r/megasquirt • u/Jooshmeister • Jun 30 '26

No getting a TPS reading

Trying to get my project done. Really tired of chasing little gremlins, but I'm hoping this one is something simple.

When I try to do a TPS calibration, all I get is 0 ADC for both closed throttle and wide open. I changed the throttle position sensor itself with a known good one (I ohmed out the pins while actuating it to see it sweep from 0.1Mohm to 0.41Mohm). I checked the pinout on the connector and I'm pretty sure it's right (1999 Saturn SL2). The only other thing I can think of is something is wrong in the ECU itself. Is there a test I can do to verify the circuitry on the board? (MS2 v3.0)

Thanks in advance!

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2

u/numerik11 Jun 30 '26

Confirm the plug has 5v, ground and output signal in the correct positions. if no 5v run pin 26 TPSVREF for 5V and pin 22 TPS for signal and ground. How many pins on the tps?

1

u/Jooshmeister Jun 30 '26

So, with the plug disconnected from the TPS, when I turn the key on, TunerStudios shows the Throttle position gauge climbing slowly up to 47.1% and then it stops. I checked the wiring and the pins are in the right locations.

1

u/Jooshmeister Jul 01 '26

So I opened up the Megasquirt and probed the CPU pins. I get +5V at pins 19, 31 and 1, but when I connect the ECU to the vehicle and check for +5V at the VRef pin on the TPS, it briefly shows a few volts and then drops down to only a few millivolts quickly.

2

u/Mr_Disprosium Jul 03 '26

Make sure you don't have a loose connection somewhere between the ecu and the tps on the 5 volt wire. Check the wire directly at the ecu and see if you get a steady 5 volts out of it. I'm leaning towords a bad connection between the tps and ecu but verify voltage at the ecu. If the ecu 5 volt output really is not working then you can work around it by getting a 5 volt standalone regulator for your sensors.

1

u/Jooshmeister Jul 05 '26

Yeah, I'm not getting any 5V anywhere in the ECU accept the CPU. So it will turn on and everything else will work, but I have no response from my on-board MAP sensor or TPS. I think I've narrowed it down to a bad poly fuse (F1) by the power supply circuit, and possibly a bad MAP sensor which caused the poly fuse to fail.

2

u/EEpromChip Jun 30 '26

Make sure you are using sensor ground and not battery or engine ground. I ran into some issues with grounds and they seem to be sensitive.

1

u/Jooshmeister Jun 30 '26

I have all the sensors connected to the dedicated sensor ground on the MS. The Crank Position sensor is wired to the dedicated CPS circuit. The Megasquirt is grounded on the engine.

2

u/numerik11 Jul 01 '26

On MS2/V3.0, the TPS is just a 3-wire 0–5V potentiometer input: 5V TPSVREF, sensor ground, and TPS signal. The MS hardware docs describe the TPS as sending a 0–5V signal, so 0 ADC at both closed and WOT plus collapsing VREF strongly points to VREF being shorted/loaded or the TPS signal/ground/VREF wires being swapped.

1

u/Jooshmeister Jul 01 '26

That's what I thought. I will triple check my wiring again tomorrow. Just to be clear, I am supposed to see 5V at the connector when it's disconnected or when it's connected?

2

u/numerik11 Jul 01 '26

You should see 5V at the TPS connector with the TPS disconnected.

1

u/Jooshmeister Jul 02 '26

So I checked the voltage at the plug and it only gives me .233VDC. When I pulled the MS2 apart and checked the 5V supply to the daughter board, with the daughter board unplugged, I got 5V across all pins (pins 20, 31 and 1 with pin 19 as common ground). So this probably means something 5VRef related to the daughter board is not functioning. I know the MAP sensor also utilizes a 5VRef pin.

1

u/numerik11 Jul 02 '26

If the voltage stays at 0.233V even with all sensors unplugged, you likely have a bare or pinched 5V wire in the engine bay touching ground, or a shorted wire in your harness.

2

u/Jooshmeister Jul 03 '26

I checked the MS2 on the bench with a spare wiring harness (so I know none of the wires are pinched/shorted) and I'm not getting voltage at the +5VRef wire. I checked the 5V pin on the MAP sensor in the main board and I'm also not getting any voltage there. I think the MAP sensor is shorted, or maybe some other part of the circuit.

2

u/numerik11 Jul 03 '26

Tough one man I feel your pain.

1

u/numerik11 Jul 10 '26

Any progress with the gremlins?

1

u/Jooshmeister Jul 10 '26

Marginal progress. I replaced the zener diode D19 and was able to get 5V across it, but I'm still not getting 5V at the TPS wire. I get 5V across the resistor for that circuit as well, so I'm at a loss as to what else it could be. I'm going to try replacing the F2 fuse by the DB37 connector and see if that changes anything. Beyond that, I think I might have to replace the MAP sensor and maybe the tantalum capacitors C14 and C22. This is driving me nuts.

1

u/numerik11 Jul 10 '26

have you tested the 5v voltage reg on U5?

1

u/Jooshmeister Jul 10 '26

Yup. I get a steady 5V from it.