'84 Carrera Non-Starting

schmaz356 · 06-01-2019, 04:17 PM

I have a 1984 911 Carrera that was running fine when I put it away last year. Now for some reason it will not start. I have heard that one culprit may be the DME relay which I had another but would not fix the problem. I also did a few checks on my original DME relay and it seems to check out fine.

Here is what I know;
All the time there is power to terminal 30. By the way these tests were done with the relay partially inserted so I could still put a test light on the terminals.
When the ignition is turned on terminal 86 become hot and the relay "clicks". I have also checked the ground at terminal 85 and it seems good.
When the ignition is on there is also power at terminal 87. While this provides power to the fuel pump the pump does not go on and there is no power at the fuse. When I jumper the fuse or at the DME relay the pump does whir so I don't think the pump is bad. I suspect there is some issue with the the control circuit from terminal 87b.
Lastly, I believe that when I jumper terminal 85b to ground the fuel pump does run.

Is there a way to check the DME without sending it out? Any ideas? I still plan to do the spark plug check and see if there is a spark but based on what I've done so far I doubt it will. Thanks

schmaz356 · 06-06-2019, 12:00 PM

I have been doing some more research and while I don't know why there is no spark I may have made some progress on the fuel. Right now it seems as if DME Relay at terminal 85b is not going to ground as it should when the ignition is turned on. This is keeping the power from the battery through DME #30 from the battery from activating the fuel pump. Short of dismantling the DME box is there a way to check if the circuit connecting to pin 20 on the DME is working properly? Could I have some issue with the DME which is causing this issue and the no spark issue? I do have power however to the coil.
Any help is appreciated.

Arne2 · Arne2's Garage

Still could be a relay issue, I'd bypass it to check.

Quote:

Originally Posted by Arne2

OK. First, the disclaimer. This jumper should NOT be used as a normal replacement for a DME relay. It should be considered as an emergency bypass to get you home in event of a DME relay failure on the road. Be warned that with this bypass in place, the fuel pump will run whenever the ignition switch is in the Run position. (This differs from the actual relay which only energizes the pump while the starter is cranking or the engine is running.)

That said, the jumper is simple. I will not provide a diagram, as looking at the picture below should be enough. The male bullet terminals are the common .157" size, and you should use a relatively heavy gauge wire. Mine is 12 or perhaps 14 gauge. (I can't recall for certain now.)

To use the bypass, you'll need to unplug the harness from the failed DME relay, and then plug the jumper wires into the terminals that would normally connect to the relay terminals marked 30, 87 and 87b. (I put a dab of white-out on the connector to mark the three proper terminals so I wouldn't have to figure it all out again while stranded on the side of the road. See picture below.)

If the DME relay has failed, but everything else is OK, this will get you home. And it shouldn't go bad from the vibration of rattling around in the glovebox for a couple of years, either.

If/when my next relay fails, I may build a replacement instead of buying yet another fragile dual relay. Looking at the schematic and such, I could build a simple replacement to duplicate all the factory functions using a pair of standard 20 amp lighting relays. Parts for that would be available at any FLAPS. That's a future project.

uwanna · 01:04 PM

Quote:

Originally Posted by schmaz356

I have been doing some more research and while I don't know why there is no spark I may have made some progress on the fuel. Right now it seems as if DME Relay at terminal 85b is not going to ground as it should when the ignition is turned on. This is keeping the power from the battery through DME #30 from the battery from activating the fuel pump. Short of dismantling the DME box is there a way to check if the circuit connecting to pin 20 on the DME is working properly? Could I have some issue with the DME which is causing this issue and the no spark issue? I do have power however to the coil.
Any help is appreciated.

Here's briefly how the system works. With the key in the on position
power is supplied to the DME "main" relay. (i.e there are two relays inside the DME relay unit). The main relay provides 12V to the ECU and also to the injectors and coil. When You crank the motor the flywheel reference sensor
provides pulses to the ECU, the ECU sees the pulses and "comes alive" it then sends 12v (or ground, can't remember) to the second relay mounted in the DME relay unit. That second relay provides the power (or perhaps ground) to the fuel pump, and voila, the motor starts! I suspect you are having a problem with the flywheel sensor's signal not making it to the ECU. Just my best guess, but check your sensors. Years ago I had a 3.2 motor and had your very problem and fixed by adjusting the sensor gap. Or you could have a bad sensor.

schmaz356 · 06-06-2019, 12:30 PM

I have verified the issue is not the DME relay. I have two and they both do the same. I have also tested the DME relay and it checks out perfectly. The jumper does start the fuel pump but I still do not have a spark. I suspect there may be some issue with the DME ECU but I would hate to spend $700 and find out it is not the problem. On one of the forums I did see a hierarchy of likely issues as follows;
1 - DME Relay - Mine checks out fine.
2 - No power at DME terminal 86 - I have power.
3 - Bad main power line - Mine is fine and delivers power to terminal 30.
4 - Bad speed sensor - I have not checked this out. This may be a possibility as the post said that without a good signal, the DME will shut off the fuel pump. These sensors are not easy to get to.

uwanna · 01:08 PM

If you have access to a Bentley Carrera service manual, it has the complete test procedure for the reference and speed sensors.
If you don't have that manual, I suggest you acquire one,
sure helps solve a lot of problems.
I highly doubt you have a bad ECU.

76FJ55 · 06-06-2019, 01:09 PM

If you don't have the manual you can always just compare the resistance values between the two sensors. the reference and speed sensor are identical sensors, so the respective pin to pin resistance values should be the same, and it is unlikely that they both would have failed simultaneously.

CCM911 · 06-06-2019, 01:59 PM

I had the same issue with mine. No spark at all. It ended up being a bad ECU. You may want to try a know good one if you can.

49willard · 06-06-2019, 03:12 PM

I had a failure to start, fortunately in my shop and traced it to one of the sensors. I Just replaced both sensors on my 86 Cab. I do have a lift. I would hate to do it without a lift. I dropped the left side 1/2 shaft for access. Like others, it was necessary to remove the aluminum holder, get it to a vice and with some level of difficulty (a mapp gas torch was used) extracted both sensors. There is a reason you would pay a tech good money to do the r & r. As I like to say "nothing is difficult for the guy that doesn't have to do it himself".

schmaz356 · 06-12-2019, 09:10 AM

Just an update. After testing the speed sensor which did no register on my Ohm meter I changed it. Luckily I din not have to take off the sensor bracket. After I got the new sensor installed, the car fired right away and started up as if there we're no issues.

Thanks for everyone's input.

Steve
1984 911 Carrera
1973 914
1973 911E
1965 356SC
1958 108k Junior