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Hvac blower disrupting oil gauge reading!

Discussion in 'Technical Q&A' started by Arturin, Aug 15, 2020.

  1. Arturin

    Arturin Karting

    Aug 16, 2009
    199
    Spain
    Hi guys,

    Just experienced what the title says. Apparently when I put my hvac blower on Max (position 4) the oil pressure gauge reads 0.5 Bar less than if it is on position 0. I’ve been testing during the trip. A/C compressor on or off doesn’t affect the reading, it’s just the blower.

    Let’s say I’m cruising at constant 3000 rpm with blower off, the oil pressure gauge reads 5 bar (middle of the gauge). Turn the blower to position 4 and oil pressure drops to 4.5 Bar. Turn the blower back to 0 and the gauge slowly starts creeping back to 5 bar again.
    Position 1 2 and 3 also affect the gauge reading but less.
    I’m not really good at electronics but any idea where to start checking or what does the hvac have to do with the oil pressure gauge???

    Thanks!
     
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  3. afwrench

    afwrench Formula Junior

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    Hi Arturin, you dont say what kind of car but do you notice a drop in rpm when you turn the AC on? Mike
     
  4. Arturin

    Arturin Karting

    Aug 16, 2009
    199
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    Oh! It’s a F355. Yeah the A/C works fine. The problem is somehow related to the blower. Doesn’t matter if the A/C compressor is on or off, it’s the blower that disrupts the gauge reading.
     
  5. wmuno

    wmuno Karting
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    Some gauge clusters have a regulated power supply to provide a constant DC voltage. Since you notice a drop in oil pressure when you turn your blower to the high setting, the current draw of the blower might cause the voltage for the gauge cluster to drop resulting in an incorrect reading. The other option might be a problem with just the oil pressure gauge itself. I don't have any experince with this particular model, but this problem might be beyond a simple DIY fix.
     
  6. Qavion

    Qavion F1 Rookie
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    +1

    Do you have a Euro 2.7 or 5.2 car, Arturin?

    There is a surprising number of devices attached to the AC System fuse in the passenger footwell, including instruments. Even spider seat & roof switches are on that fuse. I'm surprised that only your oil gauge is affected. Perhaps it is more sensitive than the other gauges (either by design, or because the oil pressure wiring, sender or gauge is faulty)

    (EDIT) The blower motor itself is powered by a different fuse to the AC system fuse, but there may be a problem with device which controls the speed of the blower motor. This controller is powered by the AC System fuse. Ref: Ferrari part number 63307200.

    I have wiring diagrams for the 2.7 and 5.2 cars. Just let me know which one you have.
     
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  8. Qavion

    Qavion F1 Rookie
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    You could try cleaning the terminals on the oil pressure sender (located below the oil filter). Note that the electrical terminals are quite fragile. Make sure they are tight, but don't apply too much force.

    The terminals are exposed to the "elements" and may corrode. Ricambi (USA) sell a rubber cover for the sender.
     
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  9. Arturin

    Arturin Karting

    Aug 16, 2009
    199
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    It’s a 2.7 euro gts. I already have the wiring diagrams thanks! That blower controller thing is we’re I will start. It’s the same as in many lancias and fiats and I can see it’s a common problem part. Will keep posted with the results but not today, it’s a beautiful sunny day to drive the car :)


    I checked that first thing, they’re clean and not corroded. Maybe the wires are a little “stiff” but they’re 25 years old...
     
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  10. Arturin

    Arturin Karting

    Aug 16, 2009
    199
    Spain
    Update.

    I looked carefully and realised that the water temp gauge is algo affected and most important of all, it’s not due to the hvac. I switched the lights on and it also alters the reading. When at idle, as soon as the radiator fans start it changes the reading.
    So I checked the alternator. Charges at 13.9V and when turning the blower drops to 13.7V so it’s time for an alternator rebuild. At least this one has lasted 13 years.
     
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  11. wmuno

    wmuno Karting
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    A 0.2 voltage drop at idle doesn't seem too bad. My old Ferrari has a Lucas alternator, and the specs call for measuring the output voltage at 3000 RPMs. How old is your battery? A newer battery should be able to keep the voltage fairly constant when there is an additional current draw from lights, fans, etc.
     
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  13. Qavion

    Qavion F1 Rookie
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    Regarding voltage control, some folks have had problems with the small red wire on the alternator which runs to the battery. By the way, how old is your battery and what brand is it?

    The red wire is seen in this diagram (bottom left):

    Fig3_1994_2.7_Injection_System_Ignition

    The wire runs through the 12 pin plug just aft of the left rear shock absorber.

    Anyway, your local autoelectrician should be able to tell you if your alternator is really at fault.
     
