No Gas, No Spark

Rouxzy · 11-30-2015, 04:15 AM

Have a '77 with a 3.6 transplant. Car was running fine then all a sudden engine turns over but doesn't start. I first switched out my replacement DME relay with no changes. Put a meter on fuel pump and cranked engine but no power to pump. Checked for spark while cranking engine and again nothing. Should I pull the ECU for a rebuild or is there something I should check before hand.

T77911S · 11-30-2015, 05:41 AM

CPS

i bet that is a fun car

gomezoneill · gomezoneill's Garage

Possible reference sensor and/or position sensor at the bell housing.

wwest · wwest's Garage

Try another DME relay, one that is known to have been working.

Purschased 3, luckily the third one worked...

Rouxzy · 12-04-2015, 04:10 AM

Thought it might be a bad relay so I switched out to my spare, still nothing. Purchased another from our host and still nothing. Is there a way to test the relay to make sure it is not a case of several bad relays?

gomezoneill · gomezoneill's Garage

One more time. Possible reference sensor/position sensor!

theiceman · 12-04-2015, 05:42 AM

as stated to fire the injectors the timing of the sensors must be seen by the dme. but no power to the fuel pump is interesting .

Trog · Trog's Garage

Step 1, check fuses.

Quicksilver · 12-04-2015, 06:04 AM

To reiterate a couple of the posts...
The DME does not turn on anything until it sees that the engine is spinning. If it doesn't get the correct signal from the flywheel sensors you won't get any fuel or spark.

You can check the sensors with an ohm meter but that is a real minimal test. What you really need is to hook the sensors to an oscilloscope and see if you get a 2.5 volt signal from them when the teeth/pin passes them. (The reference sensor is one pulse per rotation and the position sensor is continuous pulses. And don't swap the sensors.)

Rouxzy · 12-04-2015, 08:22 AM

Thanks, I'll give those sensors a check and see how I make out.

Porsche 2 · 12-04-2015, 09:21 AM

Quicksilver is right on! My check of sensor resistance revealed they were OK. Never the less, same symptoms as yours was fixed by replacing both flywheel sensors, see my thread over the past few days.

uwanna · 12-04-2015, 09:38 AM

Quote:

Originally Posted by Quicksilver

To reiterate a couple of the posts...
The DME does not turn on anything until it sees that the engine is spinning. If it doesn't get the correct signal from the flywheel sensors you won't get any fuel or spark.

You can check the sensors with an ohm meter but that is a real minimal test. What you really need is to hook the sensors to an oscilloscope and see if you get a 2.5 volt signal from them when the teeth/pin passes them. (The reference sensor is one pulse per rotation and the position sensor is continuous pulses. And don't swap the sensors.)

You are correct for a 3.2 motor but a 3.6 is different due to using sequential fuel injection versus batch injection like a 3.2.
A 3.6 has only ONE sensor! The flywheel has a couple of teeth missing and the single sensor uses the teeth gap for reference sensing and the rest of the teeth for position.

ischmitz · ischmitz's Garage

Check the SINGLE flywheel sensor with a voltmeter set to AC while cranking. If you don't see about 2V AC the sensor is bad.

Futher, make sure you have power to the DME with the ignition switch in ON. Measure inside the DME connector. One other way to tell is to listen to the flap when turning the ignition on. You should head a faint click from the engine bay at the resonance flap valve is actuated.

Unfortunately there are not too many easy clues (like the vibrating ICV) with a 3.6 DME. Given that you have a SW chip your's can only be a 964 or early 993. Which one is it?

Ingo

Rouxzy · 12-04-2015, 12:26 PM

Mine is a 964 with the DME under the driver's seat.

ischmitz · ischmitz's Garage

Are you using any of the pre-made harnesses (PM, Timmins, etc.) or did you fabricate your own?

You need to ensure power is getting the the DME. It needs to "see" GND on pin 19 and +12V on pins 18 and 37. With that the DME will be turning the 2nd stage of the DME relay on and will produce spark/fuel pulses once the FW sensor sends a proper signal.

Ingo

Rouxzy · 01:02 PM

Sorry for the delay, but with work, family, and holidays, today was the first time to get checking things out. I have ground to pin 19, and 12-13 Volts on pins 18 or 37. Checked the FW sensor and got nothing. Pulled the sensor out and it looked like someone used vice grips on the body of the sensor, teeth marks and gouges in it. Could be the cause of the intermittent issues I've had since day one with the engine install. Will be ordering a new sensor from our host. Ingo, I see that you repair/rebuild Motronic DME's. Seeing I have it out of my car right now is there anything I should do before reinstalling. Does it make since to have it checked out seeing that it has been around the block a few times?
Tom

ischmitz · ischmitz's Garage

Hi Tom,

The earlier 3.2 DME (and their related 924/944 versions) are known for deteriorating solder joints. I have done many of those and careful inspections with a high-quality stereo microscope will usually reveal how far along a unit is.

Fortunately the later 964 and 993 versions are much more reliable in comparison. PCB technology and solder technologies are greatly improved over the early units and many of the components are no longer through-hole but surface mount. In addition the larger components that are subject to mechanical stress due to vibration are secured with RTV.

These boxes still do fail from time to time but there isn't really any preventive actions that can be taken in my opinion. The failure modes are typically IC components on the logic board or the output stages for the injectors. Once that happens I am more than happy to help you out. I have had some interesting "corner cases" where a box was intermittent. But usually it'll either work or it doesn't work.

Besides the FW sensor pay particular attention to the O2 sensor. The DME harness has a couple of coding plugs that tell the DME whether you use an O2 sensor or not. In my case I know I have a sometimes poor connection on the stock O2 connector. Once that acts up the engine runs like crap. If you run with O2 sensor make sure the sensor is working properly. Else set the jumper to run w/o O2 sensor.

For an initial checkout of your DME I recommend getting the new FW sensor installed and properly gaped and then see if the engine runs. Ideally, you'd install the CEL (check engine light) as well. The CEL can be used to flash out fault codes from the DME and reveal hidden issues such as the Hall sensor not giving a signal. If you still have issues I'd be more than happy to do a full check on your DME. Let me know how it goes.

Cheers,
Ingo

Rouxzy · 01-30-2016, 04:50 AM

Good News, replaced the FW sensor and she fired right up. Other good news is that the searching for idle that I use to have at startup has gone, so maybe my sensor was to blame for that also. Thanks to Ingo and everyone else who pitched in for the help. This board never disappoints.
Tom

Rouxzy · 03-11-2018, 04:40 AM

Back to the drawing board. The ole girl left me stranded last fall. Same symptoms as before with it not wanting to start. With information from other threads that Ingo had contributed to I checked and found that I had no power coming from the ignition switch. The after market alarm system had reared its ugly head. I eliminated the connections from the alarm system to the ignition switch and VOILA, she fired right up. Thanks for the help again Ingo. Without even knowing it you help us.
Tom

T77911S · 03-12-2018, 02:51 AM

REMOVE AFTER MARKET ALARM SYSTEMS!!!!!

most of them cut the yellow wire to the starter that engages the solenoid.
the solenoid needs a lot of current to pull in.
anytime you cut and add connections you increase the resistance in that wire and reduce the current, then add in the fact that 9 out of 10 times the splice/crimp job is done poorly.