Another 3.2L No start

[motogman]

It seems a fairly common issue and there are many threads, but alas, after some diagnosis I am looking for input from some of you experts...

Recent symptoms/history: Car (early 84 3.2L Carrera) parked in garage last fall running fine. Would not start in Feb. Just started diagnosis this week. Would not start. After jumpering fuel pump to see if it was working car started and died. I then left the fuel pump jumpered and it started and ran once. Now it will not start.

What I have done:
• replaced DME relay (2 times)
• checked and cleaned grounds
• temperature sensor resistance checks OK
• checked voltage to coil – no voltage with key on or starter engaged
• checked spark – appears to be no spark when cranking
• when key on – fuel pump does not run for the 1 second at key on to build pressure as I understand the DME should do
• I do not have a fuel gage but when I cracked the fitting on the fuel rail above cylinder 1 and ran fuel pump I got a slow flow of fuel but not the spray I had expected for what should be about 40 psi of fuel pressure so I am not sure fuel pressure is where it should be but fuel does seem to flow to the fuel rails.
• The idle device does seem to be humming with key on – this would seem to indicate DME is doing something

So I need some recommendations for next steps and understanding of this system.

??? How does the coil get power? Should it have 12 volts all the time the key is on or is it powered by the DME? Is there a fuse or other connection for the coil? Why would the coil not have 12 volts under the scenario above?

??? I have not checked the speed sensor or reference sensor as I do not have an oscilloscope. - Is there a way to check these sensors at the connectors?

??? What checks can be made to the DME itself before taking it apart to look for solder joint failures?

Please give me your ideas, as I am getting frustrated/discouraged.

[Stutzdriver]

I just finished this whole process. My car ran fine the night before and would not start the next morning.

SEARCH is your friend but I would guess that one of the flywheel sensors has failed. You can test these with a ohm meter at the DME 35 pin connector. Do an internet search for the connector schematic showing how the pins are counted and then do a search for speed sensor failure or reference sensor failure there is a great thread on the testing and replacement. You can unplug the DME connector under the front seat and test the sensors there pretty quickly. This seems to be a common failure lately. If you need to do the replacement there is a BMW part number that is cheaper than the Porsche part number, the only difference is the wire is a bit longer which actually makes the connection a little easier IMHO.

You need the reference sensor to function at least for one pulse to get the engine started. After that the DME "occasionally" looks for the reference pulse to make sure timing is synched to TDC of the engine. The speed sensor signal is required at all times and that is what tells the DME when to fire the spark and injectors. An intermittent reference sensor is very hard to diagnose. As someone else mentioned there were some instances where an aftermarket flywheel didn't have the right material pin installed to trigger the reference sensor. To diagnose you can plug a spare sensor into the harness and wave a metal object in front of it. One pulse is enough to transition the DME into 'run' mode.

If either sensor is not present (disconnected, damaged) the engine will never start or even sputter. Not a single spark event will be issued and a single injection pulse will be issued. However, once the engine is running you could unplug the reference sensor and it would run on for a little bit.

My reference sensor was new the speed sensor was original ans bad. The reference sensor tested fine so I just replaced the speed sensor and it fired right up!

[motogman]

Jay... Thanks. I have been trying to find out how to test the sensors and will look for those threads.

However... My understanding is that the DME should run the fuel pump for about 1 second at key on to build pressure and then it shuts off until it sees a speed signal during cranking.

In my situation, the fuel pump is not being run for the 1 second. Fuel pump runs fine when jumpered at fuse block and seems to pump fuel to fuel rail.

When you had speed sensor problem do you recall if this 1 second of fuel pump operation happened or not?

[rick-l]

The fuel pump should run when the key is in the start position or the micro controller in the DME tells it to run (engine running).

When you jumper the fuel pump does the car run?
Is the idle air control vibrating with the key on? edit: never mind didn't read well enough

[JAR0023]

Another vote for checking speed and reference sensors. Search for posts by John Walker. He has some good concise posts on checking the sensors. -J

[rick-l]

Originally Posted by motogman

• checked voltage to coil – no voltage with key on or starter engaged

Are you sure about this?

Power for the coil comes from fuse #8 in the trunk but that track also powers the coil for the DME which turns on the injectors and idle air control.

Any chance there is a black 2.5 mm^2 wire loose by fuse #8?

[Stutzdriver]

The fuel pump part of the DME relay WILL not close to start the fuel pump until the flywheel sensor checks the reference sensor and indexes TDC AND the speed sensor registers the flywheel is turning. At that point the second part of the DME relay (there are 2 relays inside the DME relay) energizing the fuel pump AND the ignition....then it starts. the DME checks the reference sensor during normal running to check the timing.

No signal from EITHER flywheel sensor, no fuel, no spark. If the sensors are good and you jump the fuel pump it will start but if you pull the cover on the DME relay you can watch the closure of the relays.

I am still betting on one or Both flywheel sensors having failed.

[rick-l]

Stutzdriver I guarantee the operation of the fuel pump through the DME - DME relay is as I have described in the post above. The other relay in the DME relay is energized by the key in run or start position.

No power to the coil is a show stopper.

