What triggers the VSV to turn on?

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Still working on the EGR issue and found my VSV to be stuck open. I replaced the VSV with a new one and it is still not closing. I tested the old one and it appears to work just fine. I supose the nex step is to figure out why it is not getting juice when it is supposed to. Does anyone know what is supposed to trigger this thing? Thanks in advance for your suggestions.
 

MoGas

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Looks like differential pressure/vacuum between the intake before the throttle plate and the vacuum from the EGR modulator and an electrical signal from the ECU. This is what I gather from my 93 FSM.




Hope this helps,
Dave
 

Proven

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Some of this is thinking outloud that I happened to type. Please sift through this, but I thought it was worth typing out.


The output from the ECM controls the VSV(EGR) the input to the ECM that causes that output is from the following sensors:

1) The Engine Coolant Temp Signal Circuit
2) The Throttle Position Signal Circuit
3) The Battery Signal Circuit



Well...We assume that the Battery Signal is happening...because well..nothing would work would it? if it wasnt.

The coolant temp circuit monitors the temperature of the coolant.

The throttle position signal circuit = Sends a OFF idle signal, and an ON idle signal.

So....from my score I see two down, and one left. The TPS (throttle position sensor).

Now my opinion (READ THAT...I said OPINION) is one of the pins could be lose or corroded? or you have a harness issue. Trace the wires around and see if you find wear points or melting.

It is possible that the wire (ground) going from the VSV(EGR) is damaged. The ECU causes the ground. This means that there is power to the valve at all times on the positive side, but it is when the ground is caused by the ECM that you get the actuation (stops the vacuum flow).

So, you have another hunt through the engine harness. The VSV(EGR) according to the 1996 electrical manual has a ground wire with a BLUE wire with a WHITE stripe. The positive side is a YELLOW wire with a RED stripe. Pay attention that the VSV(FUEL PRESSURE) has a BLUE wire with a RED stripe.

So, my suggestion if you follow my mental flow is that you should be now looking for loose connections, or damage to the harness. The ECU grounds the VSV's to actuate them. With power it is CLOSED and without power it is OPEN.


:popcorn:
 
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Thanks a lot for the input. I am now leaning towards the TPS. All wiring and connectors look good. I need to decide if I am going to keep throwing parts at it or take it to the stealership.
 

landtank

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the water temp sensor is on the front of the engine under the intake manifold. I'm not sure which one as there are a few up there.

Come to think of it, I think a CEL would come on if it was faulty. I'll have to check that.
 
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It is th smaller one on the left of the oil filter.
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I got out my ohm meter and followed Proven's train of thought. I found the blue with white and yellow with red wires connected to a plug up by the throttle body. One end does run down to the vsv and some other vacuum switch towards the front of the engine. I checked the resistance on the side of the plug that runs down to the VSV and it was at 1.3K ohms, which exactly matches the resistance of the old VSV so I assume that end of the wiring is OK. The Red with Yellow stripe wire IS hot so that is OK. The Blue with White stripe is not grounding. It appears to run down into the center of the engine below the intake. My issue appears to be why the blue with white stripe wire not grounding. Not sure where to go from here. BTW, I marked the TPS and losened the screws and moved it while the engine was running. The idle speed did change as I moved it so I am thinking that the TPS is doing something. Found a very comprehensive write-up of the Toyota EGR sysem at http://www.autoshop101.com/forms/h61.pdf. From the diagram on Page 3, it appears that Coolant Temp, Vacuum Sensor, and Distributor all feed the ECM to determine whether to turn on the VSV. I unplugged the temp sensor and temp gauge quit so I think that is OK. So now I think I need to find out which vacuum line feeds the ECM and how the Distributor is used.
 
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Resurrection this as to see if you found anything out. I have a similiar issue with my '93 LC.

My VSV for EGR seems to not be closing when the vehicle is warm and I throttle up to 2,500 rpms. It passes all the bench testing requirements (resistance between connectors, no continuity between connectors and body, closes with 12 volts). It also closes when I ground the connection at the ECU.
This leads me to believe the ECU is not getting all of the inputs needed to tell it to close the VSV. What conditions are needed to close this connect?

This is what I think: coolant temperature and throttle position. The temperature gage is working and I bench tested the throttle position sensor and it worked out.

Any ideas? I replaced the EGR modulator and the EGR valve works because it kills the truck when I apply intake vacuum.
Thanks!
DT
 

Gumby

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The VSV should be completely ECM controlled based on the following criteria

The engine must be warm - ECT signal must be correct
The vehicle is moving at cruising speed - VSS must be working
The vehicle is at at least partial throttle opening, but not WOT. TPS must be working.

It may well take in engine load information from the manifold pressure sensor and/or MAF and it may well calculate run time and not open it until the engine has been running for more than a set amount of time.

If you can find a table of OBDii monitor enable criteria it would tell you for sure what you have to do to get it to work.
 
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Thanks for input. Truck is OBDI and i don't have a scanner. FSM states to check VSV function when warm and at 2500 rpms by attaching a vacuum gage between EGR and EGR modulator. Should see some vacuum indicating the VSV has closed.
Maybe I should route the vacuum gage to the cab and get real time info...
Thanks
 

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