355 oil pressure gauge & sender questions (also in tech)

Discussion in '348/355' started by kverges, Jun 14, 2004.

  1. kverges

    kverges F1 Rookie

    Nov 18, 2003
    Full Name:
    Keith Verges
    just to cover the bases, this is also in tech:

    Got out the 355 for a drive and noticed 0 oil pressure. The warning light would go out after startup, though, so I suspected a gauge or sender problem. All other gauges, warning lights and instrument lights worked fine.

    I checked the sender and it has two leads, one for the gauge and one for the warning light. The warning light side worked correctly, i.e. no light ever if disconnected but light works properly when wire is connected to sender (note to all - be sure you have an oil pressure warning light BEFORE startup or you probably don't have one at all!). It seems to be a variable resistance sender, and I measured under 10 ohms key on, engine off, and 80-150 ohms or so engine running, with variations in rpm. So this seems normal, although I don't know how the gauge is calibrated. The gauge did nothing, regardless of open resistance (sender wire disconnnected from sender) or shorted to ground. I also pulled the gauge cluster and checked the wire coming into the gauges. Oddly, this wire showed from under 10 ohms (engine off) to 40 ohms or so (engine running).

    I therefore suspect the gauge (it is a pair of gauges for water temp and oil pressure), but I have NEVER had any electrical gauge failure in any vehicle of any age.

    My service manuals don't seem to have a wiring diagram and I don't want to throw expensive Ferrari parts at the car.

    Anyone have any thoughts to add or similar failure? I just have this feeling that I might be missing some easy but peculiar electical gremlin that has nothing to do with the gauge. For example, why do I have a different resistance for oil pressure when I measure at the connector for the oil pressure gauge as opposed to directly at the sender?

  2. thoroughbred_1002

    Nov 5, 2019
    Bowling Green, KY
    Full Name:
    Austin E

    The car I am servicing has the same issue, only it seems that the water temp is affected too. Gauge indication is unreliable, inaccurate, and erratic..

    Any thoughts? Maybe I am just not keying in the best choices of words for my search inquiries.
  3. thoroughbred_1002

    Nov 5, 2019
    Bowling Green, KY
    Full Name:
    Austin E
    It's just two screws and a few plugs, so I went for it.

    The issue with the car I am servicing turned out to be that the nuts for the oil pressure and water temp instruments had become loose. It's imperative that they're tight for a good connection to the PCB. Very easy fix.

    Now all gauges work flawlessly. No more erratic needle behavior or intermittent operations. Both problem gauges are steady as a rock. :)
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    DaisyCutter, 308 GTB and Qavion like this.
  4. kenneyd

    kenneyd Formula 3

    Sep 30, 2014
    NE FL
    Full Name:
    The culprit on mine with the tiny brass nuts on the back of the gauge. They simply were lose, I snugged then up and have had zero issues since then.
    308 GTB and thoroughbred_1002 like this.
  5. Carmellini

    Carmellini Karting

    Jul 16, 2019
    great work….keep it going....any idea how far the needle on the speedo will travel? post pics. LOL
  6. thoroughbred_1002

    Nov 5, 2019
    Bowling Green, KY
    Full Name:
    Austin E
    There are better candidates for those samples than me! :p

    When the windshield wipers start slapping the glass, I run out of will to push it any further. This usually happens at far less than 181mph.

    Seems like this could be a common issue. I wonder how many F355 owners have been charged four figures to find and fix the loose nut issue(s). It is no more than a half hour job.

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