won't start C2

[IDGAF27]

Everything was working perfecton my 91 964 C2, until one day was cranking engine to the point that battery died and engine didn't start. Hooked up with jump cables to my truck and it start. Then again , I stoped at traffic light and it stalled. After few attempts to start,I pushed car to the curbe. Let it sit for a while whit open engine lid. 10min later crank the engine and it started rightaway. Got through the weekend with no problems.
Then last weekend came in and I couldn't start again, and again and again. Rotates like crazy.82K miles, Battery fully charged. DME relay NEW. But no crank ,no Wroom wroom

My train of thoutghs: check fuel pump, check fuel pressure, check spark .

Anything else I should look for?

[Hamptonsam]

Sounds like a good start of things to check.

[sms1305]

I had what sounds like a similar problem. Intermittent starting and non-starting. It turned out to be a bad ground wire connecting the batter to ground. It's a braided wire that looked ok, but was all corroded in between the strands. I haven't had a problem since I replaced it. Good luck! Steve

'89 964/C4 - Intermittent Total Electrical Failure

[IDGAF27]

OK, here is some things I did to start it up:
New DME (suppose to work) , check DME Fuse- current comes/ current goes.
Checked Fuel Pump - no action, both wires shows ground.
disconnect Fuel Pump wires and hot wired directly from battery - Pump Works.
checked Fuel Pump Fuse- is good, but no matter what position Ignition Key is , fuse shows only ground.
What should I look to get juice going into pump?

might anyone have electrical chart for fuel pump.


Please help,I miss riding it a lot .

[ischmitz]

The fuel pump relay (second stage of the DME relay) is controlled by the DME and ONLY switches on when the DME detects motion of the flywheel (cranking, engine running). You need to check with a voltmeter while you crank. If the pump does not run while you crank it could be a bad flywheel sensor.

If you put your hand onto the DME relay while using the other hand to crank you shoud be able to feel the relay switch while you turn the key. Turn it to ON and the first stage switches on. Crank and the second stage switches on. Release the key and the second stage switches off.

Ingo

[IDGAF27]

Sorry, but please corect me if I am wrong, shouldn't pump start rotating when ignition kye is on, and flywheel sensor have to do only with pump presure adjastment when car is running? there is pressure reading:
engine OFF, pump ruuning ( I asume engine is NOT rotating, Ign Key ON, Flywheel STILL/NO movement) - 3.8 +- 0.2 bar ( 55+- 3 psi)
Engine at Idle - ~3.3 bar (~48psi)

All I remember, from long time being around the cars, that first thing how to check pump is by listening is it makes noise when you turn ignition key on.

[my1st911]

As far as I know, there is no car, or shall I say 99.9 percent of cars do not run the fuel pump just because key is on. It may prime on most cars, but your car should turn pump on when it sees RPM. If flywheel sensor has gone south, then car doesnt see RPM's and doesnt kick the fuel pump on

[ischmitz]

Originally Posted by IDGAF27

Sorry, but please corect me if I am wrong, shouldn't pump start rotating when ignition kye is on, and flywheel sensor have to do only with pump presure adjastment when car is running? there is pressure reading:
engine OFF, pump ruuning ( I asume engine is NOT rotating, Ign Key ON, Flywheel STILL/NO movement) - 3.8 +- 0.2 bar ( 55+- 3 psi)
Engine at Idle - ~3.3 bar (~48psi)

All I remember, from long time being around the cars, that first thing how to check pump is by listening is it makes noise when you turn ignition key on.

No, starting with the 3.2 the DME controls the fuel pump via the DME relay. In the 3.2 cranking is detected by a simple signal from the ignition switch. On the 964 and later cars the DME monitors the flywheel sensor to determine whether the fuel pump it turned on. This is a safety feature. In case of an accident where the engine stalls the fuel pumps shuts down.

