Guages jump when lights are on?

I've heard other folks on f-chat talk about the same occurrence. Seemed pretty common from the responses.

 

You need to improve the grounding of your instrument pod's electrical system (what's happening is, when you turn on the lights, the "ground" at instrument pod is getting pulled up to some non-zero value). It's almost impossible to eliminate all needle motion, but, typically, it would be more like 1/2 a needle width, or maybe 1 needle width -- but sounds like you've got more than that.

 

The schematic in your euro 308QV OM shows that the grounds for these three gauges are all connected together and then there is a single connection to the chassis. Unfortunately, I don't know the physical location of this ground connection (and sometimes the symbology on a schematic for grounds doesn't always match the physical reality -- i.e. a bunch of wires might be shown connected to a single common ground point, but, in actuality, it really only means they are all connected to the metal chassis somewhere), but you might have a look around under the dash area -- could be as simple as a metal eyelet held by a screw to the metal chassis (and has become a little loose/corroded). Another option is to just add another ground connection from the (usually black) ground wires interconnecting these three gauges directly to the chassis (the ground terminal on the back of the gauges is usually labeled with a small "-" symbol).

Is your oil temperature gauge in the center console showing this same (bad) behavior?

 

Don't know how much this will help, but I do happen to know that on my Euro GT4 one of the front sidelights, the diagonally opposite tail light and number plate light, and the instruments, all share the same power input wire on the fuseblock. There is one input wire, with a riveted metal contact on the back of the fuseblock that splits the current between 2 fuses and output wires.

My guess would be that one of the lights has a bad earth and is returning current via the ground for the instruments.

 

Your schematic shows that the oil temp gauge and the others ground to the same point so, in this case, it must truly be the reality. If the access to the oil temp gauge is easier, you could experiment with adding a better ground there to see if it helps -- just a thought...

 

Yes, but you must have something odd/modified with the wiring going to the heater fan motors, or a combination of problems, or the wrong blower motors (yours are sort of special ones with three terminals -- two "+" terminals, one with a built-in resistor for low speed, and one ground terminal), etc., because they should be wired in parallel (so I'd check there before going after the switch, and by measuring there you can confirm/deny if the switch is doing the right thing):

When the switch is in Pos I = the MN (brown/black) wire at both fan blower motors should be +12V and both fan blower motors should run low speed.

When the switch is in Pos II = the MB (brown/white) wire at both fan blower motors should be +12V and both fan blower motors should run high speed.

Good hunting!

 

How unusual! Since the +12V signal going to both motors, for both the high and low speeds, comes from a common source at the switch, I don't see how the switch could cause your symptom (so I have no better advice to offer than that given before -- check the voltages at the motor connectors). When you have the "problem", do both motors run at the same speed (even though they don't run at the same time)? Can you tell if it's high or low?