Lights Vs Temp Sensor

[MN69Grande 203]

So since I got the new Aluminum radiator and the Contour fans my car runs about 182 on my digital gauge.  70 degree days 182, 80 degree days 182, 90 degrees 182.  Pretty good this summer.  But I always drive during the day.  Tonight I stayed at a friends house a little later and turned the headlights on, on the way home not to see but to be seen.  Anyway I look down at the gauge 187.  Ok not to hot but higher than usual.  Get into my neighborhood and shut the lights off, immediately drops to 178 then 182.

So my question is could running the headlights (sealed beam) have caused the car to run 5 degree's hotter.  Or is it possible turning them on affected the way the gauge was reading?  The wires are no where near each other.  Anyway Strange happening wondered if this happens to anyone else.

[barnett468 418]

The lights put more load on the engine but not enough to affect the temp. Turn your lights on while driving after engine is up to temp and see how quickly the temp changes then turn them off after a couple minutes and see how fast it drops back down.

[1969Fstback 58]

Could it be lights using amps and less amps to the fans?

 

 

[Brian Conway 264]

 

[danno 128]

If the temperature dropped immediately as soon as you shut off your lights, then it is possible that there is a ground issue.  Somehow ( I cannot imagine it) but if the headlights use the same ground return as the temperature gauge, that could explain it.  More current in this ground wire (because the headlights are on) means more of a voltage drop in the ground. A higher voltage drop look to the temp gauge like a higher temperature. 

Try it during the day, turn on your lights and see if the temperature goes up when they are on and down when they are off.  It is also possible to have a drop in your battery voltage when the lights are on that is effecting your temp gauge, but that is a real stretch.  You might put an accurate voltmeter ( not the voltmeter you might have on your instrument panel) on your battery while driving to verify it is ok.

[Midlife 814]

7 hours ago, danno said:

If the temperature dropped immediately as soon as you shut off your lights, then it is possible that there is a ground issue.  Somehow ( I cannot imagine it) but if the headlights use the same ground return as the temperature gauge, that could explain it.  More current in this ground wire (because the headlights are on) means more of a voltage drop in the ground. A higher voltage drop look to the temp gauge like a higher temperature. 

Try it during the day, turn on your lights and see if the temperature goes up when they are on and down when they are off.  It is also possible to have a drop in your battery voltage when the lights are on that is effecting your temp gauge, but that is a real stretch.  You might put an accurate voltmeter ( not the voltmeter you might have on your instrument panel) on your battery while driving to verify it is ok.

It's not a real stretch at all; he has an aftermarket digital temperature gauge, one that probably runs directly on ACC voltage rather than the CVR voltage which is regulated down to 6 V.  Thus, the gauge may be reflecting actual input voltage variations.  

[MN69Grande 203]

Thanks for the ideas everyone.  I'll try to do a test tomorrow.

[LiLMike 42]

I think Danno is on to something. I had a couple of electrical problems and it was that I had too many circuits tied to the same ground. I split the ground points and everything cleared up.

[MN69Grande 203]

1 hour ago, LiLMike said:

I think Danno is on to something. I had a couple of electrical problems and it was that I had too many circuits tied to the same ground. I split the ground points and everything cleared up.

This is interesting.  I do have 1/2 dozen small leads tied together at one point.  The Tach, AFR, Oil Pressure, Temp, Power Locks, and Radio are for sure grounded in the same spot.  AND! now that I think about it the spot is only a foot or so from the Light switch.  Well honestly as long as the engine isn't overheating and I know the reading is a little high with the lights on I can live with it for now.  I'm not pulling the dash apart for that.  Still need to test during the day.

[RPM 1,190]

3 hours ago, MN69Grande said:

  Well honestly as long as the engine isn't overheating and I know the reading is a little high with the lights on I can live with it for now.  I'm not pulling the dash apart for that.  

Smart, very smart.

[Brian Conway 264]

+1   I take every opportunity to avoid going behind the dash or to delay that adventure for as long as possible.  Brian

[69NC 21]

Another place to check is your ground strap between the motor and the chassis.

