1976 Scamp fuel gauge problems

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76Scamp

76Scamp

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As guessed, I have a 1976 Plymouth Scamp and today I finally decided to figure out why the fuel gauge doesn't work. I probed the sender and I got 61.2 ohms, so, figuring it was low on gas I went and filled it and checked after I got back. Still 61.2 ohms coming out of the tank. I probed the wire that plugged onto it and I got battery voltage of 12.5 volts and 14.2 with the engine running. If I'm right, this means the regulator in the dash has failed. My question would be, if I'm getting voltage back to the tank is the gauge still good and only the sender burned out? I always thought that it might've been a bad wire somewhere because the temp gauge still works properly because it work off the the regulator as well, doesn't it? I just got the car and from the 13 years the last owner had it, the fuel gauge never worked. I'm wondering if I should try shorting the wire at the tank to ground to see if the gauge pegs. Do you think this would be a good idea or would I risk burning up a gauge that may still work? This car is a little bit crust and the whole cluster other than the temp, odometer, seat belt light/buzzer, and an oil light that's stuck on is totally dead. But like I said, if the temp and fuel both work off of the dash regulator, why would one be working and the other not? Is the '76 12 volt gauges or are the 6 like the older models? Any help on what I should try would be greatly appreciated. Thanks.
 
If you have the old "pulsing" instrument regulator I'm not exactly sure what you would read on a modern digital meter. For certain the sender is bad or stuck
 
^^^ As said, digital voltmeter will give erratic results when checking the pulsing instrument regulator voltage. If the temp guage is accurate, then the regulator is good.

The tank sender looks to be stuck or the arm broken off and it is setting at one spot. You can ground the wire from the fuel guage for a few seconds with low risk, just don't leave it on for long.
 
That's what I figured, but I grabbed a couple regulators from the junkyard today anyway. With my meter on them, they were jumping between 7 and 0, so although difficult, it is possible to tell that they work with a digital meter. But now, before I go and play much more with this, should I get full battery power back at the gas tank or should it be the lowered, regulated voltage? I went to the Shack and bought what I need to change over from the mechanical to a solid state regulator, so I'm definitely going to do that. Now, as far as the sender, would this be a good one? http://www.autozone.com/autozone/pa...N-io1zdZ8vdfx?itemIdentifier=709606_775335_0_ I haven't purchased it yet, but I'm thinking about this one because it's the cheapest of the three I can get at AutoZone. Plus, I get a discount since I work there.
 
You should see the lowered, regulated voltage with the wire disconnected from the sender; it will be lower than that if the guage and sender are connected. A higher voltage into the guage in the dash will give higher guage readings.
 
Well, I got to poke around in it some more today. I get full battery voltage at the wire at the tank and the gauge does not move when I short that wire to ground. I will say that when I have my meter connected to the tank and bounce the back of the car, the reading does move, but it still shows close to empty with a full tank; sunken float. But now, if the fuel gauge is burned up as it seems with full battery back to the tank, wouldn't the temp gauge be the same way? The temp gauge works just fine even though I thought it runs off of the regulator. Either way, I'm gonna pull the cluster when I can and see what's happening. Right now I'm making payments and I won't official own the car for another couple of months, so I can't do anything too big to it just yet.
 
That sender should do nicely. Something else to check. There is a short piece of rubber hose that connects between the fuel line running to the front of the car and the line coming out of the tank at the sender. There is supposed to be a flat metal strap that clips to the fuel lines on either side of the rubber hose. If this strap is missing/rusted out your fuel gauge won't work since it is grounded through the fuel line. If your strap is missing and you can't find a replacement, you can bridge the hose with a couple of hose clamps and a piece of wire...
 
