Headlamps on = Hotter Engine??

[DocDirk]

My 533 BB Ford runs about 190 degrees average, depending on the usual weather and speed factors. I usually drive daytimes but recently came home later and when I turned on the headlamps I noticed an immediate slight rise in the temp gauge, about 5-10degrees, then gradually continued to rise to about 220 (ambient temp about 90 degrees F). Since then I've tried the headlamps at other times and the engine temp does exactly the same thing regardless of the ambient temp. I have a huge 4000 cfm shrouded fan pulling through a large aluminum radiator.... 140 amp alternator, high end Prima battery and no headlamp malfunction when operating... could they possibly be drawing enough to reduce current flow to the fan with resultant reduced cooling efficiency? Even at highway speeds the temp doesn't return to "normal" and turning the headlamps off results in a quick return to normal temp readout on the dash. The voltmeter moves from charging to slightly in the red when the lights are on. They're standard stock sylvania nothing-special lamps. Or am I looking at some odd wiring problem between the gauges? Unfortunately for me, ekletrikelty is one of my weak suits...

Thanks!

Dirk

[strictlypersonl]

You've got electric gauges, right?

Look for inadequate grounds on the gauges or some strange (inappropriate) connection between the gauge power lead and the dash lights.

[Rick Parker]

Dirk where have you been hiding?

What you are experiencing is an uptick in line voltage from the voltage regulator when the lights are turned on. It then causes the gauge to "Indicate" higher when in fact there is no actual change. That is why when the lights are shut off the gauge returns to "normal" right away.

[DocDirk]

Appreciate the feedback guys! Rick I've been hiding in the forest of raising kids, being the best husband I can be, pursuing an active career as a Doc and resisting the severe temptation to change over from my C6 auto tranny to a 5-speed with paddle shifters! Right now it's so bloody hot in Merced that driving ain't much fun unless I'm up at the crack of dawn or overnight. Are you anywhere nearby?

Dirk

[PDUB]

Originally Posted by Rick Parker

What you are experiencing is an uptick in line voltage from the voltage regulator when the lights are turned on.

A good regulator is supposed to hold its output voltage constant as long as the input voltage remains above a certain design threshold limit (whatever that may happen to be for the particular part in question, but typically 1.2V for an electronic regulator, IIRC) for a rated current load . If what Rick suggests is the actual cause, and it may well be (but I would have guessed a voltage drop rather than an uptick), then a couple of things:

1) You should see the temperature fluctuate within a few seconds by turning the lights on and off as the regulator falls out of regulation then comes back online.

2) You should be concerned that the alternator output voltage could be sagging under the load... thus causing the regulator to fall out of regulation.

3) Some gauges read higher when their input voltage drops. This could have been caused by the alternator not keeping up with demand, and the battery, acting as the backup, began to lose its charge thus causing the system's overall voltage to sag and the gauge reading to climb over time.

Test it by cycling the lighting load on and off with a 50% duty cycle and a period of 30 seconds to see what results and how fast the temp reading changes. If it moves quickly, then get the alternator and regulator checked out. I'd be surprised if it was a regulation problem if the temp moves slowly...

Good luck!

[James Stern]

Dirk, I get the same thing when I turn on the stereo. Now I just listen to the engine and sidepipes.

Jim

[undy]

The voltage drops as lights go on. This is especially prevalent with a marginal (typical Cobra) electrical system. Your electrical temp gauge will reflect higher temps as the voltage drops in the gauge circuitry.

Also, as the voltage drops across the board your electrical engine cooling fan will run slower (DC motor) and actual temps will raise too.

You need to measure all your voltages with the lights on and everything running. I'd hazard a guess things aren't up to snuff...

[Tommy]

Assuming Rick is correct that this is an electric gauge problem, let me add that this is not normal. However, without a look at your wiring schematic, I can't suggest exactly what to do to fix it. ... On many temp gauges, the gauge is actually showing the electricity flow through the temperature sensitive resistor that is the sending unit. If changing the current output of the alternator or the current draw of the headlights affects the current flow through the temp gauge, I'd suspect a less than adequate ground somewhere else is causing electricity to take that route. .. One quick test would be to find the ground for the headlights and run a temporary ground from there directly to the negative pole of the battery. Then turn on the lights and see if the temp gauge changes. If it doesn't, then you have found the problem.

[DocDirk]

Thanks, I'll check those things out and post my findings when I get them.

Jim.....how LOUDLY were you playing that stereo??? I've one in my car but never put in the speakers once I realized how loud the engine and pipes were.

Again my gratitude for the great feedback!

Dirk

[James Stern]

Dirk, I've had hearing loss for some years so how loud the stereo is is relative. Besides, my tiny weenie block is not all that loud compared to a big block.

Jim

[vettestr]

Hey Dirk,
The gang has you on track but thought this might help you check the ground and voltage. If you look at the headlamp plug here is the ground reference. Colors never seem to match so twist connector so one pin is horizontal when the other two are vertical. The horizontal pin should be the ground . If the ground is on the bottom the plug the pin to the right is high beam (looking at plug from the bulb side of the connector. Check the left also to prove I am correct.

Test voltages at the pins then move the negative meter lead to another known good ground like the alternator case. If the voltage changes you have a poor ground. If no change move on to voltage checks. System voltage with engine running and lamps on hi beam with cooling fans on high should show a min. of 13.9 to 14.2 VDC at say 2000 RPM. happy hunting Jeff c