If the motor is not well grounded, the added load of the lights will create a differential that could show up on the temp gauge. 

Ideally you should have two ground straps between the motor and the chassis.

 

[MN69Grande 203]

11 hours ago, 69NC said:

Another place to check is your ground strap between the motor and the chassis.

If the motor is not well grounded, the added load of the lights will create a differential that could show up on the temp gauge. 

Ideally you should have two ground straps between the motor and the chassis.

 

Thanks for the idea.  I've actually got pretty good grounds.  A four gauge from the battery to the block in front and a braided strap from the block to the firewall in back.

[danno 128]

I am curious about what ever happened with this?  Was it a bad ground? 

[MN69Grande 203]

Hey Danno,

I haven't actually dug into this any more.  I didn't need to drive at night again, so I didn't see the results reproduced.  And since the next thing to trouble shoot requires removing the dash... well I figured it just wasn't bad enough to take the time.  If I have to get in there for something else then I'll move the ground and see what happens.

[danno 128]

I thought that another place to check for a poor ground is between the flexible circuit card on the gauges and The chassis. If that is bad, when the dash lights are on the constant voltage module will put out a different voltage than when they are off. So make sure you have good connection between the chassis and the screw by the constant voltage unit.

[jmlay 80]

I infer the temp you are seeing is from the aftermarket gauge. Run temp wires from the sender as well as ground directly to the engine block or the battery, bring the engine up to temp, turn the lights on, monitor the temp reading. If this is a ground issue bypassing the car wiring will show it.

[MN69Grande 203]

2 hours ago, jmlay said:

I infer the temp you are seeing is from the aftermarket gauge. Run temp wires from the sender as well as ground directly to the engine block or the battery, bring the engine up to temp, turn the lights on, monitor the temp reading. If this is a ground issue bypassing the car wiring will show it.

You are mostly correct.  Yes I'm using an aftermarket gauge.   I've already run a direct line from the sender to the gauge.  The ground is attached to the lower dash close to headlight switch.  But to get at that ground would require removing the Dash pad.  Hence my hesitation. 

[Midlife 814]

6 hours ago, danno said:

I thought that another place to check for a poor ground is between the flexible circuit card on the gauges and The chassis. If that is bad, when the dash lights are on the constant voltage module will put out a different voltage than when they are off. So make sure you have good connection between the chassis and the screw by the constant voltage unit.

Huh?  The dash lamps do not share anything with the constant voltage module except the ground plane.  The dash lamps get their power from the headlight switch through the fuse box.

[danno 128]

1 hour ago, Midlife said:

Huh?  The dash lamps do not share anything with the constant voltage module except the ground plane.  The dash lamps get their power from the headlight switch through the fuse box.

Exactly.  Suppose if there was a bad connection between the "ground" of  the constant voltage module and chassis, with a higher than normal resistance. Now if you turn on the dash lights, you get more current through the ground. More current creates a higher voltage drop between the "ground" of the circuit card and the chassis ground.  This would mean less voltage drop on the temp sensor resistor and a lower reading.   At least it made sense to me yesterday; maybe not today. 

But now he says it is an aftermarket gauge. So forget most of what I said, it probably does not apply. With aftermarket it could still be a bad ground, but without knowing a lot more, I cannot say.  

[MN69Grande 203]

Actually now that this topic is back up I started to wonder if there is another possible culprit.  The aftermarket gauges have a dimmer feed that I hooked up by tapping into the feed out of the switch.  I wonder if this could cause the reading to be off somehow.  I wouldn't lose much by disconnecting this feed.  On the digital gauges I have (Temp and Oil Pressure) they only seem to dim from a level 10 to 9.5.   Seems to work on the mini tach.  If I remember right I used bullet style connectors and electrical tape so killing the feed should be easy enough.  So next year when it gets hot again I'll try recreating the issue then disconnect that feed and see if that might fix the issue.  Maybe I'll get lucky.  If does I can just wrap the connectors and tie them off.