Help With New Instrument Cluster, please

[pjmoreland]

I just tested a factory gauge, and it goes directly to the empty mark when the tan wire post is connected to ground and the other posts are connected to power and ground.

These gauges generally stay where they were last positioned when power is removed. Under normal circumstances, I don't see how the gauge would end up pointing off to the left like yours. It should be somewhere between empty and full, and possibly as far clockwise as 3:00 if the tan wire is disconnected. Makes me think your gauge isn't getting either power or ground.

[Zonaman]

Originally Posted by pjmoreland

If it were me, the next thing I would do would be to verify the presence of 12 volts across the power and ground posts on the gauge with the key in the run position. I would also make sure there isn't continuity between the input post (connects to tan wire) and ground.

So if want to test this in the truck (cluster plugged in and battery connected) I'd clip my red multimeter to the key on post and clip the black wire to the ground post and turn key to on position to check voltage. Then key off and clip multimeter to ground post and input posts and look for no continuity ? Just want to be sure I'm not going to short anything out this way. Thanks

[pjmoreland]

Yes on the voltage test.

For the continuity test, I thought about some more, and it would be better to measure resistance between the tan wire post on the gauge and the ground post. This should be done with the ignition off. You should get somewhere in the ballpark of 45 Ohms (maybe less), depending on the position of your sender. I would also take this same measurement with the brown wire disconnected from the sender. The resistance should be close to 100 ohms.

[Zonaman]

Originally Posted by pjmoreland

Yes on the voltage test.

For the continuity test, I thought about some more, and it would be better to measure resistance between the tan wire post on the gauge and the ground post. This should be done with the ignition off. You should get somewhere in the ballpark of 45 Ohms (maybe less), depending on the position of your sender. I would also take this same measurement with the brown wire disconnected from the sender. The resistance should be close to 100 ohms.

The tan post to ground on back of gauge key off read about 57 Ohms. There's only a tan wire on the sender no brown did you mean that I should disconnect that one at the sender and repeat the same test ?

[pjmoreland]

Originally Posted by Zonaman

The tan post to ground on back of gauge key off read about 57 Ohms. There's only a tan wire on the sender no brown did you mean that I should disconnect that one at the sender and repeat the same test ?

Yes, sorry, tan disconnected at sender

[Zonaman]

OK my turn to mess up. The first reading was with tan wire off sending unit. It read 33 Ohms wire on. that's on my Cen-Tech meter set to 200 in the Ohms section.

Here's what else I know:

Fuel Gauge Circuit Tests
New Instrument cluster, new fuel sending unit. Condition key off: gauge needle 9 o’clock (off top scale). Key on gauge needle moves toward 10 o’clock position.
• Battery: Good
• Fuses continuity: Good
• Wire (tan) continuity from sending unit to fuel gauge fuse block: Good
• Ground at instrument cluster : Good
• Ground from cluster wiring harness to cab: Good
• Ground at sending unit on tank: Good
• Tan wire off post at tank, key on: gauge does not move from 10 o’clock.
• Tan wire on full ground, key on: gauge does not move from 10 o’clock.
• OHMS at new sending unit installed in tank (approx. half tank of fuel): 44

[pjmoreland]

The resistance measurements seem a little low, but that doesn't necessarily indicate a problem. It's good that you get two different values with tan wire connected vs. disconnected.

Did you measure the voltage between the power and ground posts on the gauge with the ignition in the run position?

[Zonaman]

My apologies if I'm over cautious as I'm not too well versed in auto electric so to test this in the truck (cluster plugged in and battery connected) I'd clip my red multimeter to the key on post and clip the black wire to the ground post and turn key to on position to check voltage?

[pjmoreland]

Yep. Just make sure the clip on the key-on post is touching only the post and nothing else

[Zonaman]

13 volts

[pjmoreland]

You've got power to the gauge.

You've got a seemingly good connection to the sender.

The resistance of the sender seems good.

I hate to say it, but I think you have a bad gauge.

[pjmoreland]

Also, the 57-Ohm measurement you took of the gauge's internal resistance should be closer to 90 or 100 Ohms.

[Zonaman]

Well those readings plus the grounds being good (as per a test light) and the gauge not reacting to be having the tank input wire (tan) fully grounded and completely disconnected, as well as the the new sending unit reading 44 Ohms (right for half a tank in addition to reading correctly on the old gauge for the old cluster) would seem to point the gauge itself? Am I interpreting this correctly? Are there other tests I should conduct?

