Gas Gauge Failure - 39 LImited

[WHSEWARD]

I think the gas gauge on my original 39 Limited has given up. I checked the sending unit by disconnecting it and the gauge reading went to "full"; I think that indicates that the sending unit is OK? But when the ignition switch is turned on I get no reading even with half a tank of gas. I put a little jumper wire between the ignition switch and the + side of the gas gauge, thinking that if the wire were bad I would get the gauge to move when I turned the ignition switch on, but there was no change. The car starts and runs OK, and from what I see in the wiring diagram, the car wouldn't start if the ignition switch were bad (unless it was hot-wired).

This is a recent problem ( it was reading OK up to 2 weeks ago) and I have not messed with the wiring under the dash since I got the car, although I have had the car in the shop for other work since I got it last October. This morning I moved some wires back there and there was some sparking, so I think I need to get the instrument cluster off and find/fix the short(s).

Questions:

1. I need to know how to release and remove the instrument cluster. I can see several screws that look like the right ones, but the cluster looks like it releases out the front rather than going back through the dash. I don't want to get this wrong because the wiring looks to be mostly original so it doesn't need to be pushed around or bent more than is absolutely necessary. Diagrams or photos would be appreciated.

2. I thought I could hook up an ohmmeter across the 2 terminals of the gas gauge and that would tell me for sure if the gauge were bad, wouldn't it? I assume that a reading of zero would indicate empty which I know is not true. What would be the half-full reading?

3. Who sells NOS AC gas gauges, in case that is the problem?

I have 38 and 39 Shop Manuals and a Motor Vintage Car Repair Manual, so if I am missing something in them, please let me know and I'll go look there some more.

Thanks to everyone.

[jeff]

If you saw a full scale reading with the wire disconnected, then your guage is good. The sending unit varies from high resistance (full tank) to nearly zero resistance (empty) on the low side of the circuit (connection to ground).

So, if you take the sending unit wire to ground and see the guage read empty, then pull it off of ground and see the guage swing full scale, you have proven the guage to be OK.

In your case, it sounds like you have a short to ground in the wire leading from the guage to the tank sending unit.

If your wires are that "crumbly", you might look into investing in a reproduction harness. They are quite nicely made, and go a long way toward preventing catastrophic fires. If the '39 is like my '38, there really are no fuses to protect against harness shorts. (except the clock and heater, and the thermal circuit breaker for the lights)

Jeff

[WHSEWARD]

Jeff,

OK, I think I see...a short to ground in the sending unit wire will drive all the current through the "empty" coil to ground prio to encountering the resistance in the tank unit. That's consistent with the short circuit I obesrved under the dash this morning. Now, all I have to figure out is how to get the instrument cluster out of the dash so I can look at the wiring without being upside down and half-blind.

Thanks,

[Guest Grant Magrath]

The good news is that from memory, there are only 2 nuts to undo on either side to get the cluster out. But you don't need to take the whole thing out. You could contort yourself to unscrew the 2 screws that hold the gas gauge in. If you don't have a radio installed, it's easier to take the speaker cover off, and get at the gas gauge that way.

Cheers

Grant

[WHSEWARD]

Well, things became much more interesting since my last post this afternoon. I verified the "short-to-ground" theory and almost set the car on fire. It looks as though the dash wiring has deteriorated inside the existing harness and to what extent, I don't know yet, but as of now I cannot even turn the disconnect switch on without sparks and smoke (BTW, if anyone reading this does NOT have a battery disconnect switch installed yet on their old car, DO IT NOW). I have seen pictures of a car consumed in flames on a recent tour, and I know of a vintage Packard that burned up in a separate incident. If I had not switched the disconnect back off 3 seconds after switching it on, my Buick might have been a statistic instead of a car.

At any rate I am now in the market for a replacement wiring harness. Any recommendations?

[Guest Grant Magrath]

Rhode Island Wiring. Beautiful stuff, and fits fantastic!

I see your in Katy. Go the Texans! I have a very good friend in Katy who's rodding a 39 Special two door sedan. Bought his front bumper on eBay, and stayed in touch ever since.

Cheers

Grant

[WHSEWARD]

Thanks, Grant, for the recommendation on RI Wiring. I saw them on the Internet along with YnZ and Harnesses Unlimited. Does anyone have any feed back on any of these guys. If so you can PM me at the e-mail below.

BTW, the instrument cluster does unscrew with the 4 screws, but it comes out the back. With the congestion caused by the harness there's no way to get it out without taking out the entire harness and unhooking everything.

