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Black/Yellow should be grounded. Your black lead should be on that.
The red lead should be on the Green/Yellow.
Neither of the two numbers would make sense. It may have been rotated out of bounds.
Can you keep the multimeter connected and see if you can get to a 0.65v reading while rotating the TPS with the engine off?
Ah, I took readings off the two of them separately, with a ground on a metal bit in the engine bay. I’ll do it properly tomorrow, red lead on the Green wire, black lead on the Black wire.
Thank you both for your help tonight, it’s been an eventful session of testing. 10 o’ clock now, time for me to eat something! I’ll be back with more tomorrow.
1. Nevermind the voltage reading from the TPS. It was 0.60v when new, and from the factory. Over time, as things get gunked up, the ECU learns and adjusts the expected base idle voltage, so blindly setting the TPS to a specified voltage is fruitless. The TPS reset is simply a program that tells the ECU to accept whatever voltage is currently being seen at idle. If it was ever done before, that 0.60v would be null. Everytime the TPS is reset, it basically creates a brand new baseline for the ECU, so you don't have to ever worry about messing up future TPS resets, whether you do it by a device, or manually.
You can either have the TPS reset by a device, or attempt to manually reposition the TPS so that the idle is somewhere between 700-720rpm, warm.
2. My throttle body does not have a idle screw, so I can't speak to this.
3. According to my XJS wiring diagrams, it shows Coolant Temp should read 1.2V at 72F, 0.65V at 190F
i did not think any x300 had idle screws.
i thought idle was controlled by the IACV only which is inplace of an idle screw adjustment and is controlled by the ecu in part (amongst other things) due to readings of the TPS, coolant temp sensor,
Idle speed should also be affected by rich fuel mixture ( also effected by ECT sensor triggering open / closed loop engine regulation ) which will also effect exhaust emmisions , unburnt hydrocarbons as most prevalent
His ECT sensor is in question ( test readings ) and has failed emissions in opening post
ECT should be a PID ( parameter ID ) option to configure the device to as an important / common item on all engines to see
STFT ( mixture ) should be one to look at also after confirming closed loop ( 88 C at ECT )
But he did pull his idle in range and there may be overcompensating considerations to ariving at the correct idle speed
He can test command the IAC to the apparent proper idle position but then lose it and back to square one
To bad the IAC does not have a position sensor , but you can read the command signal as 8 volts dc on all 4 IAC wires ( I'm not square on why that but what paper says )
he could always get the IACV stepper motor set to the correct position to obtain the correct idle and then leave it unplugged.
the car will then idle at the correct speed.
just might then have some difficulties with cold start and need to keep the accel down on a cold start to maintain enough idle.
treat the iacv as one would treat an old school idle adjustment screw
The IACV is not the problem. It has been tested successfully.
I believe the TPS is the issue. I'd like tobywood to get the OBD2 to read that the TPS is somwhere around 12.6-12.9% at idle. Hopefully that would correspond to a voltage reading of about 0.63v or something thereabout.
Other suspects continue to be the:
1. Coolant Temp Sensor. Should be 0.41v at 90C, and rising with coolant temperature rising.
2. MAF Sensor. Should be 1.2v at idle and increases with RPM increase, according to the wiring diagrams.
3. Oxygen Sensors. Not really any good way to test that they are reporting correctly/efficiently. As long as they are swinging constantly, that would mean they are working, but could be lazy, or just failing.
The IACV is not the problem. It has been tested successfully.
I believe the TPS is the issue. I'd like tobywood to get the OBD2 to read that the TPS is somwhere around 12.6-12.9% at idle. Hopefully that would correspond to a voltage reading of about 0.63v or something thereabout.
Other suspects continue to be the:
1. Coolant Temp Sensor. Should be 0.41v at 90C, and rising with coolant temperature rising.
2. MAF Sensor. Should be 12v at idle and increases with RPM increase, according to the wiring diagrams.
3. Oxygen Sensors. Not really any good way to test that they are reporting correctly/efficiently. As long as they are swinging constantly, that would mean they are working, but could be lazy, or just failing.
I’ll try and get the throttle position to 12.9% at warm idle, as it’s about the only thing my OBD will actually tell me! It’s hard to tell when moving the TPS doesn’t do anything to the idle speed.
CTS is still top of the shopping list. So far I’ve only read resistance, do you read voltage across the two terminals or is it one of the terminals and an external ground?
Similar question for the MAF, how do you read the voltage?
O2 sensors are highly suspect, as my emissions are so out of whack. Also, I had to replace my downpipe recently so the O2 sensors could have been damaged during the removal and reinstallation process.
