Piggyback power

[Dan_Gyoba]

I'm looking at adding a couple of things to my '90 ECU, and was curious as to the best strategy for power.

I don't want to overload the fuse to the TCCS, and I'm having some trouble finding power requirements for things. On the other hand, I really want to avoid relying on an otherwise noncritical fuse for the correct operation of my engine electronics.

So what I have are an Apexi AVC-R, an Apexi Neo, and a Zeitronix wideband/EGT/datalogger.

There are 4 places that get +12V at the TCCS. BAT is constant power, so that's out. IGSW gets power directly from the switch (7.5A IGN fuse), +B and +B1 get power through the main EFI relay. (15A EFI fuse) These are in parallel, so it doesn't matter which of these I use, if I do.

Alternately, I could run an ignition power line from somewhere else to things.

The Apexi units are quite particular about their ground requirements (2 wires must be attached to the same ground wire on the ECU wiring harness) but a little less specific about their power requirements, aside from that it be ignition power (As opposed to constant, or accessory power.) The included wiring diagrams seem to indicate that they be connected to +B, but both the AVC-R are of course diagrammed as though they are the only additional units. The Zeitronix wiring diagram doesn't specify if power or ground should be the same as ECU power.

It may be more convenient to use +B1 for one and +B for another unit. The TSRM and TEWD indicate that these are tied together after the EFI main relay, so I can't imagine that it should matter which I use, if either is suitable.

Apexi diagrams want to connect to E1 (Computer ground) instead of E2 (Sensor ground) This makes sense, but so does a good chassis ground. I suppose that using +B and E1 ensures that you get the same voltage references as the TCCS at all times.

Anyway, advise and information appreciated.

 

[grimreaper]

apexi neo to B+ and E1.

Run the wideband off another circuit. The fuel pump puts enough load on B+ as is. Widebands can pull an amp or two depending on the manufacture. Looks like your setup will require a bit more. The zt2 shows a 4a reset-able fuse according to the instructions.

In JB#2 there is an additional port coming off the efi main relay, connector 2F, pin 3. Can be useful to keep things in control of the efi main relay. http://www.cygnusx1.net/Supra/Library/TEWD/MK3/manual.aspx?S=Main&P=19

 

[Dan_Gyoba]

Well, but that's still coming off of the main EFI relay, same as +B and +B1 unless that were used to trigger yet another relay (Not that I'm totally averse to this, it just seems inconvenient, and I'd rather do this closer to the ECU if this is how it's going to be.)

If anything, I'd rather re-wire the fuel pump to run off of its own circuit.

So If I work from the diagram at TEWD Page 46 the B-R wire should be coming from the main EFI relay. If I interrupt that at pin #2 of the circuit opening relay in relay block #4 (Or at connector B1/Pin 9) and use it to close a relay to fused battery power, that should effectively remove the load of the fuel pump, but still have it controlled by the main EFI relay. the diagrams aren't clear as to when that circuit branches off, but given that it's between the connector to JB2 and RB4, and RB4 is so close to the ECU, I'd hazard a guess that it's close to the TCCS.

The other option is to interrupt the L-B wire at pin #3 of the fuel control relay (Probably just bypass the whole thing here for constant +12V anyway, once the AFPR is installed and the J tube is bypassed) and use a relay to switch +12V to the fuel pump. (Or supply the power from a relay using that additional trigger in JB2.)

All in all though it would have been nice to have found an elegant solution not involving another relay.

Guess I ended up answering my own question. Mostly, I couldn't find a decent reference as to how much power these pieces draw. I just don't want to overload the factory circuits with aftermarket stuff.

 

[grimreaper]

Dan, its true the load would remain on the efi main relay circuit. When i rebuilt the harness I found several power circuits that utilized the parallel wire from a series source layout. At the time, I foolishly thought nothing of it. Thankfully I only noted ignoring this on the B+ circuit.

Essentially you have 3 resistors. One in series feeding two in parallel. With parallel circuits, voltage is constant but current can be varied across each path. The first 6 inches to 1 foot of 14/16ga wire out of the efi relay T's off to 2 14/16ga wires to make the 8-10 foot run to the ecu. The first section of wire can handle a much larger load with out a noticeable voltage drop because of its length (less resistance). The parallel runs simulate a larger diameter wire to maintain a low voltage drop and still handle the load needed for the rest of the run.

So adding another path/wire/run at the efi main relay should not negatively affect the original B+ circuit. The power supplied to the efi main via the alt is more then capable of handling additional load. You may consider adjusting the fuse value. I believe Jetjock stated some time back that fuse ratings are for the wire, not the load.

Adding an additional load at the ecu side of B+ will certainly increase the voltage drop though. The best solution will be to get it from the source ( assuming a high load). The tsrm electrical diagrams are not very accurate in splice locations on the actual circuit. Take the abs light blue power feed from the 60a fusible link. That is a single 3mm wire that branches into 2 3mm wires after about a foot.

I know I could have said series into parallel and you would have understood, figured others with less understanding might make use of the drawn out version. Plus it helps me solidify my thoughts and understanding as well. Feel free to add critiques or details.