Headlights on fuel gauge goes up

after 25 years of owning my Dino I actually had to drive it the other day in the dark!

Sneaking back home on side lights I played safe and put on the main headlamps, all worked fine but then noticed all the gauges had increased.

Now i dont mind the fuel gauge going up when i turn the headlights on, but the temperature gauge rising caused mild palpitations. Switch off the headlights everything is working normally.

My guess is a bad earth on the gauge cluster, anyone notice their car misbehaving in a similar fashion.
Tony

 

Hi Tony,

I have the same issue on my car, but the difference between lights on and off is not that much that the temperature gauge makes me worry.
I am always wondering if the gauges are showing right readings with lights on or off!?

Best regards
Götz

 

My headlights also add fuel to the tank.

It must be a safety device to make sure you have enough petrol to get home in the dark.

 

I am wondering that as well!!

Sounds like we need to do some investigating

Tony

 

Install a relay to power the instrument circuit ONLY. Switch the relay with the key as normal, and get a direct 12 volt source to power the instruments through the relay. No more problems. Common Lamborghini Espada and Jarama problem. With those cars, the turn signals cause the gauge needles to wiggle back and forth.

 

Thanks

Found this on the internet, seems like its a common problem on the Pantera too

12V is applied to the temp gauge. The temp gauge divides this voltage and sends part of it to the temp sender in the engine. The temp sender is a resistor connected to chassis ground. Inside the gauge the divider circuit and a light bulb are both connected to a terminal on the gauge case. The terminal on the gauge case is connected to a like terminal on all the other gauge cases ("daisy chained") and eventually connected to chassis ground.

When you turn on the headlight, you also turn on the lights in all the gauges. The current drawn by all the light bulbs in the gauges will cause the ground terminal on each gauge to be at a slightly higher voltage than the ground terminal at the sender for that gauge. This difference in voltage between the two ground points for the temperature gauge will cause the temperature needle to move up scale.

See http://www.panteraplace.com/El...20Temp%20Fig%203.jpg for the details of the gauge circuitry. Just imagine that the ground connection on the right hand coil in the gauge (which is the ground terminal on the gauge case) and the ground connection from the sensor (which is on the engine, via the ground strap at the ZF) are at different voltage levels. Elementary, my dear Watson.

So there you have it.

To solve this problem, connect a wire directly from the ground terminal on the temp gauge to a solid chassis ground. The factory connected the gauge grounds in a "daisy chain" because it was simple and easy. Each gauge should have a direct connection to ground. In fact, the lighting circuit should have a separate power and ground from the sensing circuit ..... sort of like the space shuttle does it.

adding a second ground connection to the daisy chain at which ever gauge has a single black wire on it's ground terminal. That fix might work, can't hurt to try.

 

Can't blame them (the auto manufacturers) too much for this -- every ground wire can't be a giant wire size (because of the added cost/weight). If the upward needle motion is, say, 1~1.5 needle widths = that's pretty typical for a normal F system working properly. If it's a lot more = then that's something that needs fixing. If you want less = another good ground added directly to the existing ground at each gauge should reduce it.

 

Tony: You have identified ANOTHER problem with Italian cars. They usually ground the gauges in a "daisy chain". Running separate grounds for each gauge is an improvement, but will not completely solve the problem first stated in this thread. You have to separate the "loads" so the gauges are fed from an "dedicated" power supply that is not also required to feed other "loads" such as headlights, turn signals, etc.

 

Thanks, the needle moved enough to get my attention!

I think the issue is compounded by the replacement temp sensor as well. It reads slightly higher than it used to so guess the needle will move more.

Sounds like a winter project to investigate, I'll try equalizing the earth (ground) path for the gauges. I seem to recall many years ago when i had the dashboard off I fitted a daisy chain set of grounds to each gauge already and connected them direct ot the chassis.

 

Thanks for your input, makes sense. I'll check out the wiring diagram and see how easy it is! If not I'll live with it
Cheers

 

Would the dash rheostat cause an issue with the gauges?

 

Tony: I doubt it. The final solution is to provide a dedicated power source (via a relay) to the gauges that does not flow through other switches or feed other components.