[T2] Gas gage weirdness

[c.dreike]

Thanks for the tips guys. Yes the "vibrator" assembly is a feature on 
bay window buses, however only sometime after 1972. The function of the 
gizmo is to stabilize the voltage for a consistently accurate reading. 
The device is actually a voltage regulator. If you look on The Samba you 
will find some descriptions of what it is and what it does. This does 
however give me an additional idea for my operational amplifier design. 
I'll add a voltage regulator for more stable operation.

I'll whip one of these together over the next few days and let everyone 
know how it works out. In the mean time maybe someone will pop up with 
another idea of what I might be doing wrong. I mean, VW built these 
things for years and they worked just fine. I'll take a look in on my 
splitty and see what that's doing. That gas gauge is working fine.

Cheers,
Chris
71 Sunroof
64DD Kamper Kit

On 2/21/2019 4:47 PM, c.dreike wrote:
> Started testing my gas gage and sender. The sender worked when I 
> installed it. Currently the tank is very low and the sender reads 
> about 80 ohms. According to The Samba this is about correct. The 
> senders run from about 3-5 ohms at full to about 70-80 ohms at empty.
>
> When connected, both of the gauges I have read full and a little past. 
> The battery voltage is about 11.98. The voltage across the gauges are 
> 6.57 on the original gauge and 7.98 on the reproduction gauge.
>
> On the test bench connected to a bench supply, the original gauge 
> reads full at about 6.5V and the reproduction gauge reads full at 
> about 7.2V. As I reduce the voltage to the gauge, the reading goes 
> down. At 0 volts the gauges read empty.
>
> Per the test specs in Bentley, the gauges should read full at about 
> 12V, battery voltage. Both my gauges peg when attached to full battery 
> voltage.
>
> I have double checked the wiring against the schematics and all looks 
> correct. The circuit looks simple enough. +12V on one side of the 
> gauge. The other side of the gauge connects to the sender (5-80 ohms) 
> and the other side of the sender connects to ground.
>
> I'm an electrical engineer with lots of experience with simple stuff 
> like this and this stuff makes no sense to me. I must be missing 
> something. (Kinda like mistaking the wire stripe colors on the turn 
> signal)
> Anyone have ideas on what I should be looking for or doing 
> differently? I'm stumped. I'm about to build an inverting operational 
> amplifier circuit that would drive the gauge so that it would see 0 
> volts at empty and about 7V at full.
>
> TIA
> Chris
> 71 Sunroof
> 64DD Kamper Kit
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