Caution on repurposing outputs on relay board

[JPSeuropa]

Back when I started fooling around with Megasquirt on my Europa, I thought I would use the unused fidle output on the relay board for my second ignition output. The relay was jumpered and I thought all was fine. However, the polyfuse mounted on the relay board that was in series with the signal traces can cause issues when out for a long drive once things get fully heat soaked. It will cause the spark to drop out at anything much above an idle speed. It does allow enough switching to let the engine idle normally though. I ran into this on both of my Megasquirted cars as the wiring schemes were similar. It was a giant PITA trying to figure out why the car ran great around town, but would crap out on a trip longer than 20 miles in warm weather. Having it idle normally did not help figure out the problem either... I am much happier when things that don't work stay not working until actually fixed. I picked up the issue once I connected a scope to the ignition triggers and noticed that the one with the polyfuse had a very short pulse once heat soaked...basically a spike rather than the square wave showing on the other channel. The signal looked normal until it got to the relay board which narrowed things down for me. Of course at that point I had pulled the ECU out of the car, disturbed wiring, interior carpeting, etc. Also, the signal looked normal before things got hot. 3 inches of wire jumping around the polyfuse made the problem go away.

[DaveEFI]

I'm confused here. The Fidle relay normally supplies +12v or ground, according to the jumper?

[JPSeuropa]

I am not using it as fidle. No soldered in jumper on board, no relay. It has been repurposed at the relay board as an ignition trigger by connecting pin 36 at the Megasquirt to pin 30 at the relay board. In my original configuration, I just connected the input and output of the relay together with a jumper wire. By doing that, I ended up with the polyfuse in series between pin 30 and the fidle connection. That was a blunder. I now have pin 30 directly connected to the connection labled "fidle", but it is actually "spark A". I am sure I am not the only user that considers this sort of thing to simplify wiring. All I am pointing out is that if you have the polyfuse in a circuit that does not call for it, it might cause issues. It did for me.
I hope this is more clear.

[jsmcortina]

Best really is to avoid the relay board altogether.

James

[DaveEFI]

Polyfuses sometimes get used for loudspeaker protection, so it's not that they're DC only. Not sure about the different varieties, though. So my guess it is must be getting hot.
But I agree with Jim. The relay board is an old design really meant for early very basic installations. I've never used one as I don't like the idea of adding extra connectors on principle.

But your findings are interesting. Did you try changing the polyfuse to see it it was possibly faulty?

[rickb794]

The title of this thread should be "another reason to not use a relay board"

[JPSeuropa]

Polyfuses sometimes get used for loudspeaker protection, so it's not that they're DC only. Not sure about the different varieties, though. So my guess it is must be getting hot.
But I agree with Jim. The relay board is an old design really meant for early very basic installations. I've never used one as I don't like the idea of adding extra connectors on principle.

But your findings are interesting. Did you try changing the polyfuse to see it it was possibly faulty?

No, I just jumpered around it. Problem gone. Heat is definitely the issue.
...and yes, I would never use the relay board again. On the other hand, I am not going to pull one out of a working car.

[DaveEFI]

Just me, but I'd have been curious to find out just what the polyfuse was doing.

[JPSeuropa]

This exact same failure occurred on both of my megasquirted cars...and it drove me completely crazy finding it the first time as it is an intermittent issue. I have the relay boards mounted in the engine compartment in both. The problem never showed up in cool weather, strictly a warm weather problem. It took about 45 minutes for the problem to show up and if the car was left to cool completely, you would have about 45 minutes of running time to hi-tail it back home. If I had know what was going on the first time this happened I could have saved myself a flatbed fee with an ice cube. Looking at the specs for these types of devices I can see that they typically recommend that the maximum operating temperature is in the 85 to 90°C range. If the relay board were mounted somewhere other than the engine room, then I probably never would have experienced the issue.