Won't idle down
Fleet Owner
Joined: Oct 2005
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From: Kentucky
If the idle returns to normal by unplugging the IAC then one of two things is the cause. First is a stuck IAC, but you already replaced it. The second is a sensor telling the PCM to kick up the idle. I suggest you check for codes since the latter is happening
New User
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From: socal
I'm not sure if your 302 is efi but i had a similar problem with my 90 460 efi I changed throttle position sensor and it fixed my problem it's like a 20$ part and I haven't had the problem since hopefully that'll help you
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Freshman User
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Thanks. My Shop manual shows how to remove and reinstall the TPS, but the adjustment procedure is found in the Engine/Emission diagnosis manual. Is adjustment difficult? Should I replace the TPS before trying to find any vacuum leaks? My check engine light does not come on so I'm guessing now the vacuum may be OK as nothing seems to be wrong except for the neutral idling problem.
Fleet Owner
Joined: Oct 2005
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From: Kentucky
Let's re-cap this situation. You already replaced the IAC without testing anything ($$). So now you want to do it again with a TPS ($$$)? Invest in a cheap meter then test the components you already have. Many times the aftermarket "equivalents" introduce new problems.
In the event you do need to replace the TPS there is no need for adjustment. The old wive's tale of adjusting it to 0.97-0.99 VDC at closed throttle is just that...a fairy tale. If the closed throttle value is in the 0.6 to 1.0 VDC range leave it alone.
In Post#16 I stated if the idle returns to normal with the IAC unplugged then you have an electronic issue, not vacuum. That was assuming you did replace the IAC already.
In the event you do need to replace the TPS there is no need for adjustment. The old wive's tale of adjusting it to 0.97-0.99 VDC at closed throttle is just that...a fairy tale. If the closed throttle value is in the 0.6 to 1.0 VDC range leave it alone.
In Post#16 I stated if the idle returns to normal with the IAC unplugged then you have an electronic issue, not vacuum. That was assuming you did replace the IAC already.
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OK will do. My barn is like an oven so it may take a day or two. First I will test idle with idle air actuator disconnected. I could take off the new valve and return it, but I figure after 24 years the old one might be ready to fail anyway.
Do I merely take the TPS connector off and measure the voltage with the ignition switch on or does the engine have to be running?
Thanks again for all the help.
Do I merely take the TPS connector off and measure the voltage with the ignition switch on or does the engine have to be running?
Thanks again for all the help.
Fleet Owner
Joined: Oct 2005
Posts: 20,785
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From: Kentucky
Post #1 in this thread: Ultimate Throttle Position Sensor (TPS) Thread - FSB Forums
There is a bunch of chatter throughout the write-up but the best place to start is the info in Post #1
I prefer to test the TPS with the Key On Engine Off then measure the voltage with the TPS connected.
There is a bunch of chatter throughout the write-up but the best place to start is the info in Post #1
I prefer to test the TPS with the Key On Engine Off then measure the voltage with the TPS connected.
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OK I got back at it for a few minutes. I unplugged the IAC and now the only way I can keep it running in idle is to give it a bit of gas. So I guess that means I don't have a vacuum issue.
The TPS is behind a bunch of hoses, but I can see it has three different color wires (red, green, and black?) going into the connector. So now should I grind the probes on my voltmeter to needle-sharp so I can penetrate the insulation to measure the voltage while the TPS is plugged in? Which color wires do I probe across to get the 0.6-1.0 that rla5000 says is normal?
The TPS is behind a bunch of hoses, but I can see it has three different color wires (red, green, and black?) going into the connector. So now should I grind the probes on my voltmeter to needle-sharp so I can penetrate the insulation to measure the voltage while the TPS is plugged in? Which color wires do I probe across to get the 0.6-1.0 that rla5000 says is normal?
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Fleet Owner
Joined: Oct 2005
Posts: 20,785
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From: Kentucky
Motorcraft TPS Wire Colors
VREF - Orange
SIG RTN - Black
TP SIG - Green
Measure the TPS output between the Black and Green wires. VREF should be close to 5.00VDC (+/- 3%) whenever the key is on.
Some other helpful info : Fuel Injection Technical Library » Throttle Position Sensor (TPS)
VREF - Orange
SIG RTN - Black
TP SIG - Green
Measure the TPS output between the Black and Green wires. VREF should be close to 5.00VDC (+/- 3%) whenever the key is on.
Some other helpful info : Fuel Injection Technical Library » Throttle Position Sensor (TPS)
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While waiting for O'reilly's to open tomorrow I checked all the fuses in the two blocks under the dash. All good.
High idle problem occurs both in Park and Neutral.
I'll email Ian a pic of the diode he was wondering about.
High idle problem occurs both in Park and Neutral.
I'll email Ian a pic of the diode he was wondering about.
Fleet Owner
Joined: Oct 2005
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From: Kentucky
No way the signal output can be greater than the reference signal (VREF). Recheck your readings. Please take a look at the plug diagrams I listed at OldFuelInjection.com.
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OK I will try again after I hook up to O'reilly's machine tomorrow.
I sharpened the probes on my voltmeter so I could penetrate the insulation on the three wires on the TPS just behind the plug. Is there a better way? You said the TPS must be connected no? I'll go back and look at the plug diagrams. According the the chart, 1.58 volt signal should give me >2K RPM or am I reading it wrong?
I took a picture of that diode that Lead Head wanted to see, but could not contact him.
I sharpened the probes on my voltmeter so I could penetrate the insulation on the three wires on the TPS just behind the plug. Is there a better way? You said the TPS must be connected no? I'll go back and look at the plug diagrams. According the the chart, 1.58 volt signal should give me >2K RPM or am I reading it wrong?
I took a picture of that diode that Lead Head wanted to see, but could not contact him.
Lead Driver
Joined: Jul 2007
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From: Rhode Island
TPS has nothing to do with engine speed per say. It's purely throttle position.
I'm guessing you were able to probe the TPS Signal properly, but you didn't quite get through the insulation for V-Ref.
I'm guessing you were able to probe the TPS Signal properly, but you didn't quite get through the insulation for V-Ref.