grumpuscreature said:
That sender should do nicely. Something else to check. There is a short piece of rubber hose that connects between the fuel line running to the front of the car and the line coming out of the tank at the sender. There is supposed to be a flat metal strap that clips to the fuel lines on either side of the rubber hose. If this strap is missing/rusted out your fuel gauge won't work since it is grounded through the fuel line. If your strap is missing and you can't find a replacement, you can bridge the hose with a couple of hose clamps and a piece of wire...
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Okay, I'll go ahead and purchase that sender then. I did notice the strap that you speak of and I was wondering what that was. Thanks for letting me know that. From what I understand, there's a member on here that possibly has the cluster for this car and I'm gonna try to buy that. But then again, there's a lot of stuff that I both need and want to buy for it. It's a factory a/c car, but all the under hood stuff was removed and put in the trunk and no one knows why. I also found the vacuum amplifier in a plastic bag in the trunk along with the original bias-ply spare, bumper jack, about 6 burned up ignition modules and 5 burned up voltage regulators. It also seems like the wiring has been cut a few times for various add-on stuff like the newish radio and an alarm that doesn't work, which is probably why the oil light is stuck on. I also found out there is no fuse in the number one spot in the fuse panel. Is there supposed to be one there? There are (I think) two other spots with no fuses, but those don't tabs in them to hold the fuse whereas this one does.
 
FYI, the voltage at the instrument panel starts out near battery voltage (approx 14 volts), gets regulated down to approx. 5 volts and splits off to the gas gauge and the temperature gauge. For the gas gauge circuit the current goes through the gauge, down the wire all the way to the tank, through the variable resistor (the sending unit) and finally to ground. If there is an open anywhere in that series circuit you'll get no reading. It should be pretty straightforward to troubleshoot. Good luck.
 
Well, I went on eBay and I found several clusters and I was actually able to win one at $36 + $20 shipping. The speedometer is not completely correct and neither is the wiper switch, but it has what I need to get everything on mine working again; I hope. Also did a valve adjustment and it's a ton quieter. I drove it to heat it up and then did the valve adjustment hot and running with the idle all the way down. The tightest I found before I started was .060", loosest around .080" and they're all now just slightly out of spec somewhere around .012" intake and .025" exhaust. Next will be the cluster and then I might start doing something about the rust.
 
You do realize that 76 is a one year only electrical system, and the cluster from any other year won't work. There are some real oddities to that system in relation to all other Mopars from that era.
Considering that you're finding all sorts of burned up parts in the trunk, and you're going through the trouble of changing out clusters, you might want to consider converting the car to more common 1975-back electronics. it may seem like a lot of effort and expense, but if you intend to keep that car, it may be a worthwhile investment,
 
whitepunkonnitro said:
You do realize that 76 is a one year only electrical system, and the cluster from any other year won't work. There are some real oddities to that system in relation to all other Mopars from that era.
Considering that you're finding all sorts of burned up parts in the trunk, and you're going through the trouble of changing out clusters, you might want to consider converting the car to more common 1975-back electronics. it may seem like a lot of effort and expense, but if you intend to keep that car, it may be a worthwhile investment,
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Yeah, I've been finding that it's definitely a one of a kind. I'm thinking of converting it to HEI so I don't have to deal with ignition boxes going bad and I may try to put a GM alternator on it off of an S10 since I have one of those under warranty at AutoZone. The main reason I bought the cluster is because the speedometer needle is bent because the magnet fell of and grabbed the cup; at least that's what it looks like. So I don't have a clue how fast I'm going, but the odometer still rolls over so I know how many miles are on it and that's how the previous owner kept track of how much gas was in it. I will say that the cluster I bought appears to be identical even though it came out of a 72 Duster according to the listing.
 
76Scamp said:
Yeah, I've been finding that it's definitely a one of a kind. I'm thinking of converting it to HEI so I don't have to deal with ignition boxes going bad and I may try to put a GM alternator on it off of an S10 since I have one of those under warranty at AutoZone. The main reason I bought the cluster is because the speedometer needle is bent because the magnet fell of and grabbed the cup; at least that's what it looks like. So I don't have a clue how fast I'm going, but the odometer still rolls over so I know how many miles are on it and that's how the previous owner kept track of how much gas was in it. I will say that the cluster I bought appears to be identical even though it came out of a 72 Duster according to the listing.
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The difference is not in the looks of the cluster, it's the way everything is wired in that car..and the burned up electrical parts are also a victim of that. In 76 they revised the wiring so that the Alt. gauge is no longer a direct part of the charging system circuit, so in essence they reinvented the Mopar electrical wheel. It's been my experience in working around these cars for 37 years, that 76 A Bodies are always prone to goofy electrical issues. Yours seems to be about what I would expect.
 
