87 911 wont start, low voltage supply to ECM

[prairiedawgshoo]

Hello All...I've been fighting with my 87 and getting her to start up...I finally learned how to use my multimeter and was going thru the diagnosing process for the ECM and ran across low voltage supply to the ECM...it's pins 18 and 35 which should show 12 volts, but I'm only getting 2.7 volts...I tested the battery and it is at 12.4 volts...what do I do now? all the other connectors show the correct resistance and continuity. Apparently the ECM was repaired by Specialty ECM repair shop by the previous owner in 2019 and they talked me thru the testing of the ECM.

I'm somewhat glad that I know what the problem is, but now what?

thanks!

Update! Oops...didn't read the instructions fully...I needed to have the ignition on and now there are 12.4 volts going thru the ECU, now I still don't know why she won't start...

[brianlay]

You need to follow regular no-start troubleshooting.
Check for spark
Check for fuel
Do a search…there are many no-start threads.
It was running fine and just wouldn’t start one day?

[mysocal911]

Originally Posted by prairiedawgshoo

Hello All...I've been fighting with my 87 and getting her to start up...I finally learned how to use my multimeter and was going thru the diagnosing process for the ECM and ran across low voltage supply to the ECM...it's pins 18 and 35 which should show 12 volts, but I'm only getting 2.7 volts...I tested the battery and it is at 12.4 volts...what do I do now? all the other connectors show the correct resistance and continuity. Apparently the ECM was repaired by Specialty ECM repair shop by the previous owner in 2019 and they talked me thru the testing of the ECM.

I'm somewhat glad that I know what the problem is, but now what?

thanks!

Update! Oops...didn't read the instructions fully...I needed to have the ignition on and now there are 12.4 volts going thru the ECU, now I still don't know why she won't start...

No-Start info; http://forums.pelicanparts.com/porsche-911-technical-forum/621262-911-3-2-no-start-troubleshooting.html

[prairiedawgshoo]

Thanks…I’m going thru the trouble shoot list and no luck so far…I just shipped the ECM to be checked…I hope it something there…Diego at the ecu repair shop told me that since the ecu has been previously repaired, some part of the board might be bad, which he says is a common fault…thanks for the suggestions

[mysocal911]

Originally Posted by prairiedawgshoo

Thanks…I’m going thru the trouble shoot list and no luck so far…I just shipped the ECM to be checked…I hope it something there…Diego at the ecu repair shop told me that since the ecu has been previously repaired, some part of the board might be bad, which he says is a common fault…thanks for the suggestions

Guessing wastes time & money. Some shops will always find a "problem" to repair.

[prairiedawgshoo]

Thanks for the info…got the ecu back…there were some cracked solder joints, but it passed their testing…I had them fix it and it started once when I re installed it and nothing after that…I did follow their diagnostic checklist and it might be the top dead sensor when I tested the pins for resistance…has anyone had trouble with theirs? Does this sensor going bad cause intermittent starting issues? Thanks for everyone’s help!

[mysocal911]

Originally Posted by prairiedawgshoo

Thanks for the info…got the ecu back…there were some cracked solder joints, but it passed their testing…I had them fix it and it started once when I re installed it and nothing after that…I did follow their diagnostic checklist and it might be the top dead sensor when I tested the pins for resistance…has anyone had trouble with theirs? Does this sensor going bad cause intermittent starting issues? Thanks for everyone’s help!

Sounds like you need to find a reliable ECU repair shop. Try the ECU repair shop noted in the post #3 link.

[prairiedawgshoo]

I'm sorry Dave...I think I mis-spoke...I tested the connectors on the car which determined the TDC sensor does not have any resistance, based on Specialized ECU repair shop's diagnostic checklist indicating the TDC sensor is bad. Do you have any experience with this sensor going bad? does it "come and go" causing an intermittent starting problem?

thanks for your time

Roger

[scarceller]

First, very simple test. Turn key to RUN but do not crank engine. With key in RUN does the idle control valve in the engine bay vibrate and hum? The ICV is right in front of the throttle body.

I repair these DMEs often and they all get road tested, not many repair shops can road test them.

[mysocal911]

Originally Posted by prairiedawgshoo

I'm sorry Dave...I think I mis-spoke...I tested the connectors on the car which determined the TDC sensor does not have any resistance, based on Specialized ECU repair shop's diagnostic checklist indicating the TDC sensor is bad. Do you have any experience with this sensor going bad? does it "come and go" causing an intermittent starting problem?

thanks for your time

Roger

The 3.2 911 sensors are a weak point in the system. The reference sensor (TDC) is only used for starting. Hopefully, you didn't pay your ECU "rebuilder" twice!

[prairiedawgshoo]

Thanks scarceller, I will plug everything back in tomorrow and try…I do know there is connectivity to the icv thru the wiring…I did replace the icv several months ago…I was assuming it still works…I do have a new dme installed

Thanks Dave….the previous owner got to pay the first time…unfortunately Specalized wouldn’t honor the warranty because of ownership change

[mysocal911]

Originally Posted by prairiedawgshoo

Thanks scarceller, I will plug everything back in tomorrow and try…I do know there is connectivity to the icv thru the wiring…I did replace the icv several months ago…I was assuming it still works…I do have a new dme installed

Thanks Dave….the previous owner got to pay the first time…unfortunately Specalized wouldn’t honor the warranty because of ownership change

Typical, heard that before! Again, next time consider the rebuilder referenced in post #3 link, typically less cost with a better warranty.

[prairiedawgshoo]

thanks Dave...noted!

and the ICV does work...also when I was checking for resistance to the sensors from the harness that goes into the ECU I swapped the crankshaft speed sensor into the spot for the reference sensor and got zero for the resistance...any ideas on what that's trying to tell me?

[mysocal911]

Originally Posted by prairiedawgshoo

thanks Dave...noted!

and the ICV does work...also when I was checking for resistance to the sensors from the harness that goes into the ECU I swapped the crankshaft speed sensor into the spot for the reference sensor and got zero for the resistance...any ideas on what that's trying to tell me?

Both sensors when disconnected should measure about 1K ohms between the signal pins, and the shield wire must not indicate any resistance to either signal wire pin.

[prairiedawgshoo]

thanks Dave...what does it mean when the no resistance is measured? the speed sensor points measure 1K, but the refernce TDC pin holes don't measure anything

[mysocal911]

Originally Posted by prairiedawgshoo

thanks Dave...what does it mean when the no resistance is measured? the speed sensor points measure 1K, but the refernce TDC pin holes don't measure anything

Both sensors are identical electrically, i.e. both should measure about 1K ohms.
You have one bad sensor! Replace both.

[prairiedawgshoo]

GOtcha...thanks Dave!...so when I swapped the connectors on top of the engine (I took out the suspected bad plug and put the "good" in the TDC plug, it read zero again...does that mean anything? Also, when I get the new sensors can I plug them in and test them at the ECU where they would show 1k ohms? thanks for your help!

[mysocal911]

Originally Posted by prairiedawgshoo

GOtcha...thanks Dave!...so when I swapped the connectors on top of the engine (I took out the suspected bad plug and put the "good" in the TDC plug, it read zero again...does that mean anything? Also, when I get the new sensors can I plug them in and test them at the ECU where they would show 1k ohms? thanks for your help!

Yes.

[prairiedawgshoo]

Thanks!