I'm looking at places to mount my MS and got to thinking. Modern PCM's are in the engine bay, this makes wiring and troubleshooting the harness pretty damn easy if you ask me. Can the MS handle engine bay temps?

This is on a 2nd gen rx7, and my plan was to either put it where the stock air box used to be, or where the charcoal canister used to be, kind of in a corner out of the way. I was going to build a container/holder for the MS (box) and possibly put some insulation in it to keep the heat out. Basically have it entirely contained and just have the harness coming out of the box.

Would this be okay?

 

No, the megasquirt chip can NOT handle under-hood temps:

From http://www.msefi.com/msinfo/manual/mwire.htm

"With the relay kit, you still have to run a cable from the relay box under the hood to the MegaSquirt unit (which cannot be located under the hood), but then you have a nice terminal block for all of the engine wiring."

That is from the megasquirt manual.

Also, not all modern cars put the ECU under the hood. My last two daily drivers (and '01 Alero, and an '05 Vibe) locate the computer in the passenger compartment.

The reason you can't mount it under the hood can be found here:

http://www.msefi.com/msinfo/ms2/index.htm

There's a chart a little more than half way down the page which has the temperature range of the megasquirt processor, it can only go up to 85C or 185F. Temperatures under the hood can get a lot higher than that when sitting in traffic for example.

You might be able to get it to work if you do proper heat sheilding however... but then you also have to deal with things like water and such... the megasquirt case (even before modification for rx7's) isn't water-tight.

My opinion is that you should just stick to what Bowling and Grippo (megasquirt's designers) say and mount somewhere in the passenger compartment.

 

Thanks for the links and info! I'll read up on all of it.

 

dDuB - Just stop by any comp store online, and hit up a DB37 cable, and you only have to snake 1 cable through the firewall. The Relay board makes your life really easy, too, all of the connections to the engine can connect there.. Mine is in a high-performance racing-spec tupperwear container

 

Keep in mind that if you get the relay board, and you want to control the BAC valve, you have to jumper the spot on the relay board where the fidle relay would normally have gone.

Also keep in mind that NONE of the ignition stuff goes through the relay board... all of that has to go directly to the megasquirt.

 

oh? That's bloody annoying, isn't it? Ah well, it would have been too easy the other way.

 

yeah, the relay board has only what's needed to run the original megasquirt code in fuel-only mode.... so where it'd be useful, it's no longer the end-all solution.

 

Please keep in mind that DB37 cable is generally for logic signals and is rarely larger than 22-24 gauge. This will not be optimal for carrying injector grounds and the like.

-Mike

 

Well the standard MS board uses two DB37 pins for each bank, thus we get one pin per injector. I believe those pins are rated at 1-5 amps depending upon the source. I think the connectors can normally take up to 20 gauge wire.

From the MegaManual: "For the wires from the DB37 connector to the sensors, injectors, etc., use 18 to 20 gauge for all the connections, then bring them to a common 14-12 gauge where appropriate. The only big wires are the ground and the two injector driver pairs, 32-33 and 34-35, all the rest can be 18 gauge all the way out."

Say each injector circuit has a resistance of 12 ohms, we should get 1 amp of current at 100% duty cycle, at 80% duty it would be about 0.85 amps.

Are you saying using the standard MS connections will be a problem?

You're definitely right that we need to be careful about wire gauges--in addition to amp draw, and good connections.

Scott

 

Scott, I should have been clearer. You can purchase cable for DB37 connectors that is usually 24 gauge wire - when I say "cable" I'm referring to the intervening multiconductor cable. The DB37 connectors themselves are fine with the currents we are carrying, as you've noted. If we suppose a common scenario, let's say, 4 2.2 ohm injectors split among 2 driver channels, we are now drawing 12.5 amps during injector open ("peak" of "peak and hold") as a result of 1.1 ohms per channel (2.2/2, ohms per injector, 2 injectors paralleled) and a drive voltage of 13.8 volts. Of course, this is assuming peak/hold operation and that current flow is just for the open period; the PWM period will have much less average current. My entire point was to make sure that people realize that the injector and coil drive lines need to be large enough to handle expected transient current peaks. Otherwise excessive voltage drop leading to slow or no injector opening or overheated/failed conductors may result. Multiconductor data cable won't be sufficient in any case.

-Mike

Edit: I just wrote what you wrote! Sorry...

 

Mike,

Yea I see where you're coming from, talking about the standard premade cable setups--mybad.

Scott