Daves1940Buick56S Posted October 27, 2013 Share Posted October 27, 2013 All:So I finally got the turn signals working properly after fixing the turn-signal-to-parking-light short in the front left fixture, and now am turning my attention to the gas gauge. First, it always shows "full." I know that means bad connection to sending unit or bad sending unit. There was a convenient place in the trunk to unhook the line going down to the tank. Voltage shows 2.9 VDC open circuit and a resistance of about 50 ohms. I bounced the car a bit to see if the resistance changes a bit as the gas sloshes about but nothing.So I decided to check out from the trunk forward and did a resistance/voltage plot.Results:2.5 ohm 0.2 V Gauge about 1/4 tank indicated5 ohm 0.4 V Gauge about 1/2 tank indicated10 ohm 0.7 V Gauge nearly full15 ohm 1.0 V Gauge peggedSo based on this I would expect the sending unit should be 0 ohms to about 15 ohms from empty to full. Is this what you all have seen? And, it appears my gauge/wiring is (more or less) good and I have a hosed up sending unit. True?Cheers, Dave Link to comment Share on other sites More sharing options...
jeff Posted October 28, 2013 Share Posted October 28, 2013 I almost agree with your assessment, but there is one thing to check first.The job of the sending unit is to complete a path to ground, adding from about 0 to 10 ohms depending on how much gas is in the tank. So, you need to make sure the resistance from the sending unit to chassis ground is zero, otherwise this adds to the apparent resistance that your dash unit sees. Since the tank is grounded via the straps that hold it in place, this is usually a problem. Rig up a test lead with a couple of alligator clips, and see if a better path to ground changes things at the dash gauge.Jeff Link to comment Share on other sites More sharing options...
Daves1940Buick56S Posted October 28, 2013 Author Share Posted October 28, 2013 Jeff:Good idea, will try soon.Cheers, Dave Link to comment Share on other sites More sharing options...
danhar1960 Posted October 28, 2013 Share Posted October 28, 2013 This has been discussed several times here. The earth to the sender unit was originally provided by the fuel line itself. It is metal all the way from the sender unit to the fuel pump. The fuel tank has rubber straps that will prevent a good earth. Do you still have the original all metal fuel line ?? Or have you fitted an in-line fuel filter with rubber hoses to connect ?? I have the original "AC Instruments Manual" that I can send you that may be of some help. PM me your email address if you want them. Danny Link to comment Share on other sites More sharing options...
Daves1940Buick56S Posted October 28, 2013 Author Share Posted October 28, 2013 Just tried checking resistance to the frame and the tank itself. Yesterday, it was appx 50 ohms to the frame, today infinite! Same with the tank. So my diagnosis at this point is bad sending unit, will schedule a tank pull party soon before the cold here makes it impossible to work.Cheers, Dave Link to comment Share on other sites More sharing options...
Daves1940Buick56S Posted October 30, 2013 Author Share Posted October 30, 2013 Tank yanked. Pretty easy, actually. Pulled off sending unit - ewww! See pics. Float stuck and pot appears to be non-functional. Also, looked inside tank - double ewwww!!! About 2 pounds of rust and cr@p in the tank. I took it to Looper's and am getting it "Renu'd." So I will take the opportunity to throw in an electric pump to give my starter a break.BTW, does anyone have any preferences to the sending units sold by either Bob's in CA or Cars in NJ? Both are about $75 or so.Cheers, Dave Link to comment Share on other sites More sharing options...
Ben Bruce aka First Born Posted October 30, 2013 Share Posted October 30, 2013 Probably made by the company. Ben Link to comment Share on other sites More sharing options...
danhar1960 Posted October 31, 2013 Share Posted October 31, 2013 What Ben said !!Danny Link to comment Share on other sites More sharing options...
Daves1940Buick56S Posted October 31, 2013 Author Share Posted October 31, 2013 Probably made by the company. BenBen:I assume you meant the same company. Good assumption, I think.Cheers, Dave Link to comment Share on other sites More sharing options...
Ben Bruce aka First Born Posted October 31, 2013 Share Posted October 31, 2013 Probably made by the company. BenBen:I assume you meant the same company. Good assumption, I think.Cheers, Dave[/QUOT Yep. Have to fire my proofreader.:mad: Ben Link to comment Share on other sites More sharing options...
FLYER15015 Posted November 2, 2013 Share Posted November 2, 2013 (edited) Dave,I bought my sending unit from "Bob's" and I'm quite happy with it.When you put the tank back up think about using a length of wash machine drain hose split lengthwise over the 2 rails that the tank nestles in, and making new straps. I used Chevy s-10 straps with home made 14 gage front hooks welded on.Also, it's a good time to run a BIG ground wire from the sending unit to the cross member just in front of the tank.And don't forget to add a BIG clear filter and your electric fuel pump back by the tank. Makes your starter last so much longer if you can prime her after a long "sit". I wired mine straight to the battery thru a toggle switch under the dash and I fused the link.Mike in Colorado Edited November 2, 2013 by FLYER15015 (see edit history) Link to comment Share on other sites More sharing options...
