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Low output voltage on tps.

R

Rushin

NAXJA Forum User
Location
Minneapolis, MN
Ok I thought I start a new thread so the issue is clear.

I am having stuttering when cruising on a highway lightly pressing one the gas.

I decided to adjust the tps and when I measured the input voltage it was at 3.9v which is lower than desired 5. When I measured output voltage it is .18 which is much lower than needed.

I need help figuring out what's wrong. I tried a new tps and the voltage is the same.

In all data testing it says to check for continuity between tps plug and tcu. Where is the tcu plug?
 
Please start your threads with the BASICS--year/engine/trans/t-case/mods that might be related.

A**uming you have an auto trans because you are asking about the TCU. The plug they want you to test to is where the harness attaches to the TCU, which--depending on the year--could be under either the driver's side or the passenger's side of the dash.
 
TCU plug at TPS is the four hole, square plug

ECU plug is the flat, three hole plug

Which plug are you using to get your readings? The two circuits in the TPS are inversely proportional--
--ECU wants roughly 17% of the input voltage with the throttle closed (and increases with throttle input)
--TCU wants roughly 83% of input voltage with throttle closed (and decreases with throttle input)

The 3.9v input in your case (5V preferred) is the first thing I would try to correct-- use the volt meter to probe the input line and ground the meter directly to the engine and verify whats getting to the TPS. 5V is norm, lower is bad connection between ECU and plug, poor grounds at the dipstick tube are common cause of low voltage in the complete circuit when 5V is actually getting to the TPS.

If you've got roughly 5V to the TPS and 3.9V when you're probing the plug, you've identified a bad ground-- clean it up/run an auxilury ground wire.
 
I was testing at the squire plug. So I might have both a ground and a supply issue? What's the best way to start testing the supply for the tps? Just poke the wire and test every few inches? I take it the 5v wire goes to tcu? So when they say test continuity between tcu and 5v tps I just test the appropriate prong between square and and flat plugs? Where is tcu box located itself?

I regrounder the big ground on the back of the firewall. And fiddle with the dipstick ground but that did not fix anuthing. I will try to clean up the dipstick ground even more.
 
Not to hijack this thread but I thought you may be able to help me out...

Shorty said:
TCU plug at TPS is the four hole, square plug

ECU plug is the flat, three hole plug

Which plug are you using to get your readings? The two circuits in the TPS are inversely proportional--
--ECU wants roughly 17% of the input voltage with the throttle closed (and increases with throttle input)
--TCU wants roughly 83% of input voltage with throttle closed (and decreases with throttle input)

The 3.9v input in your case (5V preferred) is the first thing I would try to correct-- use the volt meter to probe the input line and ground the meter directly to the engine and verify whats getting to the TPS. 5V is norm, lower is bad connection between ECU and plug, poor grounds at the dipstick tube are common cause of low voltage in the complete circuit when 5V is actually getting to the TPS.

If you've got roughly 5V to the TPS and 3.9V when you're probing the plug, you've identified a bad ground-- clean it up/run an auxilury ground wire.
Click to expand...

Shorty,

Your statement above about the ECU and TCU voltages prompts me to ask a question concerning my '99 XJ (4.0/AW4) PCM and TCM readouts on my Actron CP9180 scanner. I know I don't have an ECU and TCU but here's my situation:

Assuming module $18 is the PCU and module $D1 is the TCM

With Key On/Engine Off, the data display on my scanner shows TPS outputs as ABSLT TPS ($18) 0.0 % [throttle plate opening 0.0%) and ABSLT TPS (D1) 18.0% (throttle plate opening 18.0%). As the throttle is manually advanced to WOT, the % readings increase to: ($18) 98.8% and ($D1) to 78.8%.

Q: Is it normal to see ($D1) at 18.0% when the throttle plate is in the closed position? I would think it would be 0.0% like the PCM. Is this the "inversely proportional" situation at work?
 
Troubleshooting a 99 TPS is much different than a 90.
 
Shorty has it right. The Renix Jeeps are extremely ground sensitive. Your TPS should read 83% of your input voltage with throttle closed. You should shoot for at least 4.5V input to the TPS. Before you do anything with the TPS make sure your throttle plate is completely closed. Remove, clean, and re-attach all 3 grounds under the hood. Use a calculator to get 83% of your input voltage. If you have, say, 4.5V you should adjust your TPS output to read 3.74V. After you tighten the screws on the TPS, recheck your output in case you moved the TPS while tightening it. My '89 XJ was getting about 4V input until I cleaned all three grounds. It immediately went to 4.74V after cleaning grounds. I adjusted my TPS to 3.93V and it works great. It does not seem like much but it makes a HUGE difference!:yelclap:
 
SIDEWINDER CC: Your Jeep is not a Renix. Renix Jeeps are from 1987-1990. The Renix Jeeps are not an OBD type system, they do not store codes of any kind. Your system is completely different.
 
BigSky89XJ said:
SIDEWINDER CC: Your Jeep is not a Renix. Renix Jeeps are from 1987-1990. The Renix Jeeps are not an OBD type system, they do not store codes of any kind. Your system is completely different.
Click to expand...

Understood, but it looked like Shorty was knowledgeable in the workings of the Engine/Powertrain computer and Transmission computer systems and could make some sense of what I was talking about.
 
Remove, clean, polish and scrape the ground connections at the engine dipstick tube along with the ones at the back of the head and the firewall. Clean and shiny like a baby's hiney.
 
Curious as to your battery's static voltage. Should be about 12.5 volts. If the static voltage is too low then your reference voltage will be low.

Throw a voltmeter on the battery between the terminals and see what you have.

Then do as the other have said and clean all the grounds to bare metal, treat with an anti-corrosive, and make them TIGHT.

If you still have low reference voltage, I would start disconnecting OTHER sensors and see if the reference voltage at the TPS will jump higher.

Good luck.
 
Last edited:
Rushin said:
Will try next week. I am just going to remove the every connector and clean it.
Click to expand...

Are you talking grounds or component connectors in the engine compartment?

If it were me, I'd do both. It's a Renix , you know, and they are super fussy about grounds. The component connectors in the engine compartment are also old and possibly corroded inside. Take s about 15 minutes to unplug, flush with contact cleaner, and plug back in each and every connector you can see under the hood.
 
Rushin,
I don't think anybody ever answered your question as to where the TCU is located. On the 1990 it is under the glove compartment on the passenger side. There are 2 screws that go through the lower edge of the dssh and into the TCU mounting bracket. The TCU is inside a flat sheet metal can about 4" x 6" x 1" thick. The mounting screws are not needed for a TCU ground. You can pull them out and let it hang from the harness while you troubleshoot. After you finish with the grounds, you should go back to your original plan and check the continuity to the TCU plug.
 
Good. Ironically I was just preusing a Factory wiring diagram and found that the C101 connector, the one bolted together up behind the brake booster, carries alot of interesting signals.

TPS to trans
TPS voltage supply
Coolant temp
Air temp
Grounds for Map
O2 sensor heater
CPS ground
Injector grounds
O2 sensor input
Knock sensor
Idle air controller controls

I might have missed a few as it's kind of a bear to read. Just the same, it looks like scraping the tar out of the C101 and flushing both halves of it with contact cleaner might not be a bad idea............It was a factory recommended procedure over 20 years ago. I doubt the connections have improved with age.
 
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