Motronic harness questions on 3.2 conversion

[cubby911T]

Sal - hmmm.....I thought that since both the (+) and (-) leads to the coil are part of the DME wiring harness that the DME supplied the 12V. I'm starting to get a wee bit confused, but maybe the test light is the way to go to confirm that the coil is commanding spark.

[scarceller]

Originally Posted by cubby911T

Sal - hmmm.....I thought that since both the (+) and (-) leads to the coil are part of the DME wiring harness that the DME supplied the 12V. I'm starting to get a wee bit confused, but maybe the test light is the way to go to confirm that the coil is commanding spark.

Look at the fuses in the top left of the wire schematic you will see fuse #8 powers the coil it also is the signal that picks the DME relay (commands it on). Then the GND side of the coil gets it's ground pulses from pin #7 on the DME.

To confirm, simply disconnect the DME and the coil still gets 12V on its + terminal.

Also, if you get a test light get an LED test light they need a lot less current to get them lit and are the best for testing coil and injector signals.

Our host sells one:
http://www.pelicanparts.com/catalog/shopcart/TOOL/POR_TOOL_CAT447_pg66.htm
This is a real good one but others can be found cheaper.

[cubby911T]

Well, I don't have a test light (yet) so I tried to detect voltage to the coil with my multimeter last night. I'm pretty sure I'm using the multimeter correctly, because I can read 12V at pin 1 of the DME socket. However, I can't detect any power getting to the coil. I have the entire DME wiring harness sitting in the trunk next to my battery for easy access. I connected the lead to the battery (+) and the two leads to the coil. I then turned my ignition to "on" and checked for power to the coil - nothing. I had a helper (my wife) turn the key to "start" as I checked for voltage - nothing. Somehow, the PO got power to the coil with this harness. Is there something else I need to connect in order to properly test this?

Again, this harness has been modified but the PO says it worked "as-is" without problems in his '74. There are no wires leading to the fuse panel, but there are several in-line fuses in place within the modified harness.

[scarceller]

Originally Posted by cubby911T

Well, I don't have a test light (yet) so I tried to detect voltage to the coil with my multimeter last night. I'm pretty sure I'm using the multimeter correctly, because I can read 12V at pin 1 of the DME socket. However, I can't detect any power getting to the coil. I have the entire DME wiring harness sitting in the trunk next to my battery for easy access. I connected the lead to the battery (+) and the two leads to the coil. I then turned my ignition to "on" and checked for power to the coil - nothing. I had a helper (my wife) turn the key to "start" as I checked for voltage - nothing. Somehow, the PO got power to the coil with this harness. Is there something else I need to connect in order to properly test this?

Again, this harness has been modified but the PO says it worked "as-is" without problems in his '74. There are no wires leading to the fuse panel, but there are several in-line fuses in place within the modified harness.

First, pin #1 on the DME socket is the GND - side of the coil not the +. You say you read 12v on it, with the DME plugged in?

If all is working correctly you will read 12V on pin #1 because the DME leaves this pin floating, meaning it does not ground this pin unless it is commanding spark.

The bootom line is that in the stock configuration with the DME unplugged and the key in 'RUN' or 'START' the + side of the coil gets 12V. You are saying you don't see 12V at all on either the + or - coil studs, is this correct?

Maybe this pic will help:

[cubby911T]

Originally Posted by scarceller

The bootom line is that in the stock configuration with the DME unplugged and the key in 'RUN' or 'START' the + side of the coil gets 12V. You are saying you don't see 12V at all on either the + or - coil studs, is this correct?

That is correct, Sal - nothing registering on either coil post. The only place I can get a 12V reading is pin1 of the DME socket with the relay removed

What's this about the DME needing to be unplugged though? I have been testing with the DME relay plugged into the socket and the big connector plugged into the DME box. Should I be unplugging one of these for testing?

[cubby911T]

Correction: It was pin 6 on the dme socket that I got 12v on, not pin 1.....sorry.

[scarceller]

Originally Posted by cubby911T

Correction: It was pin 6 on the dme socket that I got 12v on, not pin 1.....sorry.

That does not sound right, pin #6 on the DME is a Ground pin that feeds ground to the Air Flow Meter. You sure you where on pin #6?

[scarceller]

Your answer is in post #13 see the picture with all the wires, wire #4 is what carries the 12volts to the coil.