  14. Arturin

    Arturin Karting

    Aug 16, 2009
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    #11 Arturin, Aug 17, 2020
    Last edited: Aug 17, 2020
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    Battery is less than two years old. It’s a Varta battery. As far as I know the alternator should charge between 14.2-14.7 Volts. It charges at 13.8-13.9 even at 3000rpm. Anyway as I have a lift and I’m a diver I disassembled the alternator yesterday and found the voltage regulator was cracked. Plastic doesn’t like exhaust header heat.
     
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  15. Qavion

    Qavion F1 Rookie
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    It may be a good idea to install a fabricated heatshield. I'm sure it has been done before.
     
  16. Arturin

    Arturin Karting

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    The alternator voltage regulator arrived and I’ve replaced it. Sadly, that wasn’t que problem :rolleyes:. Anyway, as it was cracked I’ve prevented a future issue.

    Checked the pressure sender and it’s ok.
    Took the gauge cluster off and dismantled the oil pressure and water temp gauges checked all resistances and they’re ok too. No shorts or anything strange in the circuit.

    Checked the connector that goes to the gauges. The wire that provides the 12V makes 12,4v with the car running but as soon as I turn the lights or the blower on it goes down to 11.7-11.8v. Bench tested the gauge with an external voltage regulator and a 100ohm resistance simulating the sensor at 5 bar roughly and when I drop the voltage to 11.8 the gauge reads less pressure just like it did on the car.

    So the problem is somewhere along the wiring.

    The quest continues...
     
  17. Crowndog

    Crowndog F1 Veteran
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    I’d recommend running a separate ground to the gauge cluster. I found this to settle most gauge fluctuation problems.
     
  18. Qavion

    Qavion F1 Rookie
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    To me it sounds like you still have a voltage regulation problem.

    As I was mentioning earlier, there is a sense wire running from the battery to the alternator which can cause voltage regulation problems:

    https://www.ferrarichat.com/forum/posts/147393589/

    Clean the battery terminals and, I believe there is a bullet connector in that small red wire circuit which can cause regulation issues if it doesn't have a good contact.

    Check the voltage at the battery terminals and see if the voltage at connector "41019" slightly aft of the left rear shock is the same.

    Fig3_1994_2.7_Injection_System_Ignition

    If you still have access to the alternator, check the voltage on the small red wire at that point. Also check chassis and engine earths. I found mine to be out of tolerance.
     
  19. Arturin

    Arturin Karting

    Aug 16, 2009
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    Oh didn’t say it but I checked that cable as soon as you mentioned it in your previous post. It’s ok, reading exactly the same voltage in the battery as in the alternator connection.
     
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  20. Beetle

    Beetle Formula Junior
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    Earths
     
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  21. Crowndog

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    Seriously a foot long wire with a snap connector halfway along its length (for easy gauge plate removal) attach to the chassis inside the instrument binacal and the other end to the gauge ground wire.
     
  22. Qavion

    Qavion F1 Rookie
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    #19 Qavion, Sep 4, 2020
    Last edited: Sep 4, 2020
    The wiring diagrams show a number of earths which I haven't been able to identify on my F355. There is a major earth behind the footwell relay panel. If you run out of ideas, it may be a good idea to check this.

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    I don't know if you've accessed this area before. There is one bolt holding the relay panel on (outboard side) and two locating pins.

    (EDIT: your wiring may look different to mine. I have a right hand drive car)
     
  23. Arturin

    Arturin Karting

    Aug 16, 2009
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    My vacations ended last week and I’m back to work, an awful lot of it so less time for wrenching. Haven’t touched the car since last post. Will keep updated.
     
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  24. Crowndog

    Crowndog F1 Veteran
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    Sorry for some reason I thought this was In the 308 section. My apologies.
     
  25. Arturin

    Arturin Karting

    Aug 16, 2009
    199
    Spain
    Had some time to give it a look yesterday. Had brand new battery laying around from a race car I just sold so I replaced it just in case and did some testing. So to recap:

    -Brand new battery
    -Voltage to the gauges is stable even with lights and air turned on ( with the “old” battery it dropped to 11.8 Volts) I guess the old battery wasn’t in perfect condition after all...
    -Placed a long wire from the sender straight to the gauge to rule out wire issues. Result: the gauge reading is equally disrupted.

    All I can conclude is that the problem is the gauge itself. I’ve got a lancia delta oil pressure gauge (Veglia) and luckily the circuit is exactly the same. The only difference is the two resistors and the face. I will replaced with ones with the same resistance as the Ferrari gauge and continue with some testing.

    Will keep updated.
     
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  26. Qavion

    Qavion F1 Rookie
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    Do you have an airbag, Arturin? If so, just be careful that you don't turn on power with the instrument binnacle removed, otherwise you will get an airbag fault light which needs an SD tool to reset.

    Possibly, but I still wouldn't rule out the earth. Can you piggyback an earth wire on the centre gauges?

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  27. Arturin

    Arturin Karting

    Aug 16, 2009
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    Already did yesterday, a ground straight from the gauge to the car but no luck, same behaviour.
     

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