Fuel gauge only going to 3/4 when grounded?

-
fshd4it

fshd4it

Squid inc
FABO Gold Member
Joined
Jun 9, 2016
Messages
588
Reaction score
297
Location
Winock WA
Looks like the fuel gauge in my '77 D150 has finally died. As long as I can remember, it hasn't read more than 3/4, even when the tank is full, and empty at about 1/2 a tank. Now it's reading below empty regardless. I grounded the gauge sending wire (to the ground wire from the frame), and it bounced a little higher, but still settled back at the 3/4 mark. I pulled the IVR from the board, with no change. So I'm wondering if the IVR may be sketchy as well as the sending unit in the tank... thoughts? None of the other dash gauges are in use, I bypassed the amp gauge and installed mechanical temp, oil psi, and voltage a several years ago.
 
fshd4it said:
Looks like the fuel gauge in my '77 D150 has finally died. As long as I can remember, it hasn't read more than 3/4, even when the tank is full, and empty at about 1/2 a tank. Now it's reading below empty regardless. I grounded the gauge sending wire (to the ground wire from the frame), and it bounced a little higher, but still settled back at the 3/4 mark. I pulled the IVR from the board, with no change. So I'm wondering if the IVR may be sketchy as well as the sending unit in the tank... thoughts? None of the other dash gauges are in use, I bypassed the amp gauge and installed mechanical temp, oil psi, and voltage a several years ago.
Click to expand...
 
Float on the sending unit leaking maybe??
 
AAndrews said:
Float on the sending unit leaking maybe??
Click to expand...
Sure could be, my next test is to get under there and check the resistance across the terminals. Then go make sure the tank is full, and check it again. Or just pull the bed and install a new sender.
 
You'll have to tear the cluster apart, but the gauge itself is adjustable through those holes shown in the picture below. It's just getting to the access that's a PITA. Put a 10 ohm resister on the sender wire and ground the resistor to the frame and it should read full. Adjust as needed to get it there. Then find the pin hole in your sender float..
beerestoration2018 1015.JPG
 
Great! Thanks for that, I never knew they were adjustable. Is it as simple as moving the toothed lever with a small screwdriver, or such? I should have mentioned (but didn't, obviously) that i pulled the cluster to replace the speedometer a while back, and it was sometime after that the gauge stopped reading. Not sure if I caused it, or it was just coincidence, but I couldn't find where I might have screwed it up. Now to go find a 10 ohm resistor.
 
Yes power on, one will adjust empty (72 ohms!) and the other full at 10 ohms.
 
Am I correct in assuming I need to power it through the circuit board, and the IVR? And instead of using resistors, could I hook it up to a new sending unit (outside the tank)? More than likely I'll just order one, since if mine's bad, I don't want the truck taking up shop space any longer than it has to.
 
AAndrews said:
Float on the sending unit leaking maybe??
Click to expand...
Could be that, or the float arm was never correctly adjusted in the first place...
With the tank empty, pull the fill tube and look in the opening with a flashlight. If the float is not on the bottom of the tank (or near it), then it is not dropping fully and could be why you're empty when it still reads 1/2 tank. This would also explain why it never reads over 3/4. In this case, remove the sender and bend the float arm down until it is at/near the bottom of the tank at full drop. Then check your ohms through the full range of travel, lifting the float with a coat hanger or piece of wire.

fshd4it said:
i pulled the cluster to replace the speedometer a while back, and it was sometime after that the gauge stopped reading.
Click to expand...
You may not have gotten the cluster properly grounded when you reinstalled it.

fshd4it said:
Or just pull the bed and install a new sender.
Click to expand...
Only as a last resort. Aftermarket senders are notoriously inaccurate, and you may end up no better off than you already are. Either have the original rebuilt, or you may have some luck finding an NOS sender since it's a '77.
Edit: The D150s have a fairly squared off tank iirc, so you may have some luck with the "linear" aftermarket senders after all (I'm used to A bodies, where they don't work well at all).
 
I'll take a better look at the board connections when I pull it out, as I had to rebuild a few of the pins for the big plug. I hear you on the aftermarket sender quality... I have access to 2 '70s parts trucks, one of them may still have a good sender. When I got this truck several years ago, the bed was off and water had leaked in around the sender (sometimes silicone doesn't quite cut it). I remember cleaning up the sender, but it may have finally succumbed to corrosion.
 
You need to treat this as an END TO END problem instead of thinking about "gauge" etc

THINK about the ENTIRE PATH from power to the IVR through the various connections, gauge, wiring, terminals and to the sender

If you can get the cluster out, "rig" it to the proper 12V terminal and test it with a known test resistance. These have been published on this board hundreds of times

But grounding the sender wire and only getting 3/4 is for sure a bad sign.

I would pull the cluster and test it. If it still has a problem, you just eliminated the entire sender wire/ harness/ power source

Test the OTHER GAUGES in the cluster to be sure they don't do the same thing!!! Maybe the IVR or it's connections are causing low power to ALL gauges. (I don't know if you have oil and temp both)

Since you have it out I would at minimum do the following:

1...harness connector pins are and can be problematic on the older cars, don't know about trucks. Check them, solder them if necessary

2....Same with IVR connector fingers. Solder jumpers from connector fingers to board traces or wherever they go. Again, I'm not really familiar with the later trucks

3...Strongly consider replacing the IVR with a modern solid state one

4....RE-test any gauges supplied by IVR. If all three, they all have same characteristics, IE the same test resistor on any of the three should give same readings on both/ all three gauges.

