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Old 05-31-2017, 11:41 PM   #1
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Dual Helm Temp Gauge Issue

I have existing dual helm temp gauges running off of a single, dual pronged sender unit. I thought my lower helm gauge was bad because it would not power on and the flybridge gauge reads much too high. It's been this way for some time.

There are two wires (green and black) coming from the sender. It doesn't matter which way I connect them to the sender, I get the same result at the gauges...lower dead, flybridge high.

Tonight, I decided to run a new, single wire from the lower helm directly to the sender. This causes my lower helm to function (but read too high), but the flybridge was dead.

If I pull the green wire, neither gauge works. Replacing the black with my direct to the lower helm wire causes the lower helm to read high and the flybridge to be dead.

I'm not an electrician. My simple mind figured that one line goes to a gauge, the other line to the other gauge, but that doesn't seem to be occurring. Maybe run a new wire (replacing the black) to the lower and then daisychain up to the flybridge? Thanks for any input.
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Old 06-01-2017, 07:55 AM   #2
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Hard to know....

Green could be ground and a single goes to the lower gauge then up to your upper one.

It is a resistance setup for 2 gauges, meaning the sender needs both guages to be hooked in series/parallel (not sure in your case, probably parallel in the harness) and both gauges have to be functioning....I think....

Can you find a wiring diagram on the manufacturer's site?

Has the problem developed since you have had the boat? Or did you buy it that way?

What brand instruments?
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Old 06-01-2017, 09:00 AM   #3
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I'm with you on the need for both gauges to be working for an accurate reading. They are VDO gauges, original to the boat circa 1988. I've been digging around on the VDO site and others in general, but haven't found a good diagram yet. I'll try calling VDO to see if technical support may be able to help.

The issue existed when I got the boat. Temp sensor is in a 3208 Cat if that matters.
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Old 06-01-2017, 09:25 AM   #4
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I would think that because of the sender - ( essentially a variable resistance T stat ) resistance relationship to the temp gage, you can not have them BOTH or wired together. I would think that it's one or the other.
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Old 06-01-2017, 10:01 AM   #5
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Nope...both wired together is more standard as many senders for dual station installs are only a single wire from the sender...which I have on my boat.

Plus on the floating ground senders, usually the 2 prong senders, one side is a ground and only one wire heads for the gauges.

Thats why you need either a single or dual gauge sender, they vary the residtance accirdingly.
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Old 06-01-2017, 10:05 AM   #6
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Someone may have put the wrong sending unit in. My second station was disconnected and Im keeping it this way for now. had to put a single station vdo temp sensor in to do this. The temp sensor are different for single and dual stations. The guage can be check to see if it works by grounding the sensor wire to rngine ground. If the gauge is good the sensor needle will go to Max temp.
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Old 06-01-2017, 02:38 PM   #7
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Found the same sort of issue on my recently purchased Defever 44.... The oil pressure on both JD4045s, on both helm gauges were reading around 100psi. PO said always been that way and his mechanic said "no problem"... 😒 I suspected instrument problem as 100psi would have probably blown out the oil filter gasket. So
I rigged a mechanical gauge on both engines before I closed the deal and verified actual oil pressure was normal.
Talked to M&G who built the instrument panels and confirmed that pressure senders rated for a single helm panel had been installed by mistake when boat was built in 2005. Good ol impedance mismatch....got correct sender number and will add to my reqork list. For now, I have just unpluged the lower helm oil gauge and the upper helm where we drive from, reads correctly.
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Old 06-01-2017, 05:16 PM   #8
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If its feasible, swap the gauges - if nothing changes then the gauges are ok and its either the sensor or wiring.
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Old 06-01-2017, 06:13 PM   #9
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I considered swapping the gauges, but I'm able to get the lower helm to replicate the too high reading of the upper helm when I run the new wire, which would seem to indicate the gauge is not the issue. I'm going to try to daisy chain the two together with a new sender wire this weekend.

As for the sending unit being replaced in the past, the fact that it is only reading on one gauge and is incorrect leads me to believe that it is a dual station unit. Also, I know the history of the boat for the last decade and I'm not aware of that being addressed. Honestly, it was used mainly as a floating condo, so non-functioning gauges (or steering cylinder, generator, windless, house water pump, etc) weren't really a concern. I've learned a ton in the process of making her functional again, but it scares me to take her out too much without that temp gauge working.
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Old 06-05-2017, 11:18 AM   #10
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Update: I ran a new 14 gauge wire from the sending unit to both gauges and now have a reasonable reading on both, although not the same reading. That's better than what I had before. My next concern is that all of my gauges aren't quite right since they don't exactly match at both stations. Also, when I flip on the instrument panel light, they all change. Most (oil for engine and trans and volts) drop while the temp gauge increases. The readings are generally reasonable before turning on the light, but I'm worried that the whole thing may need to be rewired. At least I have something for temp now.
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Old 06-09-2017, 09:41 AM   #11
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Adding this link for future reference. Provides wiring diagrams for marine gauges.


http://www.discountegauges.com/documents/ACockpit_1.pdf
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Old 06-09-2017, 09:54 AM   #12
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Your readings dropping when another load is put on the circuit like dash lights is an indicator that the ground to that area is insufficient. Clean the ground wire connections for the dash area or run a larger new ground. It may also indicate that a harness plug may be a little fouled and needs attention.
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Old 06-09-2017, 07:53 PM   #13
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Quote:
Originally Posted by The Other Gary View Post
Your readings dropping when another load is put on the circuit like dash lights is an indicator that the ground to that area is insufficient. Clean the ground wire connections for the dash area or run a larger new ground. It may also indicate that a harness plug may be a little fouled and needs attention.
This is another thread of many others dealing with gauge/sender failures and attempts to find a fix which, invariably leads to much guessing and often not much of a fix. Best option is to start over with new gauges, senders, and wiring. Take a look at the Isspro full sweep electric gauges. They work! And are accurate.
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Old 06-12-2017, 10:39 AM   #14
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I spent Sunday pulling out the starboard fly bridge gauges, disconnecting them, cleaning the contacts, and replacing connectors as needed. After all that, I still get a bump when I turn on the gauge lights. While the right answer may be to pull everything and start over, I'm not sure that I trust myself to slash and rewire that mess of wires. I'll just use a flashlight to read gauges in the rare occasions I'm out at night.
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