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Testing the Oil Temp Sensor - last of the dash gremlins!

Discussion in 'Mondial' started by Albert Penello, Mar 23, 2020.

  1. Albert Penello

    Albert Penello Karting

    Jul 21, 2019
    141
    Woodinviille, WA
    Full Name:
    Albert J Penello, Jr.
    Ok folks here goes on the last of my dash gremlins (for now anyway!)

    All the gauges work in my '85 QV except for the Oil Temp. I have replaced the thermistor with a PN# 24618025 from Superformance, as well as a new gauge itself and nothing changed.

    Dash lights work (so I have power and ground on the Yellow terminal), and I have 12v at the Green/White (+) wire. That only leaves the Brown wire which *appears* to be a straight shot to the thermistor itself according to the diagrams.

    If I ground the brown terminal at the gauge, the needle pegs. That would suggest the gauge is working.

    I have tried to ground the brown wire at the engine side but nothing happens. Re-reading some posts it sounds like it can be hard to find good ground down there so I may try again.

    Should there be some sort of power reading coming off that brown wire? I haven't tried a voltmeter yet but I'm wondering what kind of signal I should be expecting off the back of the gauge in order to test the brown wire. I presume it has some sort of (+) power coming from it.

    Thanks again!
     
  2. theunissenguido

    theunissenguido Formula 3
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    Jan 21, 2004
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    Guido
    Sometimes a little tap with you finger to the gauge (with a warm engine) can let it work again...sometimes !
    Is the bulb working in that gauge ? If all other gauges are working, then you have + .
    What do you mean with "so I have power and ground on the Yellow terminal" ? Do you have power at the bulb and you have ground on the other side of that bulb.
    Are you shure + and brown wires arent switched at the gauge ?
    There is a big connector C5 with most of the wires going to the back. This gives sometimes bad contact problems.
    Guido
     
  3. Albert Penello

    Albert Penello Karting

    Jul 21, 2019
    141
    Woodinviille, WA
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    Albert J Penello, Jr.
    LOL I will try the tap!

    I have power on the Yellow wire which is for the lights. So yes I have power at the bulb and ground on the other side since the dash lights are working. There are black wires which are at the top post (to provide ground for the lights)

    There is a (+) post where the green/white wires go. There is 12v at this post.

    There is another post with no label where the brown wire was connected.

    So I'm pretty sure the wires aren't crossed.

    Where is this C5 connector located? I can check it and make sure. Thank you!
     
  4. theunissenguido

    theunissenguido Formula 3
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    C5 (I think) is on the left side (driver side) where the red mark is on this drawing. At a 3.0 QV is a round connector...3.2 I'm not shure.
    Guido
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  5. Albert Penello

    Albert Penello Karting

    Jul 21, 2019
    141
    Woodinviille, WA
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    Albert J Penello, Jr.
    Perfect I have a 3.0 QV I will look!
     
  6. theunissenguido

    theunissenguido Formula 3
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    #6 theunissenguido, Mar 23, 2020
    Last edited: Mar 23, 2020
    You can also check ground for the gauge. Maybe the connection between ground bulb and gauge is faulty.
    There must be an other connector somewhere in the dash where the brown wire is connected.
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    Guido
     
  7. BillyD

    BillyD Formula 3
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    Feb 28, 2004
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    Bill
    I’m guessing a bad connection at one of the harness connectors. Which one is most likely the hardest one to access.
     
  8. Albert Penello

    Albert Penello Karting

    Jul 21, 2019
    141
    Woodinviille, WA
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    Albert J Penello, Jr.
    Welp, the fix for this would literally never have been guessed. AND, I'm actually wondering if this gauge ever worked or if this happened at the factory...

    So using Guido's suggestion to look at engine side connectors, I started doing some poking around. All the connections looked good but I could not find any continuity in the pins between the gauge and the harness. (Guido, FWIW that C5 round pin is for the upper intake sensors not the thermistor as it turns out. But C5 got a good clean and Deoxit anyway!)

    There are two bulkhead connectors in the car behind the Ignition coils. I was able to trace the connection from the Thermistor to the harness on the brown pin, but on the bulkhead side, I was unable to get the gauge to move by grounding. So the problem is from the bulkhead connector to the gauge.

    So off comes the wheel, the undershields, etc. so I can see the bulkhead connector from behind. I pull the connections, find the brown pin, ground it and BINGO the gauge pegs.

    WHAT THE HELL. Why is the gauge pegging from behind the bulkhead connector but not the front.

    Well, after an embarrassing amount of head scratching and looking at the pinouts - I realized there is an unused hole right below the brown pin on the bulkhead connector. I look at the harness side and the unused hole is above the brown pin on the harness.

    You gotta be kidding me. The pins are swapped.

    Now, I know this seems obvious but keep in mind the bulkhead connectors are behind and underneath the fender, so all my probing is sort of done upside down with a flashlight reaching in. You don't have an awesome view so you can't tell there isn't actually a pin inside there from the engine side. And of course looking up at it on the backside it's not immediately obvious since you're looking for loose or broken wires.

    What's super, super strange about this is that the wiring itself looks undisturbed - like I"m the first one to ever touch it. No sign it's been messed with and certainly no sign either of the harnesses have been replaced. I'm sure it must have been at some point, unless this mistake was made at the factory and this gauge never worked. The circumstances under which one side of the harness was re-pinned, and this one specifically, I can't figure out.

    So there you have it. I haven't taken it for a drive to determine if the new Thermistor works, but at this point I'm certain the wiring is all connected so it should be fine.

    Thought I'd share!
     
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  9. theunissenguido

    theunissenguido Formula 3
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    Because the oll temp only rises after several minutes, nobody has ever looked at it, I believe. So mistake from factory looks most probably in this case.
    Happy to reed all sorted out...for now. Just like my car, its a full time job to keep our babys on the road.:p
    Guido
     
  10. Rifledriver

    Rifledriver Two Time F1 World Champ

    Apr 29, 2004
    26,874
    Austin TX
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    Brian Crall
    During that period of time we saw a lot of electrical issues like that with the harnesses. More common was the wire was not pushed into the plug far enough to lock in place. Sounds like an assembly issue that should have been resolved during one of the several phases of QA that are supposed to happen at the factory, FNA in NJ and the dealer.
     
  11. theunissenguido

    theunissenguido Formula 3
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    Albert, did you have a look at the big connector from the battery cable to starter/Alternator ?
    The copper pins can have oxidation what result in bad starting or no start at all. This connector is placed under the C5 connector behind/under a protection plate.
    Be carefull, plastic cover of the connector can be briddle.
    Guido
     
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  12. Albert Penello

    Albert Penello Karting

    Jul 21, 2019
    141
    Woodinviille, WA
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    Albert J Penello, Jr.
    I see that but haven't messed with it. I'll shoot some deoxit inside there, but starting is brisk with my new battery.

    It actually seems like, for the most part, the electricals in this car are pretty good. I'm not finding a lot of oxidation and everything works aside from these few issues I've posted. But always good to check since I'm there!
     

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