5...Add a ground pigtail to the cluster at a common ground point, and bolt that to the dash frame/ column support
 
Thanks, Del, for giving me a few more directions to look. Last time I had the board out (a couple months ago to replace the speedo, needle broke), I refastened a few loose pins for the plug, and did a continuity test on everything I could. I only had to solder up a couple spots, everything else seemed fine. I just went out to the shop and checked the other 2 dash gauges, they both swung to 3/4 of their travel when grounded. Now, the oil psi gauge goes full tilt when the key is on (wire disconnected), but I always thought that was normal? I've got a charger hooked up to the battery, voltage is 13.3 key off, 12.9-13.1 key on. I'll start with putting a good ground wire the board tomorrow.
 
fshd4it said:
Thanks, Del, for giving me a few more directions to look. Last time I had the board out (a couple months ago to replace the speedo, needle broke), I refastened a few loose pins for the plug, and did a continuity test on everything I could. I only had to solder up a couple spots, everything else seemed fine. I just went out to the shop and checked the other 2 dash gauges, they both swung to 3/4 of their travel when grounded. Now, the oil psi gauge goes full tilt when the key is on (wire disconnected), but I always thought that was normal? I've got a charger hooked up to the battery, voltage is 13.3 key off, 12.9-13.1 key on. I'll start with putting a good ground wire the board tomorrow.
Click to expand...
No gauge should go to max with sender wire disconnected.
 
Nope, not touching the block. And when I do ground it, it creeps back down to 3/4 swing.
 
So, got a couple things figured out today. Swapped in a spare IVR, now all 3 gauges go full tilt when grounded. Fuel tank sender shows 43 ohms, but I don't know how much gas is in it, so I'm not sure if that's accurate. Also, with everything hooked back up, the gauge shows about 1/8 tank. I'll fill it up tomorrow, see what it says then. And the oil psi gauge going full when disconnected? The warning lamp (for a dash without a gauge) was in there, back feeding 12v to the signal side.
 
Yep, basically worked my way through your list. Now to get the fuel gauge dialed in. I hope.
 
fshd4it said:
So, got a couple things figured out today. Swapped in a spare IVR, now all 3 gauges go full tilt when grounded. Fuel tank sender shows 43 ohms, but I don't know how much gas is in it, so I'm not sure if that's accurate. Also, with everything hooked back up, the gauge shows about 1/8 tank. I'll fill it up tomorrow, see what it says then. And the oil psi gauge going full when disconnected? The warning lamp (for a dash without a gauge) was in there, back feeding 12v to the signal side.
Click to expand...
I've never seen a panel that had both warning lamp and gauge for oil. Warning lamp does operate on 12 volts. Only way that could work is with two separate senders and wires gto them. Anyway...
43 ohms is approx' 1/8th gauge scale. Chryslers service tester uses only 3 points of the scale, 73 ohms at empty hash mark, 23 ohms at 1/2 hash mark, and 10 at full hash mark. The quarter hash mark is approx' 34 ohms, 3/4 hash mark is approx' 14 ohms.
Those adjustments on the back of the gauge are for the factories calibration only. The end user should never need to mess with them.
 
Last edited:
That's much more of a curve than I would have thought, I expected it to be more linear. Sounds great though, it'd be nice to get this wrapped up... I'll check the resistance at the sender after I fill the tank, just so I have a number. Thanks!
 
^^Yeh we've reposted this a giga times LOL. Here is a photo of an aftermarket gauge tester and the resistances used I don't know who did this but "thanks!" This is same as factory tester, just a switchbox with resistors inside

c-3826-jpg-jpg-jpg.jpg
 
RedFish said:
I've never seen a panel that had both warning lamp and gauge for oil. Warning lamp does operate on 12 volts. Only way that could work is with two separate senders and wires gto them. Anyway...
Click to expand...
I glossed right over this when I first read your post, sorry. I'm guessing they used the same board for gauge or lamp, so it would only have one or the other installed. The speed nuts (on the gauge studs) are too close together for the lamp socket to fit into the board easily, but it can be done, if one is really intent on it.

20211029_092525.jpg
 
fshd4it said:
I glossed right over this when I first read your post, sorry. I'm guessing they used the same board for gauge or lamp, so it would only have one or the other installed. The speed nuts (on the gauge studs) are too close together for the lamp socket to fit into the board easily, but it can be done, if one is really intent on it.

View attachment 1715812120
Click to expand...
Well now I have seen it all. Well... almost, I would like to see a larger view of that circuit board trace paths. how they kept 12 volts away from gauge posts and 5 volts is too strange. I know lack of a bulb and socket would interrupt a connection but... They didn't throw similar curve balls at their own service techs often. At the same time... knowing what penny pinchers they were, one circuit board for two different assemblies doesn't surprise me. I'm glad you fingered it out. Happy moparing
 
Here's the back side of my spare ('73?) dash, with the IVR removed. Same as the '77, except the capacitors are soldered on the gauge side of the board, diffferent locations, but same orientation.

20211029_140205.jpg
 
You must log in or register to reply here.
-

Back
Top