My megasquirt 2 conversion looks like it's finally going to happen this summer. I drew this wiring schematic so it is more adapted to my situation than the generic diagrams found at a couple places:
I will be running 4x GM D585 coils in wasted spark mode. Also, I will be controlling an electric fan with the MS ECU.
After drawing all of that, I noticed that I will be running 4 injectors, 4 coils, 1 idle valve and the fuel pumps (dual-pump CIS car) through one 30A relay. I read on this a bit and found out that injectors pull something like 2A each, coils 7.5A and I don't know about the idle valve and the pumps. How do you size the relays and fuses? Are those values peak or RMS? Do I need to size the relays for peak? I saw that the MS manuals put each injector bank on one fuse. If I run a 4 cylinder, can I only use a single fuse? Should I split both ignition coil banks with 2 fuses? How do I determine the fuse amperage?
In reply to Rocambolesque :
Since the injectors aren’t all firing at once, nor the coils, it’s a mistake to simply add everything together and assume that is the peak draw.
I’m afraid you’ll have to plot each components peak draw over time.
With the injectors, we generally size their fuse for about 1.5 amps per injector. Those can all be run at once, at full current draw.
Coil dwell can overlap, but is generally about half their max rated current. I see you're running them in wasted spark - I'd probably fuse that sort of ignition for 20 amps total.
I note that your O2 controller is not wired to the fuel pump relay.
If you read your O2 instructions you will find that they do not want the sensor heater powered up with the motor off.
And you have both the coils and the O2 powered? up by 5vRef
I'm not 100% sure what "dual pump CIS" means. On the cars I mess with, the stock fuel pump is usually adequately supported by the stock circuit, with appropriate wire gauge back to the pump. But just adequately without much headroom.
Uprating the pumps can pull a lot of current.
I usually use the feed from the relay driving the fuel pump and other ignition system parts, but use it to trigger a second relay rather than actually supply the current to the pump.
I wire the second relay with its own current supply line from the alternator/battery bus, using 12 gauge wire, a 30 amp breaker (or fuse, but I like auto reset breakers better for track cars), and a 30 amp relay, and then 14 gauge wire on to the pump, or 12 gauge if there are two pumps in series.
That's maybe $30 in extra parts and a half-hour to crimp up and install. Probably paranoid but I just stress less if I take care of it that way.
Some(all?) of the 80s mercedes running CIS have two fuel pumps.
Thanks everyone for the help. Good call about the O2 sensor. I actually received it the other day and when I read the instructions I saw I made a mistake. I drew it on the printout of the diagram I have here. The sensor is a 14point7 Spartan2 so there's 4 wires + 1 LED for sensor temperature monitoring. I'll add one 5A fuse fed by the fuel pump relay for this O2 sensor.
For the injectors, I had 30# units laying around so I will be using those. They are a little big for the stock-ish engine I will be running; I calculated a duty cycle of 62%. I'll leave the wiring as in my schematic.
For the 4 coils I'll run one 20A fuse.
As for the fuel pumps, I'll play it safe and run an fourth relay as JBasham explained.
That makes a total of 8 fuses and 4 relays. Which is perfect because I have a 8 fuse box and 4 relays on hand!
bentwrench said:
And you have both the coils and the O2 powered? up by 5vRef
Not sure I understand the question. I installed 2x TC4427 in the proto area for the coils.
Thanks!
I have noticed a general tendency to put as big as a 50 amp breaker for the fuel pumps. Especially when using E 85. In turbo charged applications where demand is very high.
On the VWs with similar hardware most of this runs through the single 30A fuel pump fuse, with the only things separate being the feed for engine ecu power (low current) and the coils going to their own ignition switched supply. You can add it all up, but I would think you're still <30a for it all and what you have above is in range.
