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Low WOT TPS Voltage

K

kstopp

NAXJA Forum User
Location
Vancouver, BC
I'm tracking down a long crank time issue. All voltages below are measured with engine off, key on.

Idle Voltage 0.752V
WOT Voltage 3.608V
Reference Voltage 4.96V

I checked those voltages above with a digital multimeter and checked with an analog meter to verify the sweep looks good with no flat spots or jumps.

I was expecting a higher WOT voltage, is this normal?
 
Year, model, engine? I'll agree that that reading seems low, but kinda hard to know without that info...

Next thing I would do is start doing voltage drop tests on the wires between the TPS and the PCM. One of my instructors LOVED to show us what would happen to TPS signal voltage if you had high resistance on the signal return/ground wire...

Also- might want to be careful what you stick analog meters on. They're low-impedance and, apparently, can fry electronic circuits. Same thing with most circuit test lights.
 
The TPS is mounted on the throttle body. The TPS is a variable resistor that provides the Powertrain Control Module (PCM) with an input signal (voltage) that represents throttle blade position. The sensor is connected to the throttle blade shaft. As the position of the throttle blade changes, the resistance of the TPS changes. Along with inputs from other sensors, the PCM uses the TPS input to determine current engine operating conditions. In response to engine operating conditions, the PCM will adjust fuel injector pulse width and ignition timing.

The PCM supplies approximately 5 volts to the TPS. The TPS output voltage (input signal to the PCM) represents the throttle blade position. The PCM receives an input signal voltage from the TPS. It is best to use an analog meter (not digital) to see if the transition from idle to WOT is smooth with no dead spots. With your meter set for volts, put the black probe on a good ground like your negative battery terminal. With the key on, engine not running, test with the red probe of your meter (install a paper clip into the back of the plug of the TPS) to see which wire has the 5 volts. This will vary in an approximate range of from .25 volts at minimum throttle opening (idle), to 4.8 volts at WOT wide open throttle.

Perform the test procedure again and wiggle and/or tap on the TPS while you watch the meter. If you notice it not meeting the target values, any flat spots, or abrupt changes in the meter readings, replace the TPS.
 
Last edited:
JeepNoob

Sorry, forgot to fill in my profile here. 93 XJ with the 4.0L.

Tim_MN and dan1977p

Thanks for the info. That is the way I checked my TPS sensor. The FSM isn't very specific about it being required to reach 4.8V at WOT but I'm thinking it should be higher than what I'm measuring. Does anyone have reference values from their XJ?
 
Checked Mitchell Prodemand's service manual. All it's telling me is that output voltage should be greater than .2 volts at idle and less than 4.8 volts at WOT...
 
JeepNoob said:
Year, model, engine? I'll agree that that reading seems low, but kinda hard to know without that info...

Next thing I would do is start doing voltage drop tests on the wires between the TPS and the PCM. One of my instructors LOVED to show us what would happen to TPS signal voltage if you had high resistance on the signal return/ground wire...

Also- might want to be careful what you stick analog meters on. They're low-impedance and, apparently, can fry electronic circuits. Same thing with most circuit test lights.
Click to expand...

To my knowledge the few analog meters left around are nearly as old as me, cost about $100, and they are high impedance, about 20,000 ohms per volt. Never, ever heard that story before and I have been doing electronics DIY work since the days of tubes and WWII ham radios and TVs.

The standard tool for testing a TPS with power on or off to test for noise during the sweep is a high impedance analog multimeter.
 
Nice write up Tim!!!! I thought the WOT voltage was about 78 or 87% (I forget which) of the input voltage? Is that only true for the Renix TPS, 87-90 years?

Tim_MN said:
The TPS is mounted on the throttle body. The TPS is a variable resistor that provides the Powertrain Control Module (PCM) with an input signal (voltage) that represents throttle blade position. The sensor is connected to the throttle blade shaft. As the position of the throttle blade changes, the resistance of the TPS changes. Along with inputs from other sensors, the PCM uses the TPS input to determine current engine operating conditions. In response to engine operating conditions, the PCM will adjust fuel injector pulse width and ignition timing.

The PCM supplies approximately 5 volts to the TPS. The TPS output voltage (input signal to the PCM) represents the throttle blade position. The PCM receives an input signal voltage from the TPS. It is best to use an analog meter (not digital) to see if the transition from idle to WOT is smooth with no dead spots. With your meter set for volts, put the black probe on a good ground like your negative battery terminal. With the key on, engine not running, test with the red probe of your meter (install a paper clip into the back of the plug of the TPS) to see which wire has the 5 volts. This will vary in an approximate range of from .25 volts at minimum throttle opening (idle), to 4.8 volts at WOT wide open throttle.

Perform the test procedure again and wiggle and/or tap on the TPS while you watch the meter. If you notice it not meeting the target values, any flat spots, or abrupt changes in the meter readings, replace the TPS.
Click to expand...
 
dan1977p said:
For a long crank, though, I would be checking the battery and cables first and then probably the CPS.
Click to expand...

I agree, the CPS for sure.
 
I found this "You should have 5 volts going into the Throttle Position Sensor (TPS). It is best to use an analog meter (not digital) to see if the transition from idle to WOT is smooth with no dead spots. With your meter set for volts, put the black probe on a good ground like your negative battery terminal. With the key on, engine not running, test with the red probe of your meter (install a paper clip into the back of the plug of the TPS) to see which wire has the 5 volts. At idle, TPS output voltage should be greater than .26 volts but less than .95 volts. Move the throttle and look for smooth meter response up to the 4.49 at WOT. The other wire will be the ground and should show no voltage.

Perform the test procedure again and wiggle and/or tap on the TPS while you watch the meter. If you notice any flat spots or abrupt changes in the meter readings, replace the TPS."

Which sounds about right to me assuming a good ground and 5.0 volts input at the TPS

http://www.xjtalk.com/archive/index.php/t-9714.html
 
Ecomike said:
I agree, the CPS for sure.
Click to expand...

I actually replaced mine with a Mopar unit about 6 months ago. It solved the problem I was having before. Should be good to go for another 20 years I'd hope.

It starts up quick when warm, but a cold crank takes a few seconds longer than normal. Fuel pressure is good, doesn't leak down. Doing the poor man prime doesn't help it start up any faster.
 
I know low-impedance circuit test lights are a definite no-no on electronic circuits (one of my very first automotive instructors told us we should collectively throw ours away lol.) I could've SWORN several ASE-certified instructors said analog voltmeters were a no-no as well, but I could be wrong... I'm still in contact with a few, I'll ask around...
 
3.6 is fine, I think mine puts out about the same. The computer gradually learns wot voltages, so I wouldn't worry about this.

Testing a sensor like this is ok with an analog meter, but if you were to ever jump certain pins in some ways, it is possible le to fry computer drivers. This is why most new dmms are 10 mega ohm impedance
 
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