gas sending unit

How the vw gas guage works is a bit of mystery to me. Why does the the circuit need a "resonator" anyway ?? The senders variable resistor, operated by a float arm, makes sense. I think that is 0-80 ohms,, but is that supposed to be 80 ohms full or empty ?? I am not getting any continuity through my sender at the moment. Cheers, Alan

Kept digging,, found a good vw article. Still don't know why a vibrator/resonator is needed though. Alan

http://www.midsouthvw.com/TechTips/tech_tip_FuelGauge.htm

The resonator provides a rudimentary damping action for the gauge needle. Otherwise, the needle swings back and forth as the tank float moves with the shifting fuel volume.

80 ohms full. Oh yeah, that Bill C. article is a must read if you have any issues. Well explained and well illustrated.

Good post, and thanks Alan and Gordon for the testing article and comments. Do the VDO gas gauges require a resonator? My CMC uses all VDO gauges, and I do not believe that there is a resonator anywhere. My gas gauge does not swing while driving, but it only goes from full down to 1/2 tank, at which point it is really about empty.

Chuck

Do a search here for gas gauge sender or fuel sender and you'll find many threads on this.

~WB

All this info about VW fuel gauges pertain to just that... VW Bug gauges. They use the vibrators/resonators, not our gauges. Looking to abate the needle wagging of my gauge, I experimented with one on my 914 gas gauge with no effect one way or the other. The only thing that worked for me is the tube type sending unit for K. Ghias. The needle wags no more than an 1/8" which is WAY better than with the arm/float sender. The arm/float senders are installed on most builds because they are CHEAP(er). The tube type senders are a bit pricy at some retailers but I've found a few on Ebay that were more reasonable.

http://www2.cip1.com/ProductDetails.asp?ProductCode=VWC%2D211%2D919%2D051

~WB

Bill, thanks for the suggestion and information on the resonator. Here is a good link from a search on this site that deals with sender/gauge readings.

https://www.speedsterowners.com/forum/readmsg.asp?t=2335

Chuck

Interestingly, my donor VW had BOTH a resonator and a ghia-like tube sender like Bill's.

And while my speedster gas gauge does, indeed, sway a bit with the movement of the car, you actually get used to it over time. I've often wondered if that gauge movement could be buffered with some sort of a capacitor circuit in there, but then my innate laziness took over and I just drank another beer. The gauge IS working, after all...... BTW: There is a patent (#4838082) for that very circuit, but I wasn't envisioning using op-amps and the potential for the capacitor charging and then arcing across the sender potentiometer (with it's subsequent gas tank explosion) didn't excite me. Maybe op-amps in there (and their minimal current needs) might be cool after all....

If you want your gauge to read more correctly (even though it'll still move around a bit), it's all in how you bend the float arm. If it reads 1/2 when it's empty, then the arm must be bent upward more at the "kink". It usually takes one 3-4 tries of pulling the sender out, bending it, seeing how it looks for a given tank amount and so forth and then messing with it again (and again, and again) until it's (more-or-less) right.

All this is because the speedster gauge reads ass-backwards to a VW sender so you have to bend the float arm backwards to get it to read right. It's probably due to Cambodian Kharma that the Ghia sender works right with our gauges (or Bill is the luckiest guy in greater Harrisburg, PA.)

Personally, I wouldn't bother with the resonator, but I WOULD get in there and do some bending to make it somewhat more accurate when sitting still and level, at least. Besides, Winter is coming and you won't be driving it as much and will need something to do to get you away from "Jersey Shores" or something equally time-wasting.

gn

Thanks, Gordon. I believe in trying the simple approach first when tackling a problem.

Chuck