How to Perform the "Jump the SCS Line With the HDS" Procedure Without the HDS
I recently did some troubleshooting of my radiator/condenser fan circuits using the Factory Service Manual. If you have the Factory Service Manual then you may have noticed that when you are troubleshooting an electrical problem that involves a connection to the PCM, the procedure will often have you check wiring between the component you're testing and the PCM. To do this you need to unplug the connectors to the PCM. When that is required the procedure always says to "Jump the SCS line with the HDS." It says that this step is required to prevent damage to the PCM when disconnecting. Even when the key is turned to LOCK (0) the PCM continues to monitor the EVAP system for up to 30 minutes. Honda says that if you disconnect a harness from the PCM during this window you risk damaging the PCM. To prevent this they tell you to jump the SCS line with the HDS. This is done after you turn the key to the LOCK (0) position. They instruct you to use the HDS to jump the SCS line and then remove your PCM connectors and do your testing. Once you're done, you reconnect the PCM and then un-jump the SCS.
Well, obviously most of us don't have the HDS bidirectional scan tool for doing this. After some Internet research it's clear to me that all this procedure does is short pin #9 (SCS line) to pin #4 (chassis ground) on the data link connector (DLC.) There are multiple ways you could do this. You can use a wire and hope you don't damage the the computer by connecting to the wrong pins, and/or if connecting to the front of the connector, hope you don't damage the connector. When diagnosing electrical problems, front probing a connector is always a big no-no due to possible damage to the connectors preventing the connector from making contact when you're done. If you put your wire on the back of the terminal (called backprobing) you still risk shorting the wrong pins, or the wire could fall out while you're conducting your test.
So, in order to make an easy way to do this while troubleshooting I decided to make myself a custom SCS Line Jump connector. For less than $6 shipped I bought a cheap DLC connector on eBay:
Link to pinouts for Honda DLC:
Honda OBD II diagnostic interface pinout diagram @ pinoutsguide.com
As you can see below, I simply soldered a jumper wire from pin 4 to pin 9 on the inside of the plug. So, for less than $6, and less than 10 minutes of time, I have an SCS line jumper connector to put the computer into test (apparently that's what jumping the SCS does) so that removing a PCM connector doesn't damage the PCM. If any of you watch the South Main Auto channel on YouTube, I asked Eric O (one of the top electrical diagnostic guys, and overall mechanic, on YouTube) if he ever did this in his diagnosing and he said no, and that if he needs to pull a connector he just pulls it and has never had an issue. Though, full disclosure, he always says the big three, Chrysler, GM, and Ford, are what keep him in the auto repair business--the Hondas, Toyotas, and Nissans just don't break as often. Anyway, there's probably only one very obscure instance where a PCM was damaged by pulling a connector and that led to putting the procedure in the book. However, for not even $6 it's cheap insurance if you find yourself needing to unplug the PCM during diagnostics. I simply tossed it in the bag with my scan tool and it will be ready the next time I hit that "Jump the SCS Line with the HDS" hurdle in the book.
Some pics:
I recently did some troubleshooting of my radiator/condenser fan circuits using the Factory Service Manual. If you have the Factory Service Manual then you may have noticed that when you are troubleshooting an electrical problem that involves a connection to the PCM, the procedure will often have you check wiring between the component you're testing and the PCM. To do this you need to unplug the connectors to the PCM. When that is required the procedure always says to "Jump the SCS line with the HDS." It says that this step is required to prevent damage to the PCM when disconnecting. Even when the key is turned to LOCK (0) the PCM continues to monitor the EVAP system for up to 30 minutes. Honda says that if you disconnect a harness from the PCM during this window you risk damaging the PCM. To prevent this they tell you to jump the SCS line with the HDS. This is done after you turn the key to the LOCK (0) position. They instruct you to use the HDS to jump the SCS line and then remove your PCM connectors and do your testing. Once you're done, you reconnect the PCM and then un-jump the SCS.
Well, obviously most of us don't have the HDS bidirectional scan tool for doing this. After some Internet research it's clear to me that all this procedure does is short pin #9 (SCS line) to pin #4 (chassis ground) on the data link connector (DLC.) There are multiple ways you could do this. You can use a wire and hope you don't damage the the computer by connecting to the wrong pins, and/or if connecting to the front of the connector, hope you don't damage the connector. When diagnosing electrical problems, front probing a connector is always a big no-no due to possible damage to the connectors preventing the connector from making contact when you're done. If you put your wire on the back of the terminal (called backprobing) you still risk shorting the wrong pins, or the wire could fall out while you're conducting your test.
So, in order to make an easy way to do this while troubleshooting I decided to make myself a custom SCS Line Jump connector. For less than $6 shipped I bought a cheap DLC connector on eBay:


Link to pinouts for Honda DLC:
Honda OBD II diagnostic interface pinout diagram @ pinoutsguide.com
As you can see below, I simply soldered a jumper wire from pin 4 to pin 9 on the inside of the plug. So, for less than $6, and less than 10 minutes of time, I have an SCS line jumper connector to put the computer into test (apparently that's what jumping the SCS does) so that removing a PCM connector doesn't damage the PCM. If any of you watch the South Main Auto channel on YouTube, I asked Eric O (one of the top electrical diagnostic guys, and overall mechanic, on YouTube) if he ever did this in his diagnosing and he said no, and that if he needs to pull a connector he just pulls it and has never had an issue. Though, full disclosure, he always says the big three, Chrysler, GM, and Ford, are what keep him in the auto repair business--the Hondas, Toyotas, and Nissans just don't break as often. Anyway, there's probably only one very obscure instance where a PCM was damaged by pulling a connector and that led to putting the procedure in the book. However, for not even $6 it's cheap insurance if you find yourself needing to unplug the PCM during diagnostics. I simply tossed it in the bag with my scan tool and it will be ready the next time I hit that "Jump the SCS Line with the HDS" hurdle in the book.
Some pics:





