Motronic harness questions on 3.2 conversion

[cubby911T]

This weekend I unpacked/untangled the motronic wiring harness that came with my 3.2 engine just to see if I could decipher where everything will get plugged in. Luckily, the PO did a nice job labeling the plugs. The only thing I'm a little concerned about is that there only appear to be 2 individual wires that run to the luggage compartment: one with an eyelet for the positive terminal on the battery and one with a spade connector for the tach.

According to Marc's 3.2 conversion thread, there should be a couple more wires going up there. It would seem I am missing 2 wires in particular that would attach to the fuse block - one for power to the coil and one for pin 5 of the dme relay (not sure what pin 5 does though).

The thing is, this harness came out of another mid-year conversion car and was apparently working fine. What should I do?

[IROC]

Yeah, you seem to be missing a couple of wires. The coil wire, like you mention and the other wire is the one needed to send 12V to the DME to trick it into thinking the car has an alarm. These two, plus the tach and DME voltage wire which you mentioned should be all you need.

[DW SD]

In my conversions (3.2 & 3.6), the wires were bundled in a factory harness sleeve. Any chance you could strip the sleeve back a bit more to reveal? And several, if not all going forward terminate the DME. Can you find them there?

Doug

[PcarPhil]

Matt,

Here's a pic of the wires I'm using in the DME harness that goes to the front trunk:



None of the labeled wires in the pic above are for the tach. I believe the tach wire is black/purple in the harness and is spliced into the green wire shown in the bottom left of the pic. The green wire snakes up to the back of my tach. The red/green labeled wire is the power signal for the fuel pump. The big red wire with black covering goes to the battery. The big black and small black wire both come from the DME and terminate on opposite sides of a fuse. (This wiring works for my car. Your mileage may vary!)

I remember when I was researching 3.2 conversion wiring it seemed that there were a few different ways people have the DME's hooked up to the wiring in the front trunk. I also seem to remember that there may have been some DME wire harness changes throughout the years.

What color is the tach wire in your harness?

[cubby911T]

Scott,

Here are some pics of my wiring situation. The first pic shows the 2 wires I mentioned that went to the luggage area. You can see the PO's labels stating "battery positive" and "tach" respectively.

The next picture shows the only other wires which seem to branch out from the main DME harness. These 3 wires went to the rear of the PO's car where his fuel pump was installed. You can see one is a plug for the fuel pump, one has an eyelet that is labeled "ground" and the third is an unknown red wire that was cut after branching into 2 leads near the engine electrical panel. The third pic is of the PO's engine electrical panel (which was sent to me along with everything else). I'm not sure if this grouping of 3 wires has anything to do with the shortage of wires to the luggage area, but I thought I'd better include all data . thanks.

To Luggage Area


To Engine Compartment (branched off of main harness)


PO's engine electrical panel

[jazzbass]

For a 3.2 Motronic install, you'll need the following (note - AWG sizes are approx to the metric equivalents):

- One 10ga red wire straight from the battery to the DME harness (to the DME relay, then the DME and the fuel injectors).

- One 14ga red/green wire from the DME relay to one side of the fuel pump fuse (25A). The other side of the fuse is +12V to the pump.

- One 16ga yellow wire from the "start" signal (bus 50) to pin 4 of the DME. This is the same signal sent to the starter solenoid when starting the car. W/o this, the DME won't turn on the fuel pump when starting.

- One 18ga black/violet wire to the Tach. From pin 21 of the DME. This is the RPM signal

- One 14ga black wire from the run/start +12V bus (15). This is power to the coil.

- One 16ga black wire from the run/start +12V bus (15) to pin 86 of the DME relay.

** Optional **

- One 18ga black wire to the tach from pin 11of the DME. I think this is the upshift indicator. The functionality of these last two may be reversed, I can't remember off the top of my head.

- One 16ga green/yellow wire from the A/C compressor switch. Tells the DME the A/C is on so it can compensate for it.

Get a copy of the Bentley manual for a 3.2. It has all the factory wiring diagrams in it. You'll need it for the engine, anyways.

[IROC]

Originally Posted by jazzbass

- One 14ga red/green wire from the DME relay to one side of the fuel pump fuse (25A). The other side of the fuse is +12V to the pump.

- One 16ga yellow wire from the "start" signal (bus 50) to pin 4 of the DME. This is the same signal sent to the starter solenoid when starting the car. W/o this, the DME won't turn on the fuel pump when starting.

In some cars (like mine) these are optional also. Depending on how you have your fuel pump wired, these may not be used (I don't use them).

The advice on the Bentley manual is good. Once you sit down and study the wiring diagrams, it is pretty straightforward what needs to be done.

