| Author |
Message |
Kurt Kjelgaard (Kurtk328)
Junior Member
Username: Kurtk328
Post Number: 157
Registered: 3-2001
| | Posted on Wednesday, November 13, 2002 - 7:28 am: | |
I finally got around to check on the grounding for the instrument cluster. Up behind the center instrument pod there is a grounding point using a 6 mm bolt to fix the cableshoe (black wire). No apparent corrosion, nothing unusual to be seen, but after disconnecting, refitting and tightening, the readings on the oil pressure indicator is in line with what you other guys are experiencing - about 1/4 needle width deflection when turning up the instrument lights.
Thanks, Steve, for the clue. |
Edward Gault (Irfgt)
Intermediate Member
Username: Irfgt
Post Number: 2084
Registered: 2-2001
| | Posted on Thursday, October 10, 2002 - 5:01 pm: | |
This is the reason they made the dash lights so dim, so you do not notice the fluctuation of the gauges with the lights on. Don't tell me the Italians are not smart. Just out of curosity I checked my new Corvette and my lumpy old Corvair and the gauges read perfectly the same with the lights on or off. |
James Selevan (Jselevan)
Junior Member
Username: Jselevan
Post Number: 166
Registered: 6-2002
| | Posted on Wednesday, October 09, 2002 - 8:10 pm: | |
I'm heading back to the drawing board (after the Angels game).
Jim S. |
Michael N. (Man90tr)
Member
Username: Man90tr
Post Number: 691
Registered: 4-2001
| | Posted on Wednesday, October 09, 2002 - 5:00 pm: | |
James,
Drove 90 miles today -- On or off at temp my gauges went up 1/2 needle width as normal when lights are turned on, so Steve may be right its more "active" a connection.
Michael |
Steve Magnusson (91tr)
Intermediate Member
Username: 91tr
Post Number: 1113
Registered: 1-2001
| | Posted on Wednesday, October 09, 2002 - 2:27 pm: | |
James -- I know it sounds strange, but I just confirmed that, whether or not the engine is running, the water temp gauge in my TR moves up about 1/4~1/2 needle width (coolant warm, key "on") when the headlights/parking lights/dash lights are turned on. My guess would be that the gauges/sensors are somehow designed in a more active way (i.e., not like a conventional galvanometer) that inverts the logic you described (and frankly, I hope to never need to see the inside of any of my gauges to figure it out). I didn't mean to imply that Kurt's issue is a ground problem for sure, but I think a 25% change in the gauge reading is way abnormal and it wouldn't hurt to have a look at the instrument cluster connectors and maybe at least try reseating them. Just a suggestion... |
Kurt Kjelgaard (Kurtk328)
Junior Member
Username: Kurtk328
Post Number: 146
Registered: 3-2001
| | Posted on Wednesday, October 09, 2002 - 11:32 am: | |
Come to think about it, in my old TR4 back in the seventies (1967 TR4A) there was some sort of voltage regulator in the supply line to the Smith instruments.
It was there to keep the instrument voltage as constant as possible to prevent fluctations.
The F's apparently does not have any such regulator.
So the question now is: Are the instruments rated for 12 V or 13.8? At what voltage level are they showing correct? Or rather supposed to?
If my 7.5 kg/cm2 is correct, then I am running a higher oilpressure than advertised with the risk of provoking seal leaks.
I will have to check the actual pressure with a mechanical gauge - this would be possible to connect where the oil press light switch is located, opposite the oil press transmitter under the oil filter.
Anybody tried this? What might be the thread size?
Thanks
|
Steve Magnusson (91tr)
Intermediate Member
Username: 91tr
Post Number: 1111
Registered: 1-2001
| | Posted on Wednesday, October 09, 2002 - 10:35 am: | |
. |
Michael N. (Man90tr)
Member
Username: Man90tr
Post Number: 689
Registered: 4-2001
| | Posted on Wednesday, October 09, 2002 - 10:20 am: | |
James,
I am going to drive now and will let you know. |
James Selevan (Jselevan)
Junior Member
Username: Jselevan
Post Number: 165
Registered: 6-2002
| | Posted on Wednesday, October 09, 2002 - 10:09 am: | |
To diagnosis the cause, could someone please answer the following: With the engine at operating temperature but NOT running (with ignition switch momentarily turned on), does the oil or water temperature gauge change reading when the lights are turned on? My diagnostic thought experiment suggests that the readings will DROP, not rise. When the engine is running, the readings rise. Here is the reasoning.
The oil pressure, temperature, and water temperature transducers are variable resistors grounded on one side. The gauges are voltmeters calibrated to these temperature/pressure resistance ranges. A voltmeter reads the difference in voltage between its leads, in this case between the supply (line) voltage at the gauge and the gauge side of the transducer. In the limit, when the transducer resistance is infinite (open circuit), the gauge will read zero volts. As the transducer resistance decreases, the voltage will move towards the line voltage (let's assume 12 volts). With zero resistance (transducer is shorted to ground), the voltage will be 12 volts.