And then wire #1 also is 12Volts, it carries 12V to pick/activate the DME relay. Those 2 wires need to be wired to 12V ignition, they need power in 'RUN' and 'START'

Yup the wires are black which you may think is ground but they are not, in 911 cars brown is ground.

[cubby911T]

Okay, I dug into this modifed harness and tried to remove some of the "extra" leads added by whoever bastardized this thing. Power to the coil in the PO's car was apparently supplied by a lead that did not come along with this harness. I can run a new lead from the fuse block to the + post on the coil no problem. Does it matter where the coil is grounded from? Would it necessarily have to be grounded through the DME? If not, I'll just ground it in the engine compartment somewhere. Either way I think I'm okay on the coil topic now. Thanks for all the explanations!!

One other question. Marc's 3.2 conversion instructions mentioned another wire running from the fuse block into the #5 pin of the DME socket. What would that be for and is it an absolute requirement?

[scarceller]

Originally Posted by cubby911T

Okay, I dug into this modifed harness and tried to remove some of the "extra" leads added by whoever bastardized this thing. Power to the coil in the PO's car was apparently supplied by a lead that did not come along with this harness. I can run a new lead from the fuse block to the + post on the coil no problem. Does it matter where the coil is grounded from? Would it necessarily have to be grounded through the DME? If not, I'll just ground it in the engine compartment somewhere. Either way I think I'm okay on the coil topic now. Thanks for all the explanations!!

One other question. Marc's 3.2 conversion instructions mentioned another wire running from the fuse block into the #5 pin of the DME socket. What would that be for and is it an absolute requirement?

The + side you can just wire 12V to it. But the - side MUST go to pin #7 of the DME. Do not simply ground the - side

[Mark Crowe]

A question on the existing CDI set up, which cable supplies the CDI ( mine is an 8 pin) with power? I have tried 2 CDI units, niether will "whine" on start up, fuel pumps are running. Anyone have the breakdown of the pins connections on the CDI to determine the function/ cable?

[m_davidson]

Thanks, Sal for the circuit diagram. I have a fairly complete 3.2L harness with all the necssary bits and pieces. I plan to control my '76 fuel pump relay with the ECU if I can figure out where that relay got its signal before.

My other puzzle is the engine-mounted heater fan. What would control it in a '76? My previous engine was a 3.0L and the fan seemed to run all the time. I'd like to control the 3.2L fan with a toggle switch at the dash. Does anybody have answers or suggestions?

Mel

[scarceller]

Originally Posted by m_davidson

Thanks, Sal for the circuit diagram. I have a fairly complete 3.2L harness with all the necssary bits and pieces. I plan to control my '76 fuel pump relay with the ECU if I can figure out where that relay got its signal before.

My other puzzle is the engine-mounted heater fan. What would control it in a '76? My previous engine was a 3.0L and the fan seemed to run all the time. I'd like to control the 3.2L fan with a toggle switch at the dash. Does anybody have answers or suggestions?

Mel

Mel,
I don't have schematic for the 76 but I do for the 77 and in the 77 the blower runs all the time with key in 'RUN'. It shows it is powered by a relay but I'm not sure where the relay is (most likely at the engine bay fuse panel area). More important is it shows the relay coil is activated/picked from a wire at the ignition switch terminal #30 (shows RED wire). So if you could locate this wire at the ignition switch you could simply put a toggle switch on it at the dash.

Keep in mind that the rear blower does 2 things:
1) gives you heat in the Winter
2) if you close the heat flapper (by pushing down the red levers between the seats) and run the blower it helps cool the heat exchangers in the Summer and this reduces engine temps.

In short, I'd power the new blower from the old harness and then locate the wire that picks the relay at the ignition switch and add a toggle switch at the dash.
Or you could run it all the time as in the 76 configuration, no harm done.

Good Luck

[m_davidson]

Sal,

Thanks once again. I didn't realize the old fan was supposed to run all the time: I thought maybe it was a hash job from the PO. I'll just wire the 3.2L fan to run all the time also. I'll get the car running before installing the under-the-front-fender oil cooler. The extra cooling from the continuous running fan will be useful. We live near the Canadian border, so overheating should not be too big a problem for several months.

I believe the one round black relay on my engine compartment relay board is the engine fan relay, and I can easily check it. After I install my an cooler, I may re-visit the fan switch idea.

Mel

[m_davidson]

I guess I should have ended my last post with:

1971 911T (future project car)
1972 914-6 conversion (previous project)
1976 911S (present project)
1977 911S (daily driver)

Ain't retirement wonderful?

Mel