[PcarPhil]

Originally Posted by cubby911T

Scott,

Here are some pics of my wiring situation. The first pic shows the 2 wires I mentioned that went to the luggage area. You can see the PO's labels stating "battery positive" and "tach" respectively.

The next picture shows the only other wires which seem to branch out from the main DME harness. These 3 wires went to the rear of the PO's car where his fuel pump was installed. You can see one is a plug for the fuel pump, one has an eyelet that is labeled "ground" and the third is an unknown red wire that was cut after branching into 2 leads near the engine electrical panel. The third pic is of the PO's engine electrical panel (which was sent to me along with everything else). I'm not sure if this grouping of 3 wires has anything to do with the shortage of wires to the luggage area, but I thought I'd better include all data . thanks.

Ok, so you're dealing with a modified DME harness!

First of all I suggest running the fuel pump in the stock location up front. The pumps are better 'pushers' than they are 'pullers'.

Are the wires loose (black harness covering stripped off) all the way back to the DME? If not I'd start stripping back the black harness until you find all the wires mentioned. If it is fully stripped then you really need to start studying pinout diagrams to find the correct wires you need. Actually I'd study the diagrams anyways.

Good luck with this. Keep us posted.

[PcarPhil]

Originally Posted by IROC

In some cars (like mine) these are optional also. Depending on how you have your fuel pump wired, these may not be used (I don't use them).

The advice on the Bentley manual is good. Once you sit down and study the wiring diagrams, it is pretty straightforward what needs to be done.

I also don't use the 16ga yellow wire from the "start" signal. When I had that hooked up the fuel pump would run continuously when the key was on.

The way mine is wired the DME starts sending the +12v to the fuel pump when it sees that the engine is turning (from being cranked by the starter). It does take an extra second of cranking for the engine to fire but that's no biggie to me. Some folks say this is actually a better setup as it allows the engine to briefly build oil pressure before firing.

[cubby911T]

Guys, thanks for all the info - this is good stuff!!

I am planning to keep the '77 fuel pump in service from the stock location up front. Therefore, wouldn't my pump simply continue to function as it always has?

Aside from the fuel pump, what about the coil? Marc's 3.2 conversion mentioned the following:

The black wire (green arrow) connects the 15 terminal of the coil to the #8 fuse on the '89 panel. This is a 16 amp fuse that provides power to the coil upon start up, and while the key is on. On the '77, this wire was connected to the bottom of fuse #3 on the middle block.

[PcarPhil]

Originally Posted by cubby911T

Guys, thanks for all the info - this is good stuff!!

I am planning to keep the '77 fuel pump in service from the stock location up front. Therefore, wouldn't my pump simply continue to function as it always has?

Aside from the fuel pump, what about the coil? Marc's 3.2 conversion mentioned the following:

The black wire (green arrow) connects the 15 terminal of the coil to the #8 fuse on the '89 panel. This is a 16 amp fuse that provides power to the coil upon start up, and while the key is on. On the '77, this wire was connected to the bottom of fuse #3 on the middle block.

The black wires Marc is talking about are shown in my pic above, though I chose a different fuse location (not #3).

[IROC]

Originally Posted by cubby911T

Guys, thanks for all the info - this is good stuff!!

I am planning to keep the '77 fuel pump in service from the stock location up front. Therefore, wouldn't my pump simply continue to function as it always has?

I changed over to a Motronic pump. The CIS pump puts out way too much pressure for Motronic. I had heard of people using the CIS pump along with a pressure regulator, but in the end I went with a Motronic pump. FWIW, I mounted the Motronic pump in the rear where my CIS pump resided and have not had any issues.

Aside from the fuel pump, what about the coil? Marc's 3.2 conversion mentioned the following:

The black wire (green arrow) connects the 15 terminal of the coil to the #8 fuse on the '89 panel. This is a 16 amp fuse that provides power to the coil upon start up, and while the key is on. On the '77, this wire was connected to the bottom of fuse #3 on the middle block.

The important thing with the coil wire is to make sure it is tied into a fuse that sees 12V when the key is in the "run" and "start" positions. Don't get hung up on exact fuse numbers - use a voltmeter and make sure the fuse you tie into sees 12V during "start" and "run" - that's all that matters.

[jazzbass]

I forgot two wires - +12V to the coil from bus 15 and +12V to the DME relay also from 15. I'm in the middle of wiring a 3.6 conversion, so I'm adapting the 3.2 wiring to the DME to the 3.6 version.