The pressure regulator maintains a constant pressure difference between the intake manifold pressure and the fuel pressure. Depending on engine speed and throttle setting you will see the fuel pressure change between the values stated in the workshop manual. With the engine off a check valve maintans pressure in the system and that is what you see.

The reason for the variable fuel pressure is that the fuel injectors requires a constant fuel pressure relative to the intake manifold and not relative to the ambient. That way the flow rate is proportional to the opening time.

Ingo

[IDGAF27]

Originally Posted by my1st911

As far as I know, there is no car, or shall I say 99.9 percent of cars do not run the fuel pump just because key is on. It may prime on most cars, but your car should turn pump on when it sees RPM. If flywheel sensor has gone south, then car doesnt see RPM's and doesnt kick the fuel pump on

I could agree on carb operated cars.But any other car ,you turn key on and you hear noise coming from pump( it builds pressure), then it stops and when you crank it it ,pump starts working again.
In regards of flywheel sensor are you refering to speed sensor?

Once again in my case, DME relay terminal 86 has juices when key is on. So it sends power to pump, but no mater what I do- crank engine or engine still there is no power coming to pump.

[ischmitz]

Some cars run the pump for a short time when you turn the key to ON but the 964 does NOT. The 964 has only one sensor at the flywheel. It uses a missing tooth scheme to detect TDC. Unlike the 3.2 which has two sensors, one for TDC (reference) and one for engine speed.

Fuel pump power comes from terminal 87b of the DME realy to fuse 34. It energizes the fuel pump and the O2 sensor heater. Terminal 86 is hot when the igntion is ON or START.

[IDGAF27]

Thanks. Now we talking.

Let me understand. 87B is hot when I turn key on or all the times. coming back to your first replay- i turn key half way- I hear DME relay click once, then when the key is all the way to ON, I hear relay click second time, so I assume it is working.

Now the question- juice from DME goes first to fuse or to flywheel sensor? I am asking only because fuse 34 always shows ground on both ends it doesn't get juice transferring to pump. So if juice comes from flywheel sensor - I NEED FLYWHEEL SENSOR?! is that right?
please let me understand.

[IDGAF27]

Is it right part number for flywheel sensor?: 911-606-217-03

[ischmitz]

The flywheel sensor is connected to the DME. Think of the DME as a computer - it is the silver box under the drivers seat. The DME relay is the little black box the sixe of a matchbox. The DME has inputs and outputs. The flywheel sensor is connected to one of its inputs. One of the outputs controls the DME relay's second stage. 87b (on the DME relay) is the output that powers the fuel pump.

When you turn the key to ON (alternator light and oil pressure light comes on) you energize terminal 15 of your car. This will also switch on the first stage of the DME relays and provide power to the DME. That first stage of the relay powers things like the coil, the injectors, etc. Once you turn the key to START (cranking) the DME will turn on the second stage of the DME relay. That powers the fuel pump and the O2 sensor through 87b.

With the second stage not turned on you will see something close to GND potential on fuse 34 because both the O2 sensor heater and the fuel pump motor provide a path to GND.

Hope that helps,

Ingo

[IDGAF27]

So how do I check flywheel sensor. What kind of output it shuold give toward ECU. If i understand right with Ign.key on curent travels to Flywheel sensor wich gives reading and power,when flywheel starts rotate, to ECU and then DME relay terminal 87b gets power and feeds pump fuse and same time changes polariti on pump and makes pump rotate?

[IDGAF27]

How cool is it to be around peeps like you. My car is alive and running again. As De Niro said in "Analize this" : you good doc..yesh you are...."

Thanks for all the help Ingo "ISCHMITZ" for sharing your knowledge and Gene from ******** NJ for a part.

[ischmitz]

You are welcome.

So what was the fix - if you tell us this will help others having similar issues. What part did you replace?

[IDGAF27]

Ups, I were so exited ,yesterday, totally forgot about the part.

Replaced Flywheel Sensor .