Samdog

I'm thinking of converting my race car to a simple 8 wire electrical system and doing away with all of the old harnesses for reliability and simplicity.

The chassis is an '89 but I have a 2.5L '86 engine, engine harness, and DME in the car. I did nothing to modify the wiring when the 2.5L went in so the chassis harnesses are still '89.

I've looked at the wiring diagrams and searched around but I can't find a source that says specifically where the DME gets power and what it takes to run the car. The DME will be stock '86 and I intend to leave most of the engine harness as is (some sensors were removed for aftermarket gauges).

What connection do I need to make to get power to the DME? It looks like I need to feed battery voltage to the coil on the positive side and pin 1 of the DME tells the negative side when to fire. DME Pin 4 (Pin 7 at the DME connector) seems to be the start signal to ECU as it feeds off the wire to the starter solenoid. Is all that correct?

I'm unclear as to what DME pins 20, 35, and 18 (DME connector pins 5 & 3) do. They feed back from the DME to the DME relay but I'm not clear of their function.

Help is MUCH appreciated!

Wipeout

What we did with our '83 ChumpCar and with forum member schwank's '87 race car was to cut off the keyed ignition and wire the kill switch and push button start off those wires. That would be the easier way to go, I'd think (though I'm not sure how you're doing it currently)...but would mean keeping the existing wiring in there/building a new harness for that piece.

Samdog

Thank you but I'm really looking to do away with all the stock wiring. I thought some more about tonight while looking at the car and I think the only place power could be supplied to the DME is through the plug just behind the fuse box that you have to remove when removing the engine harness.

I think I can figure it out but I was hoping to pull from someone's experience before I dig in.

Thanks!

Will Feather

I am in the same boat as you. I want to rewire my car but don't have very much know how. I think the DME relay gives power to the DME and the fuel pump once the key is tuned on. My biggest fear is not knowing how to wire the fuel pump and DME if I try to rid the car of the DME relay, and also the other issues it may cause with starting and the reference sensors.

Samdog

When you take the engine out of the car, there are two connections that have to come off in order for the engine harness to come out of the chassis. Those connections are 1. DME and 2. Connector behind the fuse box near the brake master cylinder (this is on a late car).

There is no power going to the DME from the engine harness so it has to be getting power from that connector near the master cylinder. My theory is that I can use a multimeter/test light to figure out what pin is doing what for each key position.

Bri Bro

Power does come from the block by the brake master cylinder. That block is the long rectangular block to the right of the DME in the picture.

Pin 4 of the DME comes from the ignition switch and its function is to start the fuel pump, injectors while cranking. It does this by grounding pin 20 of the DME which turns on the second DME relay. This is ONLY when cranking. After the RPMs go above 200, the DME processor holds pin 20 of the DME low.

Pin 20 is an output (active low) from the DME and controls the second of the two relays in the DME relay. This second DME relay supplies power to the injectors and fuel pump and it is turned off by the DME if the RPMs drop below 200 (accident).

Pins 18 and 35 are power inputs to the DME and they come from the "first relay" in the DME relay. The first DME relay is controlled by the ignition switch and the alarm module.

The coil power comes directly from the ignition switch. The other side of the coil is a pulsed (active low) signal from the DME pin 1. Note it also goes through the connector by the brake master cylinder, pin 1.

If you need schematics, look here at vol 4&5:http://www.the944.com/manuals.htm

Bri Bro

DME connector pin descriptions from a Turbo. Most of the pins share the same function with the N/A

Samdog

Thank you Brian. That should be very helpful. Also, I found a diagram of the DME relay on Clarks-garage.com but I can't post it because my work access won't let that page come up

I should be able to put all of this together and actually pull this off at this point.

More to come...

Bri Bro

DME Relay:http://the944.com/dmerelay.htm

Bri Bro

For just getting power to the DME and the engine components, something like this should work. I am assuming you pull the DME relay and tap into the DME relay wires that connect to most of the the engine components. Use a relay something like this one.
http://skycraftsurplus.com/boschstyl...40ampspdt.aspx

Getting all the tail, marker and headlight to work will take some effort. The schematic link in the post above should help in that effort. Good luck.

