Optispark Troubleshooting Question
Thread Starter
8th Gear
Joined: Aug 2015
Posts: 8
Likes: 0
Optispark Troubleshooting Question
Hi everyone, first post.
I am troubleshooting an intermittent no spark condition on my 94 lt1 corvette. With the optispark connector disconnected, when I check the ground on the pcm side of the optispark connector it reads 1200 ohms. Is this a problem?
This came up because I put a makeshift scope on the low resolution output and it seems to be putting out gibberish. I just want to make sure I'm not getting an incorrect reading.
I am troubleshooting an intermittent no spark condition on my 94 lt1 corvette. With the optispark connector disconnected, when I check the ground on the pcm side of the optispark connector it reads 1200 ohms. Is this a problem?
This came up because I put a makeshift scope on the low resolution output and it seems to be putting out gibberish. I just want to make sure I'm not getting an incorrect reading.
Le Mans Master
Joined: Jan 2006
Posts: 5,620
Likes: 206
From: Orlando FL
You need to state exactly what pin and wire color. Be careful reading back to the PCM as it is a TTL output and you don't want to damage the PCM.
However your troubleshooting procedures I don't feel is going to resolve or find the problem. You have good effort and intentions but if you are going to work on you car, you really need factory manuals. They provide specific information for your year car and troubleshooting information and procedures.
There is not a great deal of information that you provided in your post. I will say the ICM is a known problem with ignition and spark failure. Sometimes replacement is the best way of verifying the problem as testing that unit is not really reliable for a come and go symptom.
The opti is a possible cause of the problem but that's just information as a possibility and not to run to it at this time.
Learn how to pull codes on the car. No special tools needed and there is lots of information on how to do it. It's a great system and many times can give you a clue as to what area to look in.
However your troubleshooting procedures I don't feel is going to resolve or find the problem. You have good effort and intentions but if you are going to work on you car, you really need factory manuals. They provide specific information for your year car and troubleshooting information and procedures.
There is not a great deal of information that you provided in your post. I will say the ICM is a known problem with ignition and spark failure. Sometimes replacement is the best way of verifying the problem as testing that unit is not really reliable for a come and go symptom.
The opti is a possible cause of the problem but that's just information as a possibility and not to run to it at this time.
Learn how to pull codes on the car. No special tools needed and there is lots of information on how to do it. It's a great system and many times can give you a clue as to what area to look in.
Thread Starter
8th Gear
Joined: Aug 2015
Posts: 8
Likes: 0
Thanks for the thoughtful reply! I can provide much more information.
I do have the factory service manual. I will reference the troubleshooting procedure. Also so you know where I am coming from, I am a systems engineer and do have some experience with this type of troubleshooting and with ttl logic, programming microcontrollers, etc.
I have checked the codes, none are set.
I have an intermittent no start/no spark problem. Sometimes it is fine, some times the spark appears weak or irregular, sometimes there is no spark. It also seems to be somewhat related to the ambient temperature in my garage.
My first post was referring to pin D on the optispark to pcm connector mounted on between the fuel injectors on the passenger side. This is the pink/black wire labeled "distributor reference low signal" in the fsm diagram. I have also seen this called the "ignition system test connector". When I measure the resistance from this pin on the pcm side of the connector to the chassis, it read around 1200 ohms.
I do have the factory service manual. I will reference the troubleshooting procedure. Also so you know where I am coming from, I am a systems engineer and do have some experience with this type of troubleshooting and with ttl logic, programming microcontrollers, etc.
I have checked the codes, none are set.
I have an intermittent no start/no spark problem. Sometimes it is fine, some times the spark appears weak or irregular, sometimes there is no spark. It also seems to be somewhat related to the ambient temperature in my garage.
My first post was referring to pin D on the optispark to pcm connector mounted on between the fuel injectors on the passenger side. This is the pink/black wire labeled "distributor reference low signal" in the fsm diagram. I have also seen this called the "ignition system test connector". When I measure the resistance from this pin on the pcm side of the connector to the chassis, it read around 1200 ohms.
Thread Starter
8th Gear
Joined: Aug 2015
Posts: 8
Likes: 0
I have tested the coil resistances across the primary and from the primary to secondary, and power to the coil. These are fine. However I understand that this is not a complete test of the coil, especially without any measurement of inductance.
I have also verified power to the ICM on terminals A and D on the ICM connector.
When I attempted to record the low resolution optispark signal, I was using a National Instruments high speed data acquisition device interfaced to some PC software I wrote. This allows logging of the data at 10k Hz. Good point about the internal resistance of the measurement device. I will be sure to use only devices with greater than 10 mega ohm internal impedance as mentioned in the fsm.
I have also verified power to the ICM on terminals A and D on the ICM connector.
When I attempted to record the low resolution optispark signal, I was using a National Instruments high speed data acquisition device interfaced to some PC software I wrote. This allows logging of the data at 10k Hz. Good point about the internal resistance of the measurement device. I will be sure to use only devices with greater than 10 mega ohm internal impedance as mentioned in the fsm.
Thread Starter
8th Gear
Joined: Aug 2015
Posts: 8
Likes: 0
Also, I realize that the fsm schematic is likely simplified where it shows pin D on the optispark to pcm connector connected directly to ground, and that it may have been a bad idea to put an ohm meter on it.
I will post the results from the fsm troubleshooting procedure as soon as I find it.
I will post the results from the fsm troubleshooting procedure as soon as I find it.
Thread Starter
8th Gear
Joined: Aug 2015
Posts: 8
Likes: 0
Thanks to pcolt94 for telling me to RTFM! After cranking for 15s, code 41 set. Then tested for pulse on terminal b of ICM connector from PCM, the pulse was there. Looks like the optispark is ok! The fsm tells me the problem is either the ICM or ICM wiring.
Le Mans Master
Joined: May 2005
Posts: 7,334
Likes: 2,260
From: Central Maryland
If the optical section of the unit is misbehaving, it should set a code, so you probably don't need to get too fancy with measurements of those signals.
If the distributor section of the unit is misbehaving, you'll likely have some misfiring/crossfiring, etc, just as you would with a failing rotor or cap on a conventional distributor.
Live well,
SJW
If the distributor section of the unit is misbehaving, you'll likely have some misfiring/crossfiring, etc, just as you would with a failing rotor or cap on a conventional distributor.
Live well,
SJW
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