'84 911 no start (no spark & no pulse at coil)

[jporsche]

Hello,

I'm debugging a no start situation on my '84 911. I parked it for the winter and was unable to get it started after it sat for 6+ months. I've done this for a few winters without a starting incident (always fired right away). The problem seems to be with the lack of pulse at the coil. no spark at the plugs and I get a reading of 12V at the coil, but no sign of pulse when I turn it over. From what I've read it's likely one of the following:

-TDC sensor failure.
-DME failure.
-grounding issue.

I've read through a number of posts on how to debug the above, but I have a couple of questions relating to the specific suggestions.

#1. TDC sensor test(s)
What are the pinouts for the TDC sensor (3 pins)? I tried simply reading the resistance with the multimeter and I get between 1000 - 1100 ohms when I cross pins 2 & 3, all others read nothing on the multimeter. (this is on both the TDC & speed sensors) I haven't tried with the oscilloscope as I'm not sure how to setup the probe (e.g. what pins provide the 2.0V sine wave).

#2. DME failure
I pulled apart my DME and I don't see any damage...no shorts or water damage..etc. I'd like to put my DME into a friends '89 911 to see if his will start as I've read somewhere that the pin-outs are slightly different but that it *should* work and fit with the existing connector. Can anyone confirm if this is possible? If not, anyone with an '84 vintage willing to test out my DME OR swap there DME for mine as a test? I live in the Ottawa, Ontario, Canada area?

For reference, I've reviewed the following posts in detail (along with a few others from pelican forums & rennlist):
3.2 sensor question
Stranded! DME Ignition Guru Help needed

Thanks in advance,
J.

[initial]

Does anyone know if there is a pinout test harness for the DME that is available to debug these issues?

My Workshop Manual has a nice picture of a breakout for the DME connector, but I don't see anything available for purchase on this site..

[kidrock]

Jamie,

this may seem redundant, but I have to ask...have you swapped out your DME relay for known, good DME relay yet?? This very well may be the problem.

[jporsche]

I had a spare DME relay in my glove box....1st thing I tried.

[Groesbeck Hurricane]

Jamie,

What type of CDI Box do you have? The permasomethingoranother sometimes goes bad and you will receive zero anything beyond the box. Mine did this even though the CDI tests out as perfect.

DME was my second thought.

Also, is the battery good?

Yep, I know, general/simple questions. All I have right now.

[jporsche]

I get 12V at the coil and it cranks at a good speed...just won't fire. Tried with my vehicle battery booster as well and no difference.

I'd be willing to buy a breakout box for the DME if ones available on pelican...

[jporsche]

found one via baum tools and they want $519.00.

The following post provides some breakout information if people are interested:
DME breakout box - where can I get one?

I'll be testing the CHT sensor tonight, skipped it in my last test as I've seen mixed comments about it causing a no-start situation. I'm currently running out of ideas outside of the DME & bad grounds causing the no pulse at the coil.

If anyone in the area is willing to swap out there DME as a test I'd appreciate it...

Thx.
J.

[techman1]

Does your car have any type of car alarm?

[jporsche]

no alarm or ignition shutoff switch.

[techman1]

Any mice problems? Thinking of a chewed through wire.

Did you do any winter projects on the car? I agree, seems like a pickup for the ignition.

Ignition...checked the cap and rotor? Have a known good to swap?

[jporsche]

I took a look under the dash and followed the wires to/from the DME (as best I could)...no damage to the wires.

None, it was in storage with a lot of other cars...no reported problems with the others (3rd year I've stored it in the same place).

Hmm...I don't have a replacement cap/rotor. Any reasons why you'd think it be the cap/rotor if I'm not getting pulse at the coil? (seems like the problem is before the rotor gets the signal)

[matt curtain]

Jamie, I have the exact same problem with my 84 Carerra. Although mine sat for about 7 years while I took my time putting in a new clutch.
I checked all grounds, fuel pressure, and then started testing sensors. The speed sensor read no conductivity, so I replaced that ( not so easy to do) , but it didn't solve the problem.
I understand the DME relay affects the fuel pressure, so if you have no spark that shouldn't matter?
I'd be happy to switch DME for a trial if we were geographically compatible.

[techman1]

Jamie,
I pulled up my pdf manual.
From DME to coil, I believe it is a green wire, goes from DME, pin #1 to the coil

Black wire from coil, should go to the fuse box, luggage compartment, fuse box 1, not sure which exact fuse.

[jporsche]

I'll check to ensure I've got voltage (5V I believe, need to check my manual tonight) across the respective fuse inside the luggage compartment...

How would i determine if I have a bad ground relating to my coil? Would I simply measure the voltage at the coil before & during the ignition being turned on to see if the voltage dips below a certain value using my multimeter?

What generates that signal from the DME is the reference (TDC) sensor I believe. I measured the reference sensor across pins #2 & #3 and get 1000-1100 ohms (same as the speed sensor on the same pins). So, I'm pretty certain the sensor is fine based on this test but I haven't hooked up an oscilloscope to see if I get the right sine wave going to the DME; which may indicate worn or missing teath OR a bad sensor OR a bad ground I'm guessing. I'd rather do the simple test which is swapping out the DME OR getting a breakout box for the DME & testing the respective pins....but will go back to this if all else fails.

[dshepp806]

I've seen coil driver transistor failures (actual device failure AND solder reflow repair success) within the DME (just FYI). Just something a bit "beyond" the simple checks, for now....just an FYI.

Good luck,..good info here.

Doyle

[jporsche]

I have a friend willing to put his '89 DME into my car as a test to see if some part of my DME is at fault. I haven't checked the connectors, but I've been told two additional pins are used in the 89 vintage vs the 84...any ideas if this will work?

[DonMo]

sure will, 84-89 are the same, the inside chip is different but it all does the same. 84-85 have a 24 pin eprom, the newer ones larger.

[ischmitz]

The first real simple test is to listen to the ICV with the ignition set to ON. It needs to hum and do a little shake every second. This will tell you the DME is getting power and its CPU is running. If not something is wrong with your wiring to the DME.

Next, while you crank make sure the fuel pump switches on. This confirms a good DME relay.

After that check if you get fuel. Smell the tail pipe after some cranking and you should smell unburned fuel. If both spark and fuel are missing it is likely a bad sensor (speed or reference). If only spark is missing it could be the DME.

Measuring the sensors (Ohm meter) will not always identify bad ones. An oscilloscope is the only sure way to decide whether the sensors are fine or not. You don't really need a breakout box.

ingo

[jporsche]

DonMo:

Will my '84 DME work in an '89...my friend is a bit hesitant about using his DME in my car :|

[steely]

Originally Posted by jporsche

...
How would i determine if I have a bad ground relating to my coil? Would I simply measure the voltage at the coil before & during the ignition being turned on to see if the voltage dips below a certain value using my multimeter?
....

Measure V at the ground at the coil - if it's spikey or high, that's an indicator of a gnd path prob - it should of course be 0v.

I think you should check the flywheel Speed and Reference sensor pair (not TDC). If the insulation on the leads near the firewall (either side) is crumbling, that's a clue they're on their way out. If either one goes bad, the DME will prevent start-up or give you a poor idle (depending I believe on which one failed).