Temp gauge rises when headlights are on

[Cruz]

Noticed this happening recently. The needle goes up by about 3-5mm when the headlights are on.

other than that the dashpod works fine. Any thoughts?

cheers, shaun

[naskeet]

Noticed this happening recently. The needle goes up by about 3-5mm when the headlights are on.

other than that the dashpod works fine. Any thoughts?

cheers, shaun

The headlamp's electrical load, might be affecting the supply voltage to your instrument panel. Check your dashboard voltmeter, before and after switching on the headlamps, whilst the engine is running.

The power to the headlamps, is supplied by the alternator, which in turn is provided by additional mechanical power from the engine.

[Cruz]

Without the lights on the voltage reading from the battery is 14v when the lights are on it drops to 13.5

[Oldiebut goodie]

Voltage stabilizer?
You need to be measuring at the instrument not the battery.

[tencentlife]

This is due to ground reference offsets. Clean up the grounding points, in this case focus on the battery ground strap connection to body, and the two rosettes up under the dash near the fuse panel, where all instruments and your headlights ground.

In every case, remove the item bolted to the bodywork, clean the attachment area of paint until it is bright metal, use a stainless serrated star washer between the eyelet or rosette and the body metal, and reattach. Use new fasteners as well if the old ones are at all corroded. It helps a lot to coat every part with dielectric grease, Vaseline, or antioxidant compound, something to protect the freshened connection from further corrosion.

This is also a reminder to service other ground connections before they start to cause you starting and running problems, especially the transmission nose cone ground strap, and the ground connections on the engine and left wall of the engine bay. If your van has the original ground straps at trans nose cone, engine cylinder head to body, and battery negative, they will by now be quite corroded internally, so you should replace them with suitable sized wire, don't worry about finding OEM-style braided strap, ready-made battery grounding wires you can find at a parts store of sufficient length for each location will do just fine. Never a bad idea to add a dedicated grounding wire from the alternator chassis as well.

[Cruz]

No problems whatsover with earthing points, straps or starting

[T25Convert]

Hi,

Mine does this - I've also noticed it affects the fuel gauge reading by a similar amount (although you have to be sitting on the flat and level for a while to notice!!) As with yours the earth straps and crowns are fine.

As Oldie but goodie suggested the voltage stabiliser may be on its way out - the gauge is sensitive to small changes in relative voltages, so there is a voltage stabilsier that is mounted behind the dash pod to 'smooth' out any voltage irreglaurities (like the extra load created by head lamps, heater blowers heated rear screens (if you're so lucky)). This doesn't affect the LEDs or instrument lighting asthey are not suceptible to small voltage change.

As my gauge moves by the same amount each time I switch on the headlamps I've just left well alone, and accept that the gauge just reads slightly higher at night.

Good luck if you do try and sort it out!

Cheers,

Alex

[tencentlife]

If the gauge rises when you add electrical loads it is ground offsets, plain and simple. You have grounds that aren't as good as you think they are.

[Oldiebut goodie]

Or voltage stabilizer.

[Cruz]

I'm unsure of where to probe with my voltage meter......ooeeer

[tencentlife]

You can use a voltmeter to pin down exactly which grounding circuits have too much resistance. If the gauge varies when headlights are switched on, for instance, you look at which grounding circuits they have in common. In this case it would be at the under-dash rosettes, and at the battery negative strap; both instrument panel and headlights have those two connections in the ground paths.

So take the front grille off so you can get to the headlamp connection, idle the engine, and measure the voltage between either of the headlamp negative connections (while still connected to the lamp, probe the brown wire from the back side of the connector) and any bare metal part of the van body nearby. Note the voltage (should be zero) and then switch on the headlights. You will probably now see a measurable voltage, I would say less than about 0.25V is OK, more than that and there is a lot of resistance in the headlamps' ground path. A lot of it could be at the grounding connection at the rosettes. It can also be along the wire itself if it is corroded internally, which a lot of 20year-old copper wire will be, especially in a damp climate, and the crimp connections also offer a lot of resistance as they age.

Then do the same check at the battery grounding wire connection, put the voltmeter between the battery negative post itself and bare body metal. Every circuit in the car goes thru the battery grounding strap, so this is a prime place for problems to develop. VW just bolted the strap to the painted metal and counted on the contact thru the bolt threads to offer a circuit. But the bolt goes thru right into the wheel well, and gets splashed with lots of water there. When the underbody coating cracks or breaks off, that bolt gets very corroded and all the circuits in the van see a voltage drop as a result. The solution is to make the grounding strap's eye contact bare steel rather than counting on the threads for a circuit. I would do this preventively even if you don't measure a big drop there, because it will be a problem eventually.

You can isolate and test any part this way, on the hot or negative side, just by measuring the difference in voltage between any two points while the circuit is under load. These resistances will also show up with an ohmmeter, but most consumer-quality ohmmeters just aren't very trustworthy at the low values these connections will likely present; put some real current thru them, though, and you see the real effect the resistance has immediately.

Every voltage drop, whether on the hot or negative side of a circuit, lowers the voltage the intended load receives, and since current is the inverse of resistance at a particular voltage, and the circuit's total combined resistance is now increased, current thru the circuit is also diminished. These losses will make bulbs less bright, and electronics act strangely, but at the same time you can measure across the battery posts and see a good system voltage; this throws a lot of people off the trail. We tend to assume that all grounding circuits and points are at zero potential but you will find that many are not. I know from experience that a poor ground connection in the grounding side of the temp gauge will cause just what you describe when you add loads like headlights that share some of the same grounding paths.