Anybody out there have recommendations for electrical shops in Houston or Katy? If so, PM me. It looks right now like I will have to do it myself - with the space congestion in my garage I am not looking forward to it but I will manage if I have to.

All the best,

mr.seward@sbcglobal.net

[Guest Grant Magrath]

Bill, I've given your email address to my friend, Dan, in Katy. He's been hot rodding for years, and will most likely know someone in the area.

Can you not turn the cluster slightly to get it out the front? You'd have to be careful not to scratch your dash though.

Cheers

Grant

[WHSEWARD]

Grant, I have been in touch with Dan and hope he will be able to help out. The wiring under the dash is so congested, and stiff with age that I cannot get enough play to get the cluster out the front. Each of the brackets is about 1-1/2" wide so I would have to move it a lot to get it out.

However, since I'm going to replace the entire wiring harness I don't need to get the cluster out anyway, so I just put it back in place.

Thanks to all for the advice.

[michaelod]

I got a complete wiring harness for my 1930 Buick Model 68 from YnZ. They were able to customize it to my specs and delivered a quality product in record time at a reasonable cost. I would give them my highest recommendation.

Thanks,

[JCK55buick]

Bill,

I struggled getting the speedo out of the dash of my 39 Limited, had it worked on and then struggled to get it back in, bent the needle and it was a pain! My gas guage also does not work, sits on E always. I'm pretty sure my wiring harness has been replaced already, so I probably need to start eliminating different items to learn the cause.

Keep us posted on your progress!

[WHSEWARD]

Jeff,

I found this site to be helpful in getting started on my diagnosis problems.

Fuel Gauge Problem Diagnosis

Also, has anyone tried the trick of using an ohmmeter (in the trunk) to substitute for the gas gauge? Seems like the resistance in the rheostat is directly correlated with the fuel level, so if you had a place to secure permanent connections and a relatively inexpensive meter, this could be a substitute for a gas gauge replacement. Has anybody compared the costs?

[WHSEWARD]

Hi, everyone...

Well, I have done some diagnostics and believe that my gas gauge (dash unit) really is bad, as opposed to the sending unit or the sending wire. I want to test this hypothesis and need to know a couple of things:

1. What is the stated resistance range of the 1939 Buick sending unit in the gas tank? empty = about zero; what is full? I nelieve sending units for 38 and 39 were the same.

2. Does anyone know of an inexpensive replacement gas gauge that I can rig up that will give me a reasonable approximation to the fuel level. I have just installed a brand-new front wiring harness in the car and am not about to tear it out to remove the instrument cluster from the dash, to get the gas gauge replaced. I believe that I can wire around the bad gauge through a replacement gauge, but the gauge and the sender should be reasonably compatible (6v, negative ground, AC-fuel gauge compatible). There are some replacement 6v dash gauges out there for old (early 50's) Chevy trucks, but I don't know if they are compatible or not.

I know I can get a replacement NOS gauge but I'd like to minimize my cost if possible. I'd welcome information and suggestions.

Thanks,

[Guest Grant Magrath]

Hi Bill.

Has Dan got anything?

Cheers

Grant

[WHSEWARD]

This is an update to my post of a couple of days ago. I wanted to offer it to those whose gas gauge has given up the ghost but whose sender is still good and well-grounded. The resistance in the sender is controlled by the fuel level float and can be measured by an ohmmeter. I disconnected the sender wire in the trunk and measured the resistance through the sending unit to ground. Then I took it to the filling station and added 2 gallons, measured resistance again; 2 more gallons, another reading; 2 more, another. So I have a 4-point curve of change in fuel volume versus resistance. I know the resistance for an empty tank is close to zero, and the tank capacity (for my Model 90) is 25 gallons. I did a little curve-fit and now have a graph of fuel volume in the tank vs. resistance. For me, when I see 40 ohms, I have about 15 gallons, and when I am at 10 ohms, I have about 3 gallons and know that I need to put in some gas. Not "accurate" and not continuous, but I can measure the resistance without the car turned on so I know before I start a trip whether I should put in some fuel or not. Next time I fill up, I will fill it all the way so I get the top of the curve as well. I've been told that the sending unit should show about 88 ohms on full. I get about 65 ohms but it will be whatever it is, for each individual's tank.

Perhaps this will be good enough for those folks who are not looking to buy a new gas gauge for their dash cluster right now. I would welcome comments and questions.

[Thriller]

Excellent suggestion Bill. Thanks for sharing your experience. With enough data, you are probably more accurate than most of the gauges on my running cars.

[Guest Straight eight]

Have used Harnesses Unlimited on two cars. They are just great! I was able to contact them with questions even at night. They couldn't do enuff to please. Great outfit.