So I'm running an AEM 30-0300 hooked up over the canbus. I have the ground ran to the ground just below the brake booster at the moment. Everything has worked perfectly fine for like 2 months now, until randomly last week I noticed that my AFR reading in TS is constantly, rapidly, jumping up and down within 1 point of what my actual gauge is reading. I even tried enabling smoothing and it didn't help at all. It almost seems like it's some form of interference, but I don't know what it could be from. I'm just confused since it worked totally fine for a couple of months, and has just randomly started acting up. The gauge itself reads totally fine and is behaving exactly the same as it always has. Any ideas? I'm not at my laptop right now, otherwise I'd post my tune.

 

Check your wiring.
Did you use twisted pair wiring ?
Is it shielded ?
Did you properly terminate the can bus ?

No word what ECU you're using. The AEM controller has no internal termination resistor.
What resistance can you measure between Can Low and High with ignition / power off ?
Should be 60 Ohm, 120Ohm parallel on both ends of the bus.

 

Oops, apologies. Running an MS3 PNP pro. Car is a 97. I'd have to check the wire I used, but I believe it's twisted and shielded. I will check the resistance when I get the chance. I opted for no resistor because I heard a lot of mixed opinions on whether or not it was necessary on the pnp pro. I'll check that, report back, and then most likely end up having to install a resistor.

 

Okay guys, bear with my potentially stupid question and poor illustration here. I have tried searching where exactly to place the resistor for a wideband and have yet to find a thread or post that actually tells you exactly where to put it, so here's me asking. Which of these two (poorly drawn) diagrams shows the correct placement of the resistor? Or have I somehow gotten neither one correct here?

image 1

image 2

 

image 1, parallel to the bus. resistor legs between can high and can low.

 

It's easy to tell if your CAN bus is properly terminated, and yes, it's important that it is terminated correctly. There are cases where you can get away without termination but they're the exception not the rule.

A properly terminated CAN bus requires exactly two 120Ω resistors, one at each physical end of the bus. With the bus unpowered, measuring resistance between CAN_H and CAN_L should read 60Ω (two 120Ω resistors in parallel).

Measured resistance → Diagnosis
60Ω → Correct, two terminators present
120Ω → Only one terminator present
40Ω or less → More than two terminators present
Open circuit → No terminators connected

Hope that helps!