[T2] Gas gage weirdness

[Bob Reuter]

if I have electrical problems I almost always check the ground first, VW 
was very good at hiding grounding locations

Bob



On Fri, 22 Feb 2019, c.dreike wrote:

> I figured it out!!!  Simple as grounding the gauge case.
> I pulled the gauge in my 64 from the dash and suddenly it didn't work either! 
> I noted that there is a ground terminal on the back and that the mounting 
> bracket would also provide ground. Once I added the ground wire it worked 
> properly. Amazing.
> I went back to my bay gauge and added the ground to the case and the gauge 
> now works correctly. Amazing!
>
> Thanks for the input.
> 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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