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1G 90 No voltage to TPS

caldixiechick

Probationary Member
17
7
Nov 8, 2021
Shingle Springs, California
My 90 Laser is very hard to start when cold (sitting overnight) and is throwing CEL code 14. When it finally starts it runs and idles just fine and will start normally until it’s cold again. The TPS was bad, so I found a good one, replaced it and adjusted it based on the info in the factory and Haynes book. After leaving the battery disconnected overnight to charge it, CEL 14 is still coming up. Further testing and there’s no voltage on the harness side of the TPS plug. According to the books it’s supposed to have 5 volts. I thought that the idle position sensor might be bad but then why would it idle fine?

What I’m trying to figure out is there something between the harness plug and the ECU, or where the wires go from the harness plug. I’m not finding a lot of info on this circuit in the wiring diagrams and I’m new at reading them. I have the 90 specific factory manuals and Haynes.

I’m hoping someone has run into the same issue and can share how they fixed it. I think my next step is figuring out if the circuit in the ECU is damaged? Or I’m totally barking up the wrong tree.

Fortunately the wiring in my car hasn’t been hacked up by previous owners which I’m hoping will make it easier to find the problem. I’ll add that I replaced the coolant temperature sensor since that’s supposed to be a common problem with hard starting. From what I can tell my car is stock.
 

caldixiechick

Probationary Member
17
7
Nov 8, 2021
Shingle Springs, California
With an ohm meter. I’m trying to learn how this works so I’m sure I’m making mistakes. My last project was a 77 Dodge Power Wagon. That was years ago, carburetor, no electronics.
 

steve

DSM Wiseman
14,447
1,390
Feb 3, 2002
St. Charles, Illinois
I don't have my 1990 Factory Service Manuals handy so the best I can offer is from the 91 versions which is different in the type of parts but the same in process. If I recall correctly a 14 fault code can be from the TPS or the IPS. I'll have to look at the ECU code again to see what the fault conditions are.

The first picture is from the 1990 DSM Technical Manual and depicts a 90 Throttle Body.

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The following are from the later service manuals.

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The +5v Sensor Power for the TPS is shared with other sensors and comes from ECU pin 23. The Sensor Ground pins are 17 and 24 and they are connected together inside the ECU.

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steve

DSM Wiseman
14,447
1,390
Feb 3, 2002
St. Charles, Illinois
I'll have to look at the ECU code again to see what the fault conditions are.

The TPS checks look to see if the TPS output voltage is below 4% or over 40% and the IPS isn't opened yet. At idle the the TPS voltage should be around 0.5V and the IPS closed (pulling the signal from the ECU to ground) so the fault conditions are pretty broad and represent a gross problem.
 

caldixiechick

Probationary Member
17
7
Nov 8, 2021
Shingle Springs, California
Thanks so much for the info Steve. I was able to find the corresponding pages in my 90 service manual to match the 91 pages. Very similar, and thanks for finding the ECU page too. I've got my work cut out for me. Aside from it taking forever to start and the CEL on, I wouldn't know anything is wrong because the car runs fine.

Can you tell me if I'm reading this right? On the first page 14-19 the diagram in the upper right corner shows 5 volts full open, as in the throttle full open? The volts only register from the ECU with the throttle open? Does the key need to be in the "on" position or the car running? I was working off the Haynes manual to do this test and it says to have the key on (car not running) and doesn't say anything about the throttle being open or closed.

I really appreciate your time and advice. My son is helping me with this project, but electrical isn't his strong suit.
 

steve

DSM Wiseman
14,447
1,390
Feb 3, 2002
St. Charles, Illinois
Can you tell me if I'm reading this right? On the first page 14-19 the diagram in the upper right corner shows 5 volts full open, as in the throttle full open? The volts only register from the ECU with the throttle open?
Does the key need to be in the "on" position or the car running? I was working off the Haynes manual to do this test and it says to have the key on (car not running) and doesn't say anything about the throttle being open or closed.

The TPS is a classic adjustable resistor (potentiometer) used as a voltage divider. One end of it is connected to ground and the other end to a voltage source, in this case roughly 5v from the green/red wire on ECU pin 23 (+5v Sensor Power). The ECU generates this voltage when it's powered up by the the Ignition switch being in the RUN or START position. (key on). The voltage in the middle changes as the wiper moves across the resistor being lower the closer to the ground end and higher closer to the powered end.

With the TPS unplugged. Looking at the 4 pin connector from the harness side you should see 5v on the green/red wire (Sensor Power), 12v on the green wire (IPS) and nothing on green/black (Ground) or the green/white wire (TPS).

Measuring resistance (ohms) on the connector attached to the TB you should see something between 3.5k ohms and 6.5k ohms between pins 2 and 3 and that value shouldn't change. Measuring between pins 2 and 4 or 3 and 4 the resistance should vary smoothly as you turn the throttle pulley and you'll note that one way the resistance goes up and the other way the resistance goes down as you turn it.

With the TPS plugged in the voltage (key on, car off) on green/red wire should continue to be roughly 5v, the voltage on green/black wire still 0v but the voltage on the green/white wire will vary with the position of the throttle pulley. At idle (Throttle Closed) it should read about 0.5v and increase smoothly to almost 5v at WOT. This it the TPS voltage that the ECU sees on it's pin 19.

You might ask, what about the green wire that goes to the Idle Position Switch that has 12v on it? When the throttle is closed the IPS grounds this wire so the ECU sees 0v and once the throttle opens slightly the voltage goes back to 12v telling the ECU that the throttle has opened.

Sorry for being long, I'm trying to be as precise and clear as I can since the discussion isn't interactive.

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1990TSIAWDTALON

Moderator
9,127
4,962
Nov 14, 2013
Independence, Kansas
We couldn't ask for a better explanation!!!
 

caldixiechick

Probationary Member
17
7
Nov 8, 2021
Shingle Springs, California
Steve, I can't thank you enough for you taking the time to give me such a detailed explanation. I'll go through this tomorrow and post an update when I'm done.
 

caldixiechick

Probationary Member
17
7
Nov 8, 2021
Shingle Springs, California
I was reminded today to never assume someone before me didn't foul something up. Following Steve's awesome instructions, things weren't matching up. Long story short, some nitwit plugged the TPS into a 4 pin harness with two red wires with blue stripe and two black wires with yellow stripe. Took me a while to find the matching pieces and correct the problem. Car started after only 3 attempts, yea!! CEL came back on so we'll see what it's not happy with another day. Thank you again for your help.
 

caldixiechick

Probationary Member
17
7
Nov 8, 2021
Shingle Springs, California
Thanks Brian. That's interesting because on my list of things to fix is to get the car to start with the key. A previous owner wired in a button that starts it. I haven't had a chance to dig into why yet. This issue and why the back up light circuit keeps blowing a fuse are going to be their own car day.
 
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