Coolant temp gauge problem

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1994redram

1994redram

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My dart is in “paint shop prison” so I bought a ‘69 D100 to play with. It’s a Mopar and pretty similar to A bodies electrically. I’m hoping you guys can help me.

My gauges didn’t work, I bought a digital IVR from a member here. I disabled the failed internal regulator in the fuel gauge and wired in the digital IVR. I’ve got a very steady 5.03 volts with the truck running. The trucks don’t have a circuit board, just wires and terminals. Easy!
The fuel gauge works great. The coolant gauge pegs hot when the wire is grounded in the engine bay. The gauge still didn’t work so I replaced the sender. Still doesn’t work. I tested the sending unit at engine operating temp and got ~40 ohms with the new sender and ~38 ohms with the old. Seems fine.
I checked continuity from the connectors at the sensor and at the gauge, wiring tested good. I “scrubbed” the firewall connections but it all looked great.
Again, ground the wire and the gauge moves. The gauge has 5V. The sending unit seems to test a little “cold” but I would think it’s within range to move the gauge.

I’ve got to be over looking something simple.
 
inertia said:
Sender base grounded ?
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The sender is threaded into the block. The battery is grounded to the block directly. The head is grounded to the firewall.
The senders checked out around 40 ohms at operating temp. I can try to jump it to battery ground just to rule that out.
 
1994redram said:
The sender is threaded into the block. The battery is grounded to the block directly. The head is grounded to the firewall.
The senders checked out around 40 ohms at operating temp. I can try to jump it to battery ground just to rule that out.
Click to expand...

Wouldn't be the first time sealer insulated the base .
Cheers .
 
I don't think sender grounding is a big deal, but does not hurt to jumper the sender hex to ground. Also be careful not to overtighten, they can be damaged

Try to double check the sender part numbers and see what else they fit. If they fit B/C/A bodies, etc, they are the same resistance range. With the engine fully warmed, that resistance reading sounds high, which would read cold "Standard" readings are:

c-3826-jpg.jpg
 
How is the dash grounded? More specifically how is the temp gauge grounded?
 
I have grounded the wire to the block and the alternator case since they're close by. I attached a long wire to the connector in the engine bay and ran that into the cab. I can ground that to the dash frame and watch the gauge go up to hot.

I don't have any sealant on the sending unit.

The gauge doesn't have a ground. It only has two posts on it, one is 5V supply the other is the wire that goes to the sending unit. I thought the sending unit supplies ground to make the gauge move.
 
1994redram said:
I have grounded the wire to the block and the alternator case since they're close by. I attached a long wire to the connector in the engine bay and ran that into the cab. I can ground that to the dash frame and watch the gauge go up to hot.

I don't have any sealant on the sending unit.

The gauge doesn't have a ground. It only has two posts on it, one is 5V supply the other is the wire that goes to the sending unit. I thought the sending unit supplies ground to make the gauge move.
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The cluster MUST be grounded. Part of the circuitry of the IVR must be grounded. I would run an extra wire direct to the cluster. You can find mounting screws near the ground points of the instrument lighting EG. I can't see and am not familiar with what you have, and realize it is different than the cars.

It's also possible in some vehicles that the body is not well grounded to battery NEG, AKA engine block. In the old days, many vehicles, not just Mopars, had poor grounding between cab/ unitbody and the battery NEG.
 
Dana67Dart said:
Where are you grounding the wire to?
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The reason I asked this is if your engine is not grounded correctly you might be getting no needle movement from the sender, but get movent when the wire is grounded to a good ground
 
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If the gauge needle sticks at off/home, a slowly climbing resistance may not cause movement. A zero ohm (short gauge to ground) can cause a gauge to do things that a within range (80-10) resistance will not. Shorting the sender wire to ground is a bit "shade tree" test but... everybody does it initially.
With only the vehicle for testing, you could remove the blue wire from the fuel gauge and put the purple temp wire there. If wiring is good your 40 ohm running temp resistance should move a good fuel gauge needle to just over 1/4 tank.
 
The gauge housing is grounded. The harness has a dedicated ground wire that attaches to the cluster with a screw. The voltage regulator is grounded to the same screw. The regulator is putting out a solid 5 volts. The fuel gauge itself doesn't have a ground at the cluster.

I used a wire wheel and cleaned the block, firewall, frame and also cleaned all of the wiring terminals. I removed the sending unit and cleaned it and the block. I ran a wire from the battery ground to the body of the sender just in case, still nothing.

Ill try to switch the coolant temp wire with the fuel gauge wire and see what happens.
 
I swapped the wires on the fuel and coolant gauges. The coolant sender worked great on the fuel gauge. It warmed up and showed just under half on a 10 mile trip. The fuel sender works on the coolant gauge, showed the same 3/4 tank. I topped the tank off and it shows full.

I swapped them back to where they're supposed to be. No coolant gauge! I don't get it, it makes no sense.
 
I admit, I am stumped.

You know where the gauges read, I assume, take notes.

Now measure resistance of both of them, the engine when warm. Do the resistances make sense?

Otherwise, it is time to scare up some test resistors and compare on each gauge. They of course I would think read the same, as the do on any other car of the era

If you have a spare tank sender you can use it, set it with the ohmeter where you want according to the tester photo above, and compare to each.

If you have any test equipment such as a generator/ alternator tester, and it has a field rheostat, you can likely use it
 
Are the gauges OEM?

I recall aftermarket classic ind guages say somewhere that you have to use their senders
 
BTW post some photos of the front and rear of the gauges
 
1994redram said:
The gauge housing is grounded. The harness has a dedicated ground wire that attaches to the cluster with a screw. The voltage regulator is grounded to the same screw. The regulator is putting out a solid 5 volts. The fuel gauge itself doesn't have a ground at the cluster.

I used a wire wheel and cleaned the block, firewall, frame and also cleaned all of the wiring terminals. I removed the sending unit and cleaned it and the block. I ran a wire from the battery ground to the body of the sender just in case, still nothing.

Ill try to switch the coolant temp wire with the fuel gauge wire and see what happens.
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I don't like to argue so just the facts... A typical 2 post gauge is not chassis grounded. If you remove one from the housing you will find the housing has a cut away area beneath the gauge. Only current path through a 2 post gauge is post to post.
A 3 post gauge is chassis grounded in the inst' housing. It even has a piece of metal on the back to help ensure this ground path. That current path is for only the mechanical limiter. You stated that you had replaced the mechanical limiter in the fuel gauge with a solid-state regulator.
You should have put a piece of tape on the back of that fuel gauge to isolate it from the chassis ground. The solid state regulator would be grounded somewhere else. We don't know that the remnants of the mechanical limiter is not wicking away a good portion of the solid state regulators output. 5.3 volts is one thing. Barely over 1 amp is another.
My best guess is a bad temp gauge that will not respond to high resistance/low heat. Do this... Use ohm meter to check the 2 gauges post to post. The fuel gauge should be 20 to 21 ohms. The temp gauge might be same 20 ohm gauge but could be a 13 ohm gauge too. The 13 ohm gauges are known to show low to nothing with low amps.
 
1994redram said:
I swapped the wires on the fuel and coolant gauges. The coolant sender worked great on the fuel gauge. It warmed up and showed just under half on a 10 mile trip. The fuel sender works on the coolant gauge, showed the same 3/4 tank. I topped the tank off and it shows full.

I swapped them back to where they're supposed to be. No coolant gauge! I don't get it, it makes no sense.
Click to expand...
Have you tried cleaning posts on back of fuel gauge and temp gauge? Sometimes they get oxidized and have a bad connection.
 
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