Daves1940Buick56S Posted November 4, 2013 Author Share Posted November 4, 2013 Mike:Thanks. My straps and hardware are good, may have been replaced at some point. I am running a ground wire from the sending unit and tying in at the same point as the ground for the new pump. Tank is ready tomorrow, hopefully the sending unit will be here by Tuesday, I have already drilled and tapped the hole to mount the pump. At this point reinstall is Wed afternoon.Cheers, Dave Link to comment Share on other sites More sharing options...
Daves1940Buick56S Posted November 16, 2013 Author Share Posted November 16, 2013 Got halfway thru reinstall, stopped by darkness. Finished today. Only real problem was that the filler pipe did not fit nearly as well as before to the point we had to shift the tank over to the left side some and use longer hooks. I dunno what happened, maybe the two tank halves were not rewelded in exactly the same relationship as before. So it is kind of jury-rigged more than I would like but it works for now. Electric pump works good and gauge now actually indicates something useful.So, is there a way of bending the filler pipe without destroying the pipe or tank? To fit properly the way it was before it would have to be moved about 2 inches at the top of the pipe.Cheers, Dave Link to comment Share on other sites More sharing options...
Daves1940Buick56S Posted November 16, 2013 Author Share Posted November 16, 2013 BTW, I had put in 10W-30 oil earlier this summer and found that my hot oil pressure at slow idle had dropped to about 5 or 6 lbs. I replaced with 20W-50 today and now the hot minimum oil pressure is about 12 lbs. Makes me feel better, anyways.Cheers, Dave Link to comment Share on other sites More sharing options...
Daves1940Buick56S Posted November 17, 2013 Author Share Posted November 17, 2013 One other thing - on testing with the new sending unit before installation in the tank I was getting a gauge peg at full with the float only 1/4 the way up. I know that this is a 45 ohm unit but my gauge seems to peg at 15 to 20 ohms (see first post). So I did some testing and ended up putting in a 100 ohm resistor in parallel with the sending unit - it's in the trunk so can be changed or removed easily. This lowers the equivalent max resistance to about 30 ohms and tested well with a peg just below max float position. Perhaps there are shorted windings or something in the gauge, I dunno.Cheers, Dave Link to comment Share on other sites More sharing options...
Daves1940Buick56S Posted November 20, 2013 Author Share Posted November 20, 2013 And BTW Mike, I did install a BIG fuel filter before the electric pump, so now there are 5 in the system: Fram G3 before the elec pump, the one that came on the pump, another Fram G3 before the mech pump, the screen inside the mech pump, and a glass filter before the carb. Think it's enough? Actually, they were added sequentially as I worked on each part of the fuel system from front to rear.Cheers, Dave Link to comment Share on other sites More sharing options...
FLYER15015 Posted November 21, 2013 Share Posted November 21, 2013 Dave,I like the multi filter approach. One cannot have too many.I have a question regarding your post # 15, in that you stated that you added a 100 ohm resister in parallel with the gage to effectively move the needle off (?) the full position.My issue is that even though the tank is full of gas, the gage needle is at 3/4 position. Your post indicates that my sending unit wire has too much resistance. I know that it is OEM and has no splices or stray grounds (read wear spots). Makes me wonder if the float rod needs to be "bent" to accomodate this discrepancy. Your thoughts ?Mike in Colorado Link to comment Share on other sites More sharing options...
Daves1940Buick56S Posted November 23, 2013 Author Share Posted November 23, 2013 Mike:OK, in my case, I have/had 2 issues: the first is that the mechanical stop for the needle is at the 3/4 tank position indicated. I don't know if the needle is bent, or maybe the gauge face is off, or what. If I get the gauge cluster out this winter I may be able to tell. (Empty with the sending unit fully extended is a bit below the "E" mark.) The second issue is, when I tested the new sending unit with the gauge and wiring, that I was pegging the needle with only about 1/3 throw of the float from empty. IOW about 15 to 20 ohms was giving me full scale, or what passes for full scale on my gauge. With a 45 ohm unit this is a problem. At this point I don't know why ony 20 ohms pegs the needle, but I assume some of the windings in the guage may be shorted.So, what I wanted to do was to spread out the 20 ohm range, in effect. I can do this by putting a resistance in parallel with the sending unit. This lowers the 45 ohms by a lot and the "empty" 1.5 ohms by only a little. I tested with various resistances (I have a full electronics workbench) and the result is that I was able to spread it properly with 100 ohms in parallel. The "linearity" is off since I am now reading a between 1/4 and 1/2 tank with 10 gallons in (remember "Full" is the 3/4 mark) but I don't care as long as it is repeatable and I can rely on it.Cheers, Dave Link to comment Share on other sites More sharing options...
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