[pjmoreland]

The definitive test would be to pull your sender out of the tank so you could move the arm and see the needle move (sender flange connected to ground). If the needle doesn't move, then you could reinstall your old cluster and see that the needle moves in the same conditions.. I think you already have enough info to say the gauge is bad though.

[Zonaman]

I'd really like to avoid pulling that sender out but might just do it. What about the amp meter. Is there a test for that I can do while it's installed to see if it the gauge is good? Again my thanks to everyone who contributed to helping me out.

[pjmoreland]

You can simulate the sender in the empty position by grounding the tan wire post on the gauge with ignition on.

Since the gauge doesn't move to 3:00 with the tan wire disconnected from the sender and ignition on, this indicates the problem is not with the sender.

I wonder if the gauge needle is mechanically stuck somehow.

[pjmoreland]

The amp meter doesn't usually move very much. Mine will twitch to the left a little if I turn on my headlights with the engine off. It will move slightly to the right for a few seconds immediately after starting the engine.

There are two fuses for the amp gauge up in the front two corners of the engine compartment. They are inside inline black rubber holders that are connected to the headlight/alternator harness.

[Zonaman]

Originally Posted by pjmoreland

You can simulate the sender in the empty position by grounding the tan wire post on the gauge with ignition on.

Since the gauge doesn't move to 3:00 with the tan wire disconnected from the sender and ignition on, this indicates the problem is not with the sender.

I wonder if the gauge needle is mechanically stuck somehow.

I did ground the tan wire out, Stay right where its. I noted that when I unpacked the cluster, the fuel gauge showed in the 9 o"clock position. Don't know if that means anything or not.

[Zonaman]

Did pull that sending unit. It went from 4 to 96 Ohms. Knew it. Plastic float is doing fine

[pjmoreland]

Originally Posted by Zonaman

Did pull that sending unit. It went from 4 to 96 Ohms. Knew it.

There's gotta be something wrong with that new gauge.

I took some more measurements on my OEM gauge. I believe there are two windings inside the gauge that push the needle in opposite directions. One winding is between the power post and tan post, and it pushes the needle toward Empty. The other winding is between the tan post and the ground post, and it pushes the needle toward Full. With the tan wire disconnected, the needle goes way past Full (3:00) and I measured 8.55V on the tan post.

Since your needle isn't moving toward Full, this might be because the empty-pushing winding is pushing too hard, or the full-pushing winding is too weak. You measured only 57 Ohms for the full-pushing winding resistance, whereas I measured 103 Ohms. That seems odd to me.

[pjmoreland]

I just noticed something interesting. The needle can be spun by hand an unlimited number of times around. If you spin the needle by hand while the gauge is powered, at first it will fight and want to return back to its original position. As you turn it farther, it reaches a point where it is no longer fighting, and then it starts pushing the other way to return to its original position. If you position the needle exactly 180 degrees out from where it wants to be, it gets stuck there until you help it along a little in either direction, and then it will finish moving to the desired position on its own.

I wonder if your gauge is stuck in this position (9:00) opposite from the normal resting place for a disconnected tan wire (3:00). Now normally if you connected the brown wire to ground, then this would unstick the needle and move it to Empty. I wonder if for some reason your gauge has a flaky connection between the tan post and the tan wire. Maybe the nut securing the gauge to the flexible printed circuit just needs to be tightened a little.

[Zonaman]

At this point, I say I've done due diligence. I've paid a premium price for a fully functioning instrument cluster. Not one that needs tweaking or requires me to tear it apart or put a replacement part in. Brother's Truck Parts has a real nice sounding statement about customer service and treating folks right when you call them. Now I'll find out if they're going to man-up an make good on this by sending a me replacement in a timely fashion.

[pjmoreland]

Sounds like a good plan

[pjmoreland]

The photo you posted of the back of the gauge shows star washers under at least two of the nuts. Is there a star washer under the tan wire post nut?

[Zonaman]

Yes. The tan wire input, and the key on power poles have two star washers and two nuts each sandwiching the gauge to the housing and the printed circuit down.
The ground has a nut and star washer as well. All are nice and snug.