Here is a picture of the stock, factory wires that go from the front of the car into the 3.2 DME harness. This is a pic from the actual harness on my 1985 Carrera:



Note:

• Busses: 15 = Run/Start, 30 = Hot always, 50 = Start.
• DMER = DME Relay
• Measurements are wire diameter in square mm.

FWIW, the factory wired the fuel pump into the DME relay so that the DME would turn off the fuel pump in the event of an accident. If the DME doesn't see a speed signal from the flywheel sensor over some threshold (400 rpm I think, but I don't really remember), it cuts the fuel pump. This is why the yellow wire from the starter is there - it overrides the DME's rule of needing speed > 400rpm to turn on the fuel pump. The idea is that is you have an accident (say on the track), the DME will cut fuel when it sees the engine stall. That way, if there is a compromise to the fuel lines, your high pressure Motronic fuel pump doesn't continue to pump gas all over the hot engine while you sit there trying to figure out why the heck you lifted in the middle of that last turn.

[cubby911T]

Guys - I corresponded with the PO of my engine/harness today and he confirmed that the only 2 wires going to his luggage compartment were the one to the battery and the one to the tach. Perhaps the harness has been modified in such a way that the coil is in fact receiving power. Couldn't I test this theory by connecting the battery wire, hooking up the coil, turning my key to the on position and then checking the coil for power?

[IROC]

Originally Posted by cubby911T

Guys - I corresponded with the PO of my engine/harness today and he confirmed that the only 2 wires going to his luggage compartment were the one to the battery and the one to the tach. Perhaps the harness has been modified in such a way that the coil is in fact receiving power. Couldn't I test this theory by connecting the battery wire, hooking up the coil, turning my key to the on position and then checking the coil for power?

Yeah, that would work. I don't know where he was getting his power to the coil, but if it works, it works. It also seems that you have the issue of getting 12V to the DME for the alarm circuit (besides the coil).

I hope it works out for you.

[cubby911T]

Okay, teachable moment for me guys. I bought a digital multimeter a while ago, but I've never used one. I've been reading online about how to test current, but I want to make sure I'm doing this right. I'm trying to determine whether there is current coming into my ignition coil when the key is in the "on" position. I have attached the positive battery lead from the DME harness, but what do I do with the probes on the multimeter (and let's assume that I have the multimeter setup correctly to measure current)? Do I touch them to the coil leads? To the coil posts? Do the leads need to be attached to the coil or can I just deal with the leads only? Help!!

[IROC]

Originally Posted by cubby911T

Okay, teachable moment for me guys. I bought a digital multimeter a while ago, but I've never used one. I've been reading online about how to test current, but I want to make sure I'm doing this right. I'm trying to determine whether there is current coming into my ignition coil when the key is in the "on" position. I have attached the positive battery lead from the DME harness, but what do I do with the probes on the multimeter (and let's assume that I have the multimeter setup correctly to measure current)? Do I touch them to the coil leads? To the coil posts? Do the leads need to be attached to the coil or can I just deal with the leads only? Help!!

Actually, you want to check for voltage and not current. Set your multimeter to "volts DC" or whatever and set the scale to appropriately read 12V if it isn't "autoscaling". Disconnect the wire (the wire you're hoping sees 12V) from the coil and touch (or attach) the + lead of the meter to that wire. Touch the - lead to some known ground point on the car. Switch the ignition to the position you want to check and you should see 12V. Hopefully.

In order to check for current, you have to put the meter in series with the circuit, but typically for what you're doing, you're only looking for voltage, so I wouldn't bother with current.

[scarceller]

This may help:



I have a better .jpg version but to large to put here, PM me if you want it.

[cubby911T]

Sal - PM'd you, thanks.

Mike - thanks for the instructions....I'll try it tonight.

[scarceller]

Originally Posted by cubby911T

Okay, teachable moment for me guys. I bought a digital multimeter a while ago, but I've never used one. I've been reading online about how to test current, but I want to make sure I'm doing this right. I'm trying to determine whether there is current coming into my ignition coil when the key is in the "on" position. I have attached the positive battery lead from the DME harness, but what do I do with the probes on the multimeter (and let's assume that I have the multimeter setup correctly to measure current)? Do I touch them to the coil leads? To the coil posts? Do the leads need to be attached to the coil or can I just deal with the leads only? Help!!

The coil gets it's 12Volts from a fuse when the key is turned to 'RUN' or 'START' - the DME has nothing to do with getting the coil 12V. Now the other side of the coil '-' gnd side gets grounded by the DME. The coil always has 12V when key in 'RUN' or 'START' then the ground side gets grounded by the DME to produce the spark. Hope that makes sence.

So if you wish to see if the DME is commanding spark then place a test lights across the 2 coil terminals and while cranking the test light will flicker.