Samdog

Brian,

I couldn't open your attachment (might be a work thing) but I'll spell out below what I'm thinking.

I don't need any lights except brakes but that is easy and a separate fused circuit. Still need fans, wipers, etc too.

I'm only concerned here with what it would take to run the engine.

The connector I was referring to earlier behind the master cylinder is Connection T21 which is the 14 pin connector shown in your diagram above coming into the DME. My n/a car is a little different than your turbo diagram based on the 89 wiring diagram I'm looking at.

It looks like power to the DME comes through T21 via Pin 3 which splits into DME Pins 35 & 18. Power to T21 comes from the DME relay when you turn the ignition switch to "on" energizing relay contact 86. Power goes to the DME from relay contact 87 which also sends power to the injectors through T21 Pin 2.

The DME relay also sends power to the fuel pump and O2 sensor heater via relay contact 87b. The DME controls activation through the ground side of that circuit inside the relay via relay contact 85b through T21 Pin 5 to DME Pin 20.

The start signal comes from the ignition switch through T21 Pin 7 to DME Pin 4. The same signal goes to the starter solenoid.

Ignition is controlled by the ignition switch powering the positive side of the coil when the ignition is switched on and the DME firing the negative side from DME Pin 1 through T21 Pin 1.

So….to run the engine in a race car, in the simplest terms you need two on/off toggle switches and a momentary contact switch. You could use just one on/off switch but I’d rather have separate ignition and fuel pump switches. Of course the circuits should be relayed but I won’t go into that.

I would probably retain connector T21 on the DME side rather than hacking into the DME harness so here is my new proposed system:

Ignition On/Off switch will control:

Power to the DME through T21 Pin 3
Power to the positive side of the coil
Power to the injectors through T21 Pin 2

Momentary switch will control:

Power to the starter solenoid
Start signal to the DME through T21 Pin 7

Secondary On/Off switch will control:

Power to the Fuel Pump
Power to the 02 sensor heater through T21 Pin 4

The negative side coil wire from the “back” side of T21 pin 1 must be maintained to the coil.

I think that would do it unless I’m missing something.

Samdog

I'm also thinking of changine the alternator output so that it only sends power to the battery, i.e. the car won't run without the battery in the circuit. Anyone see an issue with that?

schwank

Just be sure that you know the details of the rule set you want to race your 944 under. For instance, 944 Cup requires an original unmodified factory wiring harness for the ECU. It would very much suck to build your own harness only to have it make you technically illegal.

Samdog

This change would not affect the factory DME harness. The DME harness remains untouched. This actually would make it much more obvious to inspect for cheaters as there would be few wires to inspect and trace. I believe that both Cup and Spec make provisions for removing all unused wiring in the car and this falls under that provision in my opinion. It is purely a reliability move and would have no affect on performance except maybe to pull a little weight out.

I've seen several Cup cars with aftermarket fuse boxes.

Bri Bro

I popped open the 89 944 schematic when I got home and it is quite a bit different and a lot easier to read. Looks like you are on top of this. I also would use T21 as much as I could. I have a couple of suggestions.

I would make the first switch switch power to all components that require switched power, like:
-The second switch which controls the engine electric power
- Inside gauges and lights
- Brake light
- Alternator regulator
- DME power

The second switch could control the injectors, fuel pump, idle controller, O2 sensor etc. I would connect the relay ground connection to pin 20 of the DME on the relay that this switch controls. This insures that the fuel pump is cut off if the engine dies, very important if you get knocked silly in a accident and you have a fire.

I would not use the wire coming into DME pin 4 to engage the starter. The gauge going to the starter is 4mm, it is taped inside the fuse box to a 0.5mm which goes to T21 pin 7. A 0.5 mm wire isn't large enough to carry the current. It would be easy to run a new wire to the starter solenoid .