1987 911 starting problem

[a911ebayer930]

I have a 1987 911 with an intermittent starting problem. I replaced the DME module which is control by the (electrical schematic) CONTROL SW module. Question: Is the Control SW module the same as an AND gate device. AND gate device requires ALL the inputs to be HIGH or have 13 Volts on ALL the inputs to turn ON the output pin 87a in reference to ground (pin 15)?

[DRACO A5OG]

I am sorry, never seen a question like this.

So when you wrote "module" do you mean the ECU (metal Motronic Box) or the RELAY (arrowed)?

[ischmitz]

The DME (also called engine control unit) is the large silver box that is shown above. Logically, it acts as an OR. Either it needs to see the flywheel moving (with both sensors) or it needs to see the starter being engaged (key in START) to turn on the 2nd stage of the DME relays (the little black box).

That box has two relays combined and these are daisy-chained. The first stage turns on when the key is in RUN. And the first stage is a pre-requisite for the 2nd stage to be turned on. So you could say the 2nd stage is a logical AND. Once the DME determines the flywheel is moving fast enough (>200 RPM) or the starter is engaged it supplies the other input to the 2nd stage. And that turns the fuel pump on.

If you have issues starting the engine you need to determine what is missing: Fuel (does the fuel pump run), spark or both?

That will help narrow it down to whether it's the DME, the DME relay, one of the sensors, or the DME itself. Do some search here. Lot's of good material to read up about this. Good luck,

Ingo

[a911ebayer930]

Thanks for the explanation DRACO and ischmitz. However, I replaced the DME module with no luck. Therefore, based on the attached drawing it seem to be the problem. If anyone is aware of the location of this module please let me know. Once again this is for a 1987 911.

[steely]

That is your Alarm control unit.
Location could be anywhere - try under the passenger seat, in the console, behind the glove box.
If it is the culprit, you could delete it - quite a few threads here on how to do that, but do check to see if you even have one.
If you could, please look at Draco's picture and using his terminology, tell us what you think the DME module is.

[a911ebayer930]

Thanks guys for your assistance. How can I determine if this unit is defective or still in working order?
Should pin:2(61s),T indicate 13 volts in reference to ground when I try to start my 911?

[DRACO A5OG]

Oh I see, I believe that is the factory alarm system, if the switch above grounds out yes, you will not be able to start the car as it will disable the fuel pump.

Check the wiring on the key switch in driver side door, sometimes the ground wire dislodged and hits the window guides or door handle trigger and frays then grounds out and kills the car.

[steely]

Originally Posted by a911ebayer930

... How can I determine if this unit is defective or still in working order?
Should pin:2(61s),T indicate 13 volts in reference to ground when I try to start my 911?

Sorry I can't answer your questions in detail, but you could try to determine if it was the cause by disconnecting it and jumpering the alarm control connector per the instructions in this thread Alarm Bypass - help please

This is just a suggestion as I have no first hand experience with the Alarm.
Please let us know how you make out if you try it.

[a911ebayer930]

Once again. Major thanks for all the help. I will keep you posted on my findings.

Regards

[86 911 Targa]

^^^^^
If the failure mode is in the alarm module, bypassing it is "easy-peasy.

But, accessing the module is another story.

The module is located behind the fresh air blower motor assy
which must be removed.

I just replaced mine.

I'll try to find more pics.

pm me as needed.

Best,

Gerry

[DRACO A5OG]

^^^ Nice Doc G

I hate using my factory Alarm.

[86 911 Targa]

Originally Posted by a911ebayer930

Thanks for the explanation DRACO and ischmitz. However, I replaced the DME module with no luck. Therefore, based on the attached drawing it seem to be the problem. If anyone is aware of the location of this module please let me know. Once again this is for a 1987 911.

^^^^^
As you can see, there are two resistors in the alarm key CKT.

R1 is always engaged providing a resistive ground to pin # 31.

When the alarm goes "active" R2 resistor comes into play.

The reason for the R1 value is to prevent the "Bad guys" from just shorting
the circuit and "borrowing" your car.

Also, I suspect that shorting pin #31 might "smoke" the IC.

If the R1 circuit is open, your car will not start.




^^^^^
I don't think the Alarm module is your issue.

But, just in case, here are a few pics.

Good luck,

Gerry

[DRACO A5OG]

Great now the thieves will know how to by pass :-(

[steely]

not before the alarm goes off :-)

[DRACO A5OG]

lol

[86 911 Targa]

Originally Posted by DRACO A5OG

Great now the thieves will know how to by pass :-(

The resistor values are sensitive for circuit performance.

And, even by-passing the module would require:

1. Access to the alarm lock-which would take some time,
while the horn is active.

2. The ability to start the car.

Gerry

[86 911 Targa]

Originally Posted by steely

not before the alarm goes off :-)

I agree.

The best approach is an unrelated back-up kill switch.

From some of the previous theft stories, I don't
think that the factory alarm is much of a deterrent to the bad guys.

Gerry

[DRACO A5OG]

DME Relay removal

Sorry OP, off topic.

Check the ohms on your Ref/Speed sensor and CHTS.

[steely]

Originally Posted by DRACO A5OG

DME Relay removal

+1

Originally Posted by DRACO A5OG

Check the ohms on your Ref/Speed sensor and CHTS.

agreed

[a911ebayer930]

Located my already removed jumped out alarm module (911.637.104.02) socket. Applied electrical contact cleaner on terminal connector. I measured the resistance (ohms) between the battery negative terminals to the front driver side grounded cable and found a -.5 ohms (negative .5 ohms). (Resistance MUST always be a positive number) this meant there’s a voltage drop between my negative battery terminal to my chassis ground.
Started the car and listened the stereo while rev the engine and heard a variable frequency noise been generated by the alternator from the speakers. This confirmed that my problem could be caused by a defective chassis ground problem. I found the chassis ground located in the driver side language compartment on corroded.
I’m 99% certain that my problem is resolved after started the car over 50 times and added 70 miles the on the Speedo. ONCE AGAIN MAJOR THANKS TO ALL THAT PROVIDED THEIR INFO ON THIS MATTER.
If the problem reoccurred, I will repost this issue.