So this is my shopping list at the moment in descending order of how much I want to buy them:
CTS
Thermostat
O2 sensors x 2
TPS
I can buy a used TPS for £99, or a used throttle body complete with TPS and ISCV for £90. I’ll probably get that.
I’ll try and get the throttle position to 12.9% at warm idle, as it’s about the only thing my OBD will actually tell me! It’s hard to tell when moving the TPS doesn’t do anything to the idle speed. Once you get the OBD2 to show you that the TPS is in the upper 12% range, I'd like you to use your multimeter to read the voltage between the Black/Green wire and the Green/Yellow wire. You do not need the engine on for this.
CTS is still top of the shopping list. So far I’ve only read resistance, do you read voltage across the two terminals or is it one of the terminals and an external ground? External ground should work too, but lets read it using your multimeter between the two wires. Should be the Blue/Yellow wire and the Black/Brown wire. You will need the engine on for this test. Check the value cold, and then watch it rise as the engine warms up.
Similar question for the MAF, how do you read the voltage? You will need to attach the multimeter leads to the Green/Pink wire and Pink/Grey wire. Should show 1.2v with engine on, and should go up with a foot on the pedal, raising the RPM. Perhaps I can find a chart on that. Engine will need to be on for this as well. Does not matter if the engine is hot or cold, so do this after the coolant test.
O2 sensors are highly suspect, as my emissions are so out of whack. Also, I had to replace my downpipe recently so the O2 sensors could have been damaged during the removal and reinstallation process. This might move the probability of having something to do with this issue...but changing the sensors may not solve it. Either way, I'm surprised you didn't swap out these sensors with the downpipe. They aren't meant to last forever. I wonder if it's worth your while to see if the values are constantly changing. Will your OBD2 show you this? Engine would need to be hot for this.
So this is my shopping list at the moment in descending order of how much I want to buy them:
CTS - cheap and easy, no reason not to.
Thermostat - not sure why this is on the list. Thermostat will not affect idle, even if its bad.
O2 sensors x 2 - probably a good thing to change anyways.
TPS - replacing these will still require a reset. Very costly part with no known alternate/substitute.
I can buy a used TPS for £99, or a used throttle body complete with TPS and ISCV for £90. I’ll probably get that. If I were you, I would rather spend that money on a chinese knockoff Mongoose on eBay that would let you reset your TPS. I would use this device sparingly as they don't seem to last long, but should be available on eBay for less than $100. If you came across one that was an actual Mongoose by Drew Technologies brand, that would be a good investment. Furthermore, if you came across a VCM (not VCM2) that would be great too. Rotunda by Ford is the one you want, I have no experience with the chinese knockoffs, but they might last longer than the fake Mongoose? Lastly if you came across a WDS, PDU or some similar device, you would become "Master of Your Domain", allowing you to reset TPS to your hearts content.
Buying a new/used TPS will not solve the problem. The value of the TPS is tied to the ECU, so unless you bought that TPS, on the throttle body, with the ECU it was used with, there's no guarantee anything would be different.
CTS or ECT is read as resistance across both wires
I do have a chart for that as well, but I think it can be read both ways, voltage or resistance (in kilo ohms) as the engine coolant temperature is determined by the ECM by a change in resistance within the sensor. The ECM applies 5 volts to the sensor and monitors the voltage across the pins to detect the varying resistance.
Buying a new/used TPS will not solve the problem. The value of the TPS is tied to the ECU, so unless you bought that TPS, on the throttle body, with the ECU it was used with, there's no guarantee anything would be different.
True , but if absolutely needed which hasn't been proved to be the case
1. Google backprobing wires. You can also stick a pin or needle into the back of the harness connection and then you have a point where a multimeter can see what is happening.
2. The TPS is bad. It can be salvageable, but it certainly explains why we're having all the trouble we are having. Someone was certainly in here before making manual adjustments!!!
Throttle position is now at 12.9%, currently idling at 1400.
What would your next suggestions be? Time to start buying replacements?
Tweaked idle screw, managed to get it down to around 1000.
Now I would like to know what the voltage is between the two wires I mentioned previously on the TPS.
I would also like to know if the idle remains high after a car restart like you tried before.
What I'm going to ask you to do next is to SLIGHTLY rotate the TPS so that it moves towards 12.5% and see if the idle drops...it might take a restart of the car.
I am assuming the engine is warm? Because the next thing I'm going to ask you to do is take a voltage reading of the Coolant Temp Sensor wires.
06-05-2024, 03:46 PM
