19-2020, 12:10 AM   #10
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Default Re: Car wonít start -PCM Bad?

Yeah, my apologies for helping this thread "out into the weeds". Checking compression, fuel pressure, or using starting-fluid are not going to be of any use if there is clearly no spark; and live-data will be of limited use.

However, checking the 5V supply and checking for injector pulses are still valid to determine why there is no spark
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Old 03-19-2020, 01:31 PM   #11
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Default Re: Car wonít start -PCM Bad?

Thanks for the replies, guys. I typed up a reply and it didn't post. I will try again....

As stated, my main issue is no spark. I verified that at the coils with a small piece of wire between the posts and using a spark tester.

I am getting voltage to the fuel injectors. I checked with a test light. Looked really consistent during cranking. Therefore the crank sensor must be good since that will prohibit the fuel injectors from firing if it's bad (as well as the prohibit the spark, of course).

I also checked the crank sensor resistance (765 ohms) and voltage output (130 mV). Resistance was good. Voltage output was a little low but I think I had a bad connection when I back-probed the connector at the computer.

I have 5 volts at the ignition module at the brown/white stripe wire. I have 53 mV at the white wire and I have 1000 ohms at the white wire. I don't have an electrical schematic for the ignition module in the Haynes manual. Just the 95 and older ignition module, which has more wires and different colors.

The coils have 11,400 ohms on the secondaries.

The plug wires have 12,000 ohms. They are not very old.

Plugs are in good shape. Replaced within the last year or 2.

I had no stumble or signs of life while cranking, even with starter fluid. That would also indicate no spark.

I did not check fuel pressure, but I know it's pretty good because I loosened up a hose clamp and gas squirted out across the engine bay.

I did not check compression but even with low compression it would probably stumble or even start. Last year I broke 3 piston rings causing low compression. The car ran perfectly fine except for a small stumble at idle and a lot of blowby. I rebuilt the engine at that time. The car ran great for months, then I stopped driving it to work on the exhaust and fix rust. I had it in my garage all winter. I finished that up and drove it for 3 weeks. No issues at all.

When I was welding on the exhaust, I had the ground clamp on the exhaust. I was mostly working on the catback area. I did have the downpipe and header attached to the catback so there was a connection to the engine through the exhaust. I had the negative on the battery removed. Not sure if that was enough, though.

So... I have no spark. New crank sensor and new ignition module. Do you think it's the PCM?
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Old 03-19-2020, 03:30 PM   #12
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Default Re: Car wonít start -PCM Bad?

The welding with the ground back there would cause no problems.

Check that 5V from the PCM (at MAP or other). It is quick and easy to do, and if that 5V is flaky you know the PCM has no chance to work correctly. Again, that 5V should come up instantly at key-on, if it takes a couple of seconds, or doesn't even come all the way up to 5V (+/- .2V), then the PCM is having troubles. Maybe the PCM or maybe the wiring to it.

PS: it sounds like you have pulses at the injectors, not just "power" to them; is that correct? There will be 12V power to them all the time (the key is on), regardless of what the PCM or CKP sensor are doing.

PPS: I read again, and see you have 5V at the ICM, but it isn't clear to me if that 5V is coming from the same PCM supply; I would check at one of those other places too. More important, just having 5V is not good enough, the 5V has to come up promptly or else the PCM doesn't boot up correctly. That is what I found with my '94 and I expect your year might be similar...

Last edited by billr; 03-19-2020 at 03:41 PM.
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Old 03-19-2020, 06:08 PM   #13
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Default Re: Car wonít start -PCM Bad?

Wiring diagrams for '97's should help; http://www.saturnfans.com/forums/sho...control+module, post#4.

A good clue to determining if the pcm failed is if fuel isn't injected, fuel pump doesn't run or loss of spark. Presuming the new crank sensor is fine and sending signals to the pcm, fuel on plugs and spark should be seen. Removing spark plugs to check for wetness tells you if the pcm provided injector pulses. Injectors are powered up at ignition on time. The pcm switches ground on/off to each injector. Unless you use a noid light, you cannot tell if injectors are operating. Removing plugs tells you if injectors are operating or not.

Loss of spark can be; failure of one or both coils, failure of the ignition module, break in wiring or faulty pcm. Loss of spark may be a faulty pcm if all other parts and wiring are checked and in working condition.

Again, remove spark plugs and examine them for fuel. They should wreak of fuel which implies injectors are operating from commands from the pcm.

Despite what anyone says, unless you pull plugs to examine them for fuel, you don't know if injectors are operating. You verified fuel pump and pressure from descriptions but haven't verified injectors with wet plugs yet.

Loss of spark and/or loss of injectors to allow fuel into cylinders are hints of a problem. Parts replacement hasn't proven which way to proceed.

The EFI system in a nutshell; turning the engine over allows the crank sensor to output timing signals to the pcm, turning it on. The pcm using timing signals turns on the fuel pump, supplies injector pulses to each injector and provides timing signals to the ignition control module that generates spark.

Some years back, throwing parts at a problem would be be expensive. Saturn parts are dwindling and with fewer on the road, parts costs may be lower with less demand. Your choice to shop around for ign control module, coils, pcm, etc.
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Old 03-21-2020, 04:20 PM   #14
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Default Re: Car wonít start -PCM Bad?

I haven't seen mention that the OP checked the pcm/02 ground on the back of the engine block. I would also double check the EIS fuse. There are already very capable people helping here and the best start for the OP is probably to look at the first image in the thread that fdryer linked to see the wiring. I had actually already pulled my 97 fsm.

OP, do you have a oscope or know someone that does? It should not be a problem running down why you have no spark with one.

To know if the pcm is commanding spark: 1/4 coil wire is the white wire at the 5 way icm connector. 2/3 signal wire is the orange wire. The signal wires will not read steady 5v as you've seen. The brown/white wire with voltage you read is the cylinder 4 signal out.

I don't want to muddy the waters by chiming in but felt that the ground check should have been the first thing after a fuse check when no spark was seen at the towers when cranking. I apologize if it was done and I somehow missed it.
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Old 03-21-2020, 06:21 PM   #15
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Default Re: Car wonít start -PCM Bad?

What was your voltage on the pink wire?
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Old 03-21-2020, 08:24 PM   #16
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Default Re: Car wonít start -PCM Bad?

And I just reread your post. You don't check for spark between coil posts with a wire. You check for spark by cranking between posts with no wire.

I also meant to say that not always will you see 5v on a signal wire when referencing the orange, white and brown/white wires.
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Old 03-28-2020, 12:28 AM   #17
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Default Re: Car wonít start -PCM Bad?

Thanks for all your help, guys. I did finally figure it out. Of course I bought parts I didnít need including a used pcm. The wiring diagram is what helped me figure it out. Looking that over, I kept seeing the ignition control module referred to as the EIS. I wasnít familiar with that abbreviation, so I never tried pulling that fuse when I was looking for blown fuses. I went back to my fuse block and found the fuse labelled EIS and pulled it out. SOB was blown. I felt like an idiot but relieved I knew what the problem was. Didnít take to long to get the gas out of the engine and get it fired up. I always try the easy stuff first but I missed that one fuse and went on a heck of a goose chase. Thanks everyone for your help. You went above and beyond. My backup ignition parts, CPS, and pcm might come in handy some day when I troubleshooting a new reason the car wonít start.
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Old 03-29-2020, 04:49 PM   #18
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Default Re: Car wonít start -PCM Bad?

Personally I despise acronyms and abbreviations even though I know there's a reason for them and when they're used. EIS - electronic ignition system.

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