ECU warm/hot to touch

[Carlsonmj]

I am getting close in the wiring and have everything grounded. When I position the key in the ignition I can hear the fuel pump whirl. Turn to the start position and the starter relay engages properly (turns the starter while turning everything off). I do not have any of the lights or signals connected yet, but most everything else is plugged in. I do have a few connectors that need to be dieted, but was waiting for the first start to make certain of their need.

My concern is that the ECU seems warm/hot to the touch. I disconnected the battery, let it cool, reconnected, did not turn anything, and the ECU got warm again.

Do I need to delete or ground anything further from the ECU that may be causing this?

[STiPWRD]

Did you ground your engine to the frame with the provided FFR strap? Also, what do you mean by "turns the starter while turning everything off"? Is the ECU getting hot without the key even in the ignition?

[iWire]

The ECU does get "warm" but if it gets "hot" that would be bad. You should easily be able to touch it. EDIT - This only applies to when the Key is in the ON position, not when in the OFF position.

As mentioned above, check your grounds.

[Carlsonmj]

Thanks for the replies.

Yes. I have grounded the engine with the strap and sanded off a nice contact point. I have created quite a few grounding points. I was referring to the starter interrupt relay which seems to be doing its job.

"Hot" may be a relative term. It is warm; I can touch it and feel there is heat on one side. Yes, The ECU does get warm without the key. Not enough heat to cook eggs but is noticeably warm.

Thanks again in advance. I really have learned a lot on these forums.

[STiPWRD]

The ECU should definitely not be getting hot/warm without the key, this means it is drawing watts of power whereas it should only be drawing a handful of milliwatts. Perhaps something inside the ECU is causing it to short to ground. Was it working before in the donor? Have you tried hooking up a code reader and communicating with the ECU? This is an easy way to see if it is functioning properly (key must be in the on position).

As far as the starter interrupt relay - are you trying to reuse the factory security system? If not, then there's really no need for this relay (you can short it out). All it does is prevent the starter from running if the security system is enabled. I got rid of this relay on my harness.

[iWire]

The ECU should definitely not be getting hot/warm without the key, this means it is drawing watts of power whereas it should only be drawing a handful of milliwatts. Perhaps something inside the ECU is causing it to short to ground. Was it working before in the donor? Have you tried hooking up a code reader and communicating with the ECU? This is an easy way to see if it is functioning properly (key must be in the on position).

As far as the starter interrupt relay - are you trying to reuse the factory security system? If not, then there's really no need for this relay (you can short it out). All it does is prevent the starter from running if the security system is enabled. I got rid of this relay on my harness.

I didn't catch the last part about that this was happening with the key off. Absolutely it should not be warm with key off!

I have seen in WRXs that do not have the engine plugged in that the ECU will stay on with the key out which would could be similar to what you are seeing. Verify the engine plugs are plugged in and that the engine is grounded to the chassis.

[Carlsonmj]

I am pretty sure the ECU is fried. I took the advice above and plugged in a code reader and got nothing. The ECU was working prior (in the donor).

Is the ECU a plug and play or will it need to be programmed? How much does a harness cost again?!

[STiPWRD]

I am pretty sure the ECU is fried. I took the advice above and plugged in a code reader and got nothing. The ECU was working prior (in the donor).

Is the ECU a plug and play or will it need to be programmed? How much does a harness cost again?!

Sorry to hear about that. The ECU should be plug and play as long as you get one from the same model year you had before. ECU's from a different model year could work but there may be minor differences to the wiring/sensors that may cause issues.

I'd highly suggest you try to pin point what caused the ECU to fail in the first place before putting in a new one. Check all of the 12V power wires running from the fuse box to the ECU and all of the grounds. A blown fuse on one of those power wires could prevent the ECU from turning on and communicating with the code reader (this has happened to me once). Another thing you can try is removing the cover to inspect the actual board, maybe one of the components looks burned inside?

[iWire]

I am pretty sure the ECU is fried. I took the advice above and plugged in a code reader and got nothing. The ECU was working prior (in the donor).

Is the ECU a plug and play or will it need to be programmed? How much does a harness cost again?!

Sorry to hear that! We can definitely build you a fresh harness that won't fry the ECU. The OEM would plug right in and fire up. No programming necessary. Depending on what model and year we may have a replacement ECU here.