When the engine is running and alternator charging the battery, the system should be at 13.5 volts (approximately). When load is placed on the battery, its voltage will drop, which leads to a decrease in alternator field voltage and increased alternator current output, which raises line voltage (through battery charge). When line voltage rises, the difference in voltage between the line and transducer increases, and the meter mistakenly detects a higher temperature or pressure.
With the engine off, the line voltage will drop, but will not be over compensated by the alternator; thus, the voltage difference between line and transducer will decrease.
I do not believe this has anything to do with the ground connection at the gauge, unless all the gauges suffer poor connections or a poor common ground.
Jim S. |
Michael N. (Man90tr)
Member
Username: Man90tr
Post Number: 686
Registered: 4-2001
| | Posted on Wednesday, October 09, 2002 - 9:27 am: | |
My rise is only 5 - 6 degrees on water and the oil P rises about 2- 3 PSi with lights on. Hey on a range from 0 - 250+ on the oil, 240+ max on the water and 40 - 90 PSI on the oil P -- the error whichever readings are correct make no difference in diagnosing a problem with the car unless you are at the "max" end of the scale and by then the difference is no big deal because you will be doing someting long before then as not to operate in that region. The point is if the gauges are within 5% error (no matter what causes the error) everything should be fine. JMO |
Steve Magnusson (91tr)
Intermediate Member
Username: 91tr
Post Number: 1109
Registered: 1-2001
| | Posted on Wednesday, October 09, 2002 - 9:21 am: | |
(My TR is the same as Michael N.'s, and for half a needle width change I wouldn't worry about it, but) Kurt -- you have a bigger problem which probably lies in a bad ground connection somewhere. The gauges only compare a reference signal (usually a voltage) to ground so if the "ground" side of the gauge gets floated up (by turning on something else which shares the same ground), the gauge readings are affected. There's a whole bunch of shared grounds in an instrument cluster, and the grounding from the cluster itself to the chassis can also be a pretty iffy arrangement so deserves checking. Good luck with the fix...(hate to say it, but if someone asked me what tool would be used most on a F -- "multi-meter"). |
Kurt Kjelgaard (Kurtk328)
Junior Member
Username: Kurtk328
Post Number: 144
Registered: 3-2001
| | Posted on Wednesday, October 09, 2002 - 3:38 am: | |
My 328 does the same thing.
When dimming the instrument lights, the gauges increases at the same rate of dimming.
If the oil pressure indicates 6 kg/cm2, no lights,
turning on lights, instruments fully bright, will increase the reading to 7.5. That is a 25% increase and way too much for my liking.
Dimming the instrument lights will bring the reading back to 6.
Water temp, oil temp and fuel guage will also change, but not as much.
The changes seem to be greatest when the guages are indicating midscale.
All four instrument readings either increase or decrease
If the theory about grounding through the bulbs is correct, wouldn't that imply that the guages are showing correct only with the lights fully on and consequently showing too little with lights off?
Could the electromagnetic field created by the bulbs be sufficient to influence readings?
That field would vary with voltage to the bulbs.
I guess the only way to find out is to take the bulbs out of the instrument housing without disconnecting.
Comments please.
|
Amata Kulchan (Modamata)
New member
Username: Modamata
Post Number: 3
Registered: 10-2002
| | Posted on Tuesday, October 08, 2002 - 1:01 am: | |
Thanks Craig, I try to do that when I have time.. Michael, I know it really doesn't harm any but I'm slightly shock when I look at the gauge sometimes and realize that is not the right temp reading so I think I should fix it anyway,Thanks for your words, Michael... |
Edward Gault (Irfgt)
Intermediate Member
Username: Irfgt
Post Number: 2073
Registered: 2-2001
| | Posted on Monday, October 07, 2002 - 5:55 pm: | |
My 308 did the same thing. Oil pressure went up and temp went down. Just add it to the list of querks. |
Craig Dewey (Craigfl)
Member
Username: Craigfl
Post Number: 462
Registered: 1-2001
| | Posted on Monday, October 07, 2002 - 3:53 pm: | |
Sounds like turing on the lights may be making a better ground so the voltage goes higher.....You would have to check and clean all the ground point to get rid of that. |
Michael N. (Man90tr)
Member
Username: Man90tr
Post Number: 684
Registered: 4-2001
| | Posted on Monday, October 07, 2002 - 2:17 pm: | |
My TR does that also, rises about 5 degrees. Go figure. After 3 1/2 years, I think its normal. Good eyes though. I told my mechanic and he laughed and said less than 1% of Ferrari owners notice that kind of stuff, so it probably happens a lot and no one ses it. Another thing he said is that very few F-cars are driven at night, so the practice/chance of looking at your gauges the exact second you turn your lights on is not part of the ferrari driving experience. Personally, I don't think there is anything really to check -- one of the engineers on the site might have idea, but be careful aout chasing something that can't be fixed and really doesn't harm in any way. |
Amata Kulchan (Modamata)
New member
Username: Modamata
Post Number: 2
Registered: 10-2002
| | Posted on Monday, October 07, 2002 - 1:44 pm: | |
Anybody can help me solve this problem? When I turn the light on , water temp,oil temp,oil press gauge point higher than they should. When I turn off or dim the light to off position , they become normal. what should I check?? |