Hello Folks,
I was working on a 1999 Honda CRV this evening and had the scanner connected to view data parameters and codes. This particular vehicle had a random missfire aparantly since the replacement of the timing belt and other drive belts. I found a bulletin, and it's mentioned in the troubleshooter as well, that states a non-oem alternator belt can cause a random missfire code. (don't ask me how). Anyways, I thought I would scope the crank sensor at the front end of the crankshaft. It is a magnetic pulse generator. I connected the scope leads accross the sensor at the connector below the alternator by backprobing both terminals instead of connecting to the ckp+ and vehicle ground. Since I know this is an AC signal I did not pay any attention to the polarity of the leads. I couldn't get any signal at all?? I double checked my backprobes several times. I disconnected the sensor and measured its resistance and it was in spec. With the sensor disconnected, I checked the waveform again. This time it was perfect?? (The engine will run without this crank sensor). I then connected my backprobes to the back of the connector and reconnected the sensor. Again the signal was perfect this time? At first I couldn't figure out what I did wrong, thought maybe it was a connection issue or wiring problem.
I decided to scope the signals at the distributor and when I connected the scope to the ckp signal, the engine stalled instantly? I then realized I was connecting the negative scope lead to the ckp+ terminal. I was going to put the scope leads accross the sensor + and - and again since this is an AC generator the polarity of the scope leads should not matter. It appears that the ground though the scope and the ground through the scanner to the DLC are not isolated. When I disconnected the scanner data cable the short to ground disappeared.
This is the same problem the old vantage pro had if you connected the external 12 volt power supply to the vehicle you were testing. This will cause a variety of missdiagnosed sensors or circuits if you don't realize what you are doing. I don't recall the Modis having this problem.
Just thought I would mention this issue and hopefully help prevent someone else from making the same error.
I was working on a 1999 Honda CRV this evening and had the scanner connected to view data parameters and codes. This particular vehicle had a random missfire aparantly since the replacement of the timing belt and other drive belts. I found a bulletin, and it's mentioned in the troubleshooter as well, that states a non-oem alternator belt can cause a random missfire code. (don't ask me how). Anyways, I thought I would scope the crank sensor at the front end of the crankshaft. It is a magnetic pulse generator. I connected the scope leads accross the sensor at the connector below the alternator by backprobing both terminals instead of connecting to the ckp+ and vehicle ground. Since I know this is an AC signal I did not pay any attention to the polarity of the leads. I couldn't get any signal at all?? I double checked my backprobes several times. I disconnected the sensor and measured its resistance and it was in spec. With the sensor disconnected, I checked the waveform again. This time it was perfect?? (The engine will run without this crank sensor). I then connected my backprobes to the back of the connector and reconnected the sensor. Again the signal was perfect this time? At first I couldn't figure out what I did wrong, thought maybe it was a connection issue or wiring problem.
I decided to scope the signals at the distributor and when I connected the scope to the ckp signal, the engine stalled instantly? I then realized I was connecting the negative scope lead to the ckp+ terminal. I was going to put the scope leads accross the sensor + and - and again since this is an AC generator the polarity of the scope leads should not matter. It appears that the ground though the scope and the ground through the scanner to the DLC are not isolated. When I disconnected the scanner data cable the short to ground disappeared.
This is the same problem the old vantage pro had if you connected the external 12 volt power supply to the vehicle you were testing. This will cause a variety of missdiagnosed sensors or circuits if you don't realize what you are doing. I don't recall the Modis having this problem.
Just thought I would mention this issue and hopefully help prevent someone else from making the same error.
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