whitepunkonnitro said:
The difference is not in the looks of the cluster, it's the way everything is wired in that car..and the burned up electrical parts are also a victim of that. In 76 they revised the wiring so that the Alt. gauge is no longer a direct part of the charging system circuit, so in essence they reinvented the Mopar electrical wheel. It's been my experience in working around these cars for 37 years, that 76 A Bodies are always prone to goofy electrical issues. Yours seems to be about what I would expect.
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That's what I've been finding out. The Alt. gauge doesn't work, but I know that it's charging so I don't really care much. All I really want is the speedometer needle and some of the clockwork for it. If the fuel gauge will work, that'd be a nice bonus. For the Alt. gauge, I'll probably end up getting an under dash volt meter. I'm also going to put relays on the headlights because they tend to dim at stops and I'm going to attempt HEI.
 
I've replaced both the speedometer and the fuel gauge, and now the speedometer works, but is about five high. The fuel gauge is still dead, but I know it's a working gauge. I also replaced the regulator with a known working one, but the back of the circuit board is well corroded and the pins are separating, so I'm thinking I'm gonna have to replace that too. The cluster I bought is for a 72 and not a 76, so I can't use that being the layout for the ammeter is different. Anyone know where I can get one for a 76 for cheap? Thanks.
 
I know it's more work and money than you want to spend, BUT, the best possible way to fix that car would be to keep the 72 cluster as is, and swap out the dash harness for a 75 style one. You MAY need to change the position of some of the pins on the bulkhead connector to correspond with the underhood harnesses, so some schematic studying would be a good idea, but that 76 only stuff is very hard to find in good, working order.
I do have a perfect dash harness if you decide you want to go that way.
 
whitepunkonnitro said:
I know it's more work and money than you want to spend, BUT, the best possible way to fix that car would be to keep the 72 cluster as is, and swap out the dash harness for a 75 style one. You MAY need to change the position of some of the pins on the bulkhead connector to correspond with the underhood harnesses, so some schematic studying would be a good idea, but that 76 only stuff is very hard to find in good, working order.
I do have a perfect dash harness if you decide you want to go that way.
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What I'm thinking is find out what pin does what and wire directly to the corresponding gauge or light from the plug. If I don't want to cut, splice, add wire, and confuse myself in the future, I can buy one on eBay, but it's $150 before shipping and at that rate I could've bought a restored cluster for $150 with free shipping. I have the functional fuel gauge installed in the 76 cluster, but nothing is happening due to the circuit board being in such terrible shape. I think I'm gonna try to clean it up and touch up the soldering at the pins to see if that brings any life to anything. But for the next couple months, it's still not my car; I just made the first for 5 $400 payments yesterday. This car also has a major rust problem and the a/c is disassembled and put in the trunk, but it is registered and driven every now and then. But right now my main thing is to get the gauges working again and then I'll deal with the rest.
 
The car now has a working speedometer, temp gauge, and fuel gauge. The ammeter is burned up and none of the dash lights come on, but the three main gauges work along with the turn signal indicators and the oil light that's stuck on. Now the fuel gauge always reads just over empty and it should work properly once I have a new sending unit since it pegs when I short it to ground at the tank. And the worst part about buying a cluster and making one out of the two is that I stopped at a friends place, told him what I did, and then he goes and digs in his shed and pulls out a correct, potentially functional cluster for the car that he said I could have. He told me it's been sitting in there since the late 90's, so it may or may not work. Since mine does all that I care about, I don't know if I should try to mess with it much anymore.
 
nm9stheham said:
^^^ LOL isn't that always the way it works... "Oh, I had your part here all the time..."
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Yeah, that's how it normally goes. I told him I was gonna order window cranks because the knobs were broken off, and he gave me a full set for it. He said if there's anything else I need, talk to him first.
 