A 2.5 mm^2 is slightly smaller than a 12 ga wire and will melt if you put 198 Amps through it for 10 seconds and correct me if I'm wrong but I don't think that wire has a fuse.

EDIT BELOW:

The coil should be on in run or start and does not go through the DME relay.

DME circuit for fuel pump:
The fuel pump runs when pin 20 of the DME is pulled to ground energizing the DME relay (green arrow current flow).

The blue arrow is the direction current will flow when 12 volts from the yellow wire (ignition start position) is applied to pin 4 (current into the base turns on NPN darlington).

The red arrow is the direction current will flow when the DME detects the engine is running and commands the pump on (also turns on transistor).

either of these conditions will run the fuel pump.

[DRACO A5OG]

+2 ref & speed sensor, check the OHMs on both.

If you suspect your DME, ask a local 3.2 if you could put your DME in their car to remove that as a culprit. DO NOT PUT YOUR THEIR DME IN YOUR CAR.

[rick-l]

Again.... Priority #1 is to get 12 Volts to your ignition coil (and figure out why it went away). You are going nowhere without that.

[motogman]

So the no voltage to the coil was a stupid measurement error on my part. I initially thought that one connection at the coil was ground and the other battery so when I measure between them it was zero. One of the threads said both connections need to be battery voltage so when I measured each relative to ground there is power to the coil.

But battery voltage seemed a bit low, 11.65 volts so I hooked up the battery charger for a few hours even though the starter was cranking fine. After shutting down the charger, reinstalling the DME and checking the voltage again at the coil to see that it is now 12.9 V, I tried to start the car and low and behold, it started.

So now the question is... does a low slightly battery voltage cause a no start condition? Or perhaps I have an intermittent electrical problem that could leave me stranded at any time - love that thought.

I did check the resistances for the speed sensor and the reference sensor at the DME plug and it seems the speed sensor was not reading in the right resistance range. Now I need to check again.

Stay tuned to see where this goes.

Thanks for the comments and expertise.

[DRACO A5OG]

Yes, our 3.2's need 12.6v for a proper start. 11.65v is a drained battery. It may not last you unless your charger also disulcify ( spell check )/maintenance cycle like the C-Tek 3300.

[motogman]

What is it about 11.65 V that results in the DME not functioning properly?

I work with some digital electrical control systems on vehicles and in todays electrical specifications electrical units must function through a lower range of voltage - (I will go check some specs with those more expert than me).

I do think you are right that 11.65 might be causing the problem, but the question is why? Does it not power some element of the system properly or what?

I was surprised the starter turned it over very properly at 11.65 volts.

I was interested in doing a drive event tomorrow but I am a bit concerned there is another issue and I may be left stranded somewhere. I think an early morning drive evaluation is in order... :-)

[rick-l]

Originally Posted by motogman

I was interested in doing a drive event tomorrow but I am a bit concerned there is another issue and I may be left stranded somewhere. I think an early morning drive evaluation is in order... :-)

If it won't start it you don't have to worry about getting stranded.

If it will crank it should have enough power to deliver spark and fuel.

Now go out and measure those two variable reluctance sensors on the flywheel again and determine if one is open.

EDIT: mark them on the connector in the engine bay so you don't mix them up.

[mysocal911]

Originally Posted by rick-l

Stutzdriver I guarantee the operation of the fuel pump through the DME - DME relay is as I have described in the post above. The other relay in the DME relay is energized by the key in run or start position.

Rick is correct about how the fuel pump section of the DME relay operates.

[cabmandone]

I believe I'd be at +3 for speed and reference sensors. I had my wires crossed on mine from a previous owner installation and the car wouldn't start. Swapped the wires around and it fired right off.

BTW, this might have been stated already but for the fuel pump to stay on, there must be signal from the Speed and Reference sensors. If they are the problem, Pelican offers a less expensive yet the same part (or they did). I believe it was a BMW sensor that is the same as the one in our cars.

[brp914]

OK, now my turn! Went to store, came out and she's dead as a door nail. Towed home. I have found:

12V at coil
36 psi at fuel rail
no spark
swapped in spare dme box and relay - problem persists

I have not tested speed/ref sensors yet. It seems likely. Surprising though - no moving parts. They just sit there. Can these sensors be replaced with the engine in the car? Maybe loosen the trans mount and lower the engine 6 to 8 inches?

[cabmandone]

Originally Posted by brp914

OK, now my turn! Went to store, came out and she's dead as a door nail. Towed home. I have found:

12V at coil
36 psi at fuel rail
no spark
swapped in spare dme box and relay - problem persists

I have not tested speed/ref sensors yet. It seems likely. Surprising though - no moving parts. They just sit there. Can these sensors be replaced with the engine in the car? Maybe loosen the trans mount and lower the engine 6 to 8 inches?

If you search the tech forum, there's a writeup on replacing with then engine in. I think you just remove the drivers side tire. I know when I replaced mine prior to installing the engine, I about destroyed them because they were very nicely stuck. I'd suggest some penetrating oil for a few days before attempting to remove.

[brp914]

Culprit found: open circuit on ref sensor. Removed with engine in car.