355 Exhaust Bypass Valve Solenoid Wiring

I posted about this in my thread about replacing my headers, but wanted it to have its own thread for visibility and later searches.

I’ve narrowed this down to an electrical issue and could use a little help to complete the diagnosis and repair.

A little diagnostic history: I checked all of the vacuum lines and the valve itself and confirmed that all is functional there. The solenoid was reading 55ohms and the connector from the 5.2 Motronic to the solenoid reads 12V to ground on one pin and 6V to ground on the other, which is odd. To verify that the system is working correctly minus the electrical portion, I actuated the solenoid using a 9V battery with the engine running. The bypass valve opened and closed exactly as it should.

So now I’m at the point of figuring out the electrical problem. I understand it is pin 64 (yellow/blue wire) on the ECU that is controlling the solenoid.

My questions: Does the ECU provide 12V to this wire to command the solenoid open? Is the other wire at the connector grounded somewhere on the car or has something been integrated some other way to complete the circuit? Why do we think one wire is receiving 12V and the other 6V to ground (the ground being a bolt on the gearbox housing that holds a bonding strap)?

I won’t be able to look into the issue more for another week or so, but how often do the ECU’s fail on just one thing like this? The car seems to be operating fine otherwise. I’m sure I’ll have more questions as this is discussed, but appreciate any info to get me headed in the right direction when I get my hands on the car again.

 

Great info. This is exactly what I was looking for. As far as the 6V to the blue/yellow wire, I don’t know. I’m not sure which wire at the connector was reading 6V and which was reading 12V, though it would stand to reason that the yellow/blue was the one reading 6V. YelCab suggested a short somewhere, which it very well may be. Now that you’ve given me a better understanding of how the system works, I can chase down the problem more effectively. Just curious, how do you know the startup sequence so well? Is this spelled out in the WSM somewhere? Thanks.

 

I did pull the connector off the ECU the other day and didn’t see anything super obvious there that would be causing any problems. I’d like to avoid disconnecting/reconnecting this too many times, of course. Some pins did have slight corrosion.
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No check engine light. I did get a P1448 when I wired the valve open temporarily, but that was expected and I’ve since cleared it and unwired the valve. I’m kind of surprised because I would have thought the thermocouple would read that the valve is not opening when it is supposed to and throw a light, but I guess it isn’t programmed that way. I’ll check the 14-pin connector when I’m back home. Another thing I noticed while chasing a coolant leak at the thermostat housing is there is some slack in the throttle cable. I need to rectify that, but I can’t imagine it’s enough slack that I’m not meeting the conditions required to command the valve open. Plus that 6V thing doesn’t add up anyway.

 

I had considered attaching a DVM to that wire, but couldn’t think of a good, non-destructive way to do so. I may have to go this route in the end, but maybe I can figure something out another way.

For case 1:
a) Can’t rule out a broken wire completely, but it now seems unlikely given that the voltages are the same as other folks have on their cars.
b) I’ll also say it isn’t ruled out, however, I did test the valve using the 9V battery while the car was idling and it worked fine a number of times in a row. The vacuum was sufficient at idle and I also tested just the vacuum reservoir using a handheld vacuum pump and it held vacuum. So, given that vacuum is sufficient at idle and the reservoir is holding vacuum, this is unlikely to be the problem.
c) The car has a Capristo exhaust and is definitely noticeable when the valve opens. This was the reason I temporarily wired the valve open and drove the car. I wanted to make sure I knew what it was supposed to sound like with the valve open. I can definitively rule this out.

I’d call case 1 very unlikely, but I’ll never say never.

For case 2:
a) I had considered this, but have no solid way to test it at the moment. Going by memory, I think the ECU needs to see at least 85% throttle to command the bypass valve open? There is a fair bit of slack in my throttle cable that I will correct next week, but I doubt it’s enough to cause this. It would be great if it was this simple, so we’ll see.
b) The speedo and tachometer are working correctly. Is it safe to assume that if those gauges are working correctly, then the ECU is getting that signal?

It seems like I am headed down the road of needing some sort of more advanced diagnostic tool that can read TPS values, so I’ll start researching my best options. I’m open to suggestions here.

Thanks for the suggestions here. With this community, I think this problem will get solved one way or another.

 

The problem existed prior to the header install, but I just didn’t realize it. I had never wired the bypass valve open to check the sound levels until after the headers went in. After I installed the headers, I had this feeling that the car still just wasn’t as loud as it should be. During one random drive after the header install, I heard the valve open when flooring the car in 2nd gear. I looked at my wife in the passenger seat and said “that’s what it’s supposed to sound like!” It was after that I wired the bypass valve open to confirm what I’d heard. So maybe this adds to the mystery that the valve has opened a single random time since I’ve owned the car, but the issue existed before and after the header installation. I did do the headers myself.

 

The speedo is working and I did get a suspension light the other day from starting the car a few times on the lift and not moving it. As soon as I pulled the car off the lift, the suspension light went out. Seems like good confirmation that the speedo signal is being received. The engine and tach are working, so I think we’re good on the crank sensor. As I said in another reply, I think I’ll need to invest in a good scanner very soon. Suggestions on a scanner? My OBDII port seems to be functional since I can view and clear codes using a simple scanner. Not sure if the more advanced scanner would be hooked up there or if it hooks up to the 3-pin connector by the ECU, but I should be in good shape with either connection point.

 

Ok, this is going to be a stupid question, but what else is vacuum used for on these cars? I won’t rule out vacuum, but assumed (probably wrongly) that if I had sufficient vacuum to open the valve at idle and a reservoir/accumulator that holds vacuum, then I should still have enough vacuum remaining in the reservoir to open the valve during hard acceleration after a stop where the car is idling for a bit.

I’m currently over analyzing and hunting for the “perfect” scanner so I can read TPS values.

 

Secondary air system sounds like it is working properly. I can clearly hear when it shuts off. I always let the car warm up prior to any revving, and generally the coolant is to operating temp prior to even leaving the garage. Best I can tell, the engine behaves very well when warmed up.

 

Ok, so I hooked up the OBDLink MX+ and got some interesting readings for TPS. I pushed it to the floor and was getting less than 70%. I pushed harder, to the point it almost felt like I was going to push a hole in the floor and only got to ~75%, but that means the limiting factor was the floor and there was probably more throttle travel available without the floor in the way.

So, the question is, is this reading accurate for TPS absolute throttle position? If so, either the slack in the throttle cable is worse than I thought, or the TPS itself is bad, or a combo of both.
Does this look normal?

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