Okay, new sender is in, and it doesn't really do anything. From an empty tank, the gauge doesn't start to move until I have it about half full and then once it is full it's only reading 3/4 at best. Would this be caused by it being a new sender that isn't exactly like the old one is or could it be something else? On my meter, I see a nice sweep up and down the scale from 83ohms up to about 7ohms at the wiring behind the cluster.
 
From these symptoms, either the guage is off (high resistance) or the voltage limiter is not right (low output). But I am assuming that the guage and limiter operation were the same in '76 as in earlier years. The sender resistance range seems about normal, at lesat for all earlier years; some quick checks indicate that the Aspens used the same resistance range so the system look to be the same for the '76 as for the older cars..

Have you changed the voltage regulator to the solid state, and is that a 7805 voltage regulator? Do you have access to a 5V power supply? The best way to troubleshoot that will clear this up is to feed the gauge and sender with a known good steady 5v and see what you get.

Does the temp gauge seem to read right when operating and driving the car? If so, then that is an indirect indicaiton that the limiter is OK, and all that is left is the fuel gauge itself being off. These are thermally activated gauge movements; there is a heating wire coiled around a bi-metallic strip that bends when it gets hotter (fuller tank). Its movement is mechanically connected to the needle. It might be the heating wire, the strip or the needle. See here: http://www.nls.net/mp/volks/htm/fuel_ga.htm
 
nm9stheham said:
From these symptoms, either the guage is off (high resistance) or the voltage limiter is not right (low output). But I am assuming that the guage and limiter operation were the same in '76 as in earlier years. The sender resistance range seems about normal, at lesat for all earlier years; some quick checks indicate that the Aspens used the same resistance range so the system look to be the same for the '76 as for the older cars..

Have you changed the voltage regulator to the solid state, and is that a 7805 voltage regulator? Do you have access to a 5V power supply? The best way to troubleshoot that will clear this up is to feed the gauge and sender with a known good steady 5v and see what you get.

Does the temp gauge seem to read right when operating and driving the car? If so, then that is an indirect indicaiton that the limiter is OK, and all that is left is the fuel gauge itself being off. These are thermally activated gauge movements; there is a heating wire coiled around a bi-metallic strip that bends when it gets hotter (fuller tank). Its movement is mechanically connected to the needle. It might be the heating wire, the strip or the needle. See here: http://www.nls.net/mp/volks/htm/fuel_ga.htm
Click to expand...

The temp gauge works as it should, but while I had the cluster out I replaced the regulator with a known working one and I do have an electronic replacement to put in it, but haven't gotten to it yet. Is there any known way to see if the gauge itself is working properly? A way to know if it's worn out? I had to pull the cluster in the first place to repair the speedometer and the fuel gauge that was in it didn't move at all, so both speedometer (unrelated) and the fuel gauge were replaced along with the voltage regulator that came out of a mid-80's Dodge truck. The gauge came out of a 72 Dart cluster.
 
The only way I know of is to use a steady 5v source and connect up the guage in series with resistors to duplicate the sender: 10 ohm for full, 24 ohms for 1/2 tank, and 72 ohms for empty. You can usually get the resistors at Radio Shack.
 
Okay, after getting all the mechanical tuned up and this car is running better than ever, but this is still a problem. I have a new sender in the tank and the resistance goes up and down very smooth and is spot on in value with what is in the tank, but the gauge does not. The first time I filled it, it went to full and after driving it for a while I noticed it was almost on empty, so I went to fill it and it took all of three gallons to get it back to full. I have isolated the fuel gauge from the old circuit board and I put a 6v regulator on it thinking the little extra power might help. Now it reads empty after burning four gallons and the resistance read off of the line at the cluster is right for that amount. Could the gauge be that far out of calibration or could it just be worn out and I need to have it rebuilt or something?
 
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