Tps Sensor Vref Voltage Below 5 Volts

Discussion in '1994 - 1995 Specific Tech' started by k00ksta, Jul 1, 2013.


  1. k00ksta

    k00ksta Member

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    Guys I need some help here trying to figure out what is going on with the VREF voltage to my TPS sensor it basically is not getting the full 5volts it needs for proper return voltage to the computer ..

    First off if I test between the 5 volt source (bottom Brown wire) and the Ground wire (top grey wire) I only get 1.73 volts not 5 volts like I should be getting.

    [​IMG]

    So of course my voltages between the Test wire and the Ground are way below (note the picture below) 0.29v closed throttle and 1.08v wide open throttle which should be upwards of 5 volts .. but impossible to achieve when you only have a 1.73 volt REF voltage.

    What can I check on the computer side to see if the computer is actually getting the voltage/ground that it requires to distribute it to other sensors .. maybe test the voltage on the IAC connector for correct voltage? I am unsure what to do with this so any help would greatly be appreciated on tracing down why I have such an under-voltage at my TPS sensor's VREF.

    (First picture closed throttle reading between Ground & Test Wire and the other WOT)
    [​IMG]


    ** Test's all done with KEY ON ENGINE OFF! (KOEO)
     
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  2. k00ksta

    k00ksta Member

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    Ok another question .. If I probe from the VREF (Brown wire) to the battery or engine I do get 5.0 volts but if I try to go from the "ground" top wire to the VREF on the connector I come up short .. what should I look for next here ..
     
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  3. k00ksta

    k00ksta Member

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    Ok so I just took a jumper wire and went from the "GROUND" (TOP WIRE) and ran it directly to the negative post on the battery and then checked the voltage on the Signal wire and the ground wire with the TPS plugged in and now if I crank the TPS up I get it to almost 5v (WOT) and about 0.85v closed .. Also my VREF shows 5 volts as well like this .. so basically this comes down to a bad Ground wire .. Any suggestions on tracing this down as I don't want to just put some spliced ground wire here as their may be other sensors losing out from a lack of ground as well .. Thanks for any suggestions guys!
     
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  4. toyman

    SN Certified Technician

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    First make certain the battery to engine and engine to chassis grounds are solid. Remove the passenger side kick panel and check to be certain the ECU ground to the chassis is good. Back probe pin 20 for the case ground, pin 40 power ground and pin 46 the signal return ground. Check resistance to chassis. Should be close to zer0 ohms.
     
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  5. k00ksta

    k00ksta Member

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    Thanks toyman for the quick response ..

    If I jump the GREY wire" on the TPS to the chassis grounds next to the battery (see pic's below)..

    [​IMG]

    Then did (KOEO) I got the TPS to .98v idle position and 4.75v wide open throttle position.

    Idle/Closed throttle
    [​IMG]

    WOT throttle
    [​IMG]

    So I know this is the issue here and need to nail out the computers/ecu chassis grounds ..
    Ok so pin 20 - 40 I see are grounds but where is 46 the GRY/RED their? or did you mean 26 VREF?

    [​IMG]

    Also is this the ECU chassis grounds here?
    [​IMG]
     
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  6. k00ksta

    k00ksta Member

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    Does anyone know where pin 20, 40+60 and 49 terminate? .. Also has anyone seen 49 as possibly Pink and not Orange as the diagram states above?
     
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  7. toyman

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    Pin 46 is on the lower part of the diagram. 5th pin from the left. That is the ECU ground in the picture. Pins 20,40 and 60 appear to terminate at the chassis . When I get home tonight I'll see if a specific location is identified. Shouldn't really matter so long as the chassis is well grounded. The main chassis ground is from the engine to chassis and will be about a 2 gauge or braided wire. If you don't see one just add one. People keep forgetting that the grounds are the other half of the circuit and all the sensors need solid grounds.
     
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  8. toyman

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    Pin 20 is grounded at the kick panel as shown in your photo. Pins 40 & 60 show the ground location as "LH front of engine compartment". From the illustration it could be in the fender to the right of the battery. It would appear that your issue is the signal return wire pin 46 in the engine control sensor harness, located behind RH side of I/P, near grommet. If you can't trace it back I would be inclined to run another wire in it's place.
     
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  9. k00ksta

    k00ksta Member

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    Thanks toyman I been reading all your posts as I see you been down this road with others .. thanks for the help so far man greatly appreciate it .. I pulled another ground hoping I had a bad chassis ground to the pin 20 "black" that is right next to the ECU their and cleaned the existing ring terminal but no dice getting the same results. I also cleaned all the ground's under the hood on the left and right side of the radiator and double checked my engine to chassis ground which seems to pull a solid 12v from anywhere on the engine.

    Let me ask you this what exactly is the Grey/White (Pin 46) wire all my Haynes manuals state its a "Ground" and it did fix my issues with incorrect voltages (under voltages) at all my sensors when I took a jumper wire to it at the TPS sensor and grounded it to the chassis. As I see it returns to the computer on that Pin 46 does that just pass thru inside the computer to one of the 20/40/60 pin ground's or does the computer need to do something with it I would think .. Any other suggestions to help me trace this thing down?
     
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  10. k00ksta

    k00ksta Member

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    I ohm'd out the Injector Harness (un-plugged it from where it mates to the other harness under the hood near passenger firewall) and everything seems to ohm out just fine. Now when I get to the clutch safety switch which has the grey/red wire their as well before it looks like it leads to the o2 sensors which state they also go to ground .. what grounds the O2 sensors? is their another lead off of them that goes to chassis ground or is the exhaust supposed to ground them? I don't see any ohm's readings if I touch my headers to chassis ground even tho a lot of other spots on the motor appear to be grounded fine with the battery to engine ground cable.

    It actually does not show the O2's get the grey/red wire in the above picture unfortunately like it does in my Haynes manual strange ..
     
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  11. k00ksta

    k00ksta Member

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    Pulled the battery and found this Black/White pair terminated into this connector but its not plugged into anything .. is this supposed to be plugged into something or is this for some tests?

    [​IMG]

    [​IMG]
     
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  12. toyman

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    It's the ground for all the sensors with a 5v signal reference. For example, I'm running an AEM-EMS with a MAP and the MAP sensor ground had to be tapped into pin 46 for the correct signal output voltage. At first I had just used the chassis as the sensor ground and the computer was significantly miss reading the engine load value.

    Pin 20 is the ECU case ground and it is dedicated. Pins 40 & 60 are power grounds and are also dedicated.
     
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  13. toyman

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    Like the other sensors the ground is pin 46.
     
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  14. k00ksta

    k00ksta Member

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    Well I found my issue .. After trying to Ohm/beep out pin 46 to either pin 40 or 60 and getting nothing I said screw it and popped the computers case open ..
    [​IMG]

    Yup .. Traces were literally burnt to hell .. So after cleaning it up I took a jumper wire and completed the circuit to have now Pin 46 Ohm/Beep out across the ground pins on the ECU of 40 and 60. Now I could have just soldered a wire sure but instead I called up Autozone and they actually had one at their hub for 130$ (refurb'd) When I got it I Ohm'd out the pins and they all were good on the new one .. threw it in the car and walla I have 5.0 volts reference at all my sensors and the car starts right up ... :dead: finally...... haha
     
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