3.2 no start... Please help me diagnose!

[cnielsen]

This 3.2 is transplanted into a 73 chassis so I don't have an alarm hooked up and the fuel pump is wired through the chassis from the 73 chassis harness...yes, I know I need to change that.

So...the other day we were taking the 3.2 out to dinner, I noticed the tach (3.2 tach)drop to zero and then popped up to normal RPM. It did this a couple times over the course of 30-60 seconds and then the car died. No power to the motor but it would crank strong. It really felt like a electrical failure...just went DEAD

We towed the car home and I started checking the DME relay. It did appear to have a broken or weak solder bridge between two terminals so I resoldered that and still no start.

- Battery is putting out a good strong 12.5+V and more than that when I have it on the charger so I'm sure it's not the battery being too weak to trigger the DME and fuel injectors. I've never had any battery issues and the battery is fairly new.

- I swapped the DME for a new one and still no go

- no fuel smell, and I do have 2/3 of a tank of gas

- I swapped the coil and still no go

- I have a spare Motronic brain still no luck with that

Now I know it's best to carefully diagnose and not just swap parts around but I was going for the easy solution first since I had the parts

So... Ignition on...and the brain disconnected, I check the power going to the DME relay plug and I have well over 12v. The ground is strong. Now I hook the relay to the plug and I still get 12v BUT with the ignition on I only have 8.5V! I have power to all the terminals that should have power when the relay is activated but still only 8.5V to all those wires. I've got 3 relays, two are new, and they all do the same thing. I know that 8V is not enough to power the brain and certainly not enough to power the fuel injectors so I have a problem somewhere. If the relay is getting 12V going in, why am I not getting 12V going out? The weird thing is that the big red wire coming straight from the battery has 12V, but as soon as I turn the ignition on, it drops to 8v (with the relay connected). So somehow the presence of the relay is affecting the power going into the relay.

I even swapped out the ignition switch and that didn't help.

The red wire going to the battery seems very secure and in great condition.

[Tippy]

Left side rear of intake, there are 2 crank reference sensors.

Sounds like the speed one died on you. The other is the reference indicating TDC of cylinder #1.

[cnielsen]

Checked those...they test out at 1000ohms

[cnielsen]

Just ran a jumper cable from the (-) battery terminal to the ground number 1 intake runner and it fired up.

I checked that ground and it is very securely fastened. So now I am suspicious of the trans to chassis ground strap...

At least I'm making headway!

[cnielsen]

Checked all my grounds again. Trans ground strap looks new! I used scotch brite on all the connections and tightened them up. Same for the battery ground. Then tried to start and no go... Then connected my jumper cable back to the number 1 intake runner and now it won't start. Ugggh, I'm frustrated! I'm sure I have a ground issue but I can't narrow it down and now I can't even bypass it.

[JJ 911SC]

When the car was running with the jumper, how did you shut off the car?

If you use the key vice removing the jumper, you have not positively confirm that it was related to the ground path.

It could have been a collateral fix as in when you installed the jumper touch something else (i.e. the 16 pin connectors).

[vince_aust]

I agree with tippy. Sounds like reference sensors have lost alignment/moved from bell housing. You won't find that with a multimeter. They will check out ok just need to be moved closer.

[techman1]

Check and clean the main ground strap, from battery to frame. I would even go so far as to replace it if it is original.

Not sure of the level of truth in this, but I was told that over time the ground cable and its terminations can build up resistance to current. It may look good, and pass a volt test, but when high current demand is placed on it, it just can't flow that much. This paragraph is anecdotal.

I was having strange electrical gremlins on my '95 911 and replaced the cable. All the gremlins went back to Lucas, where they belong. It is possible the clean ground points of the cable solved the issue, but it is peace of mind not having to worry about it failing.

[1990C4S]

Originally Posted by cnielsen

Checked those...they test out at 1000ohms

Are they plugged in correctly? If they are crossed over it won't start (no spark).

I know this for a fact....

[cnielsen]

I did get it to start a few times. so I'm tempted to believe the sensors are fine.

When I shut it down, I did it with the key. Good idea on disconnecting the ground.

I'm off to buy new ground straps for the battery and transmission.

Would there be any harm in adding an additional ground from the motor to the ground post behind the fuel filter?

[Elombard]

Chris go ahead and by pass the DME Relay. If you search "relay jumper" I think you will find which pins to connect to take it out of the equation. I have twice seen where thumping them can get the motor to quit or start.

[cnielsen]

It runs!

The ground strap between the transmission and chassis is apparently bad...although it looks great. I ended up running a secondary ground strap between the chassis and the #3 intake runner. Now it fires up every time and I'm getting proper voltage at the DME relay. How frustrating was that!!! Oh well, at least I learned a lot about checking all the individual parts with the Motronic system.

Time to go for a drive!

[cnielsen]