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Old 06-19-2021, 04:09 PM   #1
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Question for you electronic wizards - circuit resistance

My water and trans temp dash gauges both read about 50 degrees hotter than the actual temperatures. The sending units are okay.

The way these senders work is that the "resistance drops as the temperature increases".
The specs are:
100 degrees = 450 ohms
175 degrees = 99 ohms
250 degrees = 29.6 ohms

Since my gauges are reading high (about 50 degrees too high) that means the resistance in the circuit (or internal to the gauge) is lower than it should be.

I'm not sure what would cause circuit resistance to drop. But that rules out the circuit itself (old wires or terminal corrosion) resistance - if the wiring had too much resistance the gauges would read lower not higher. Right?

My question is: how can I lower the resistance of the circuit so the gauges read more accurately? I need to lower it by around 60 ohms.

The sender unit is the "ground side" of the circuit so there is nothing I can do on that side. Could I add some resistance (a resistor) on the positive side (at the + side) of the gauges. Would that result in the gauges reading lower?

Also since both water and trans temp gauges are reading the same amount too high, I'm thinking the problem causing both gauges to read wrong is probably the same item. One thing common to both gauges is the + power feed to gauges. Could low voltage to the gauge cause it read too high?

Maybe I'm misunderstanding the operation of the circuit. Any comments or help are appreciated. I can monitor these temps through my Aladdin unit, so getting the gauges to work perfectly is not necessary - but I would still like them to be correct as possible.
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Old 06-19-2021, 04:44 PM   #2
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I dont know if they even use these any more but here's some info.

https://www.autoblog.com/2016/01/11/...age-regulator/Click image for larger version

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Old 06-19-2021, 05:10 PM   #3
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What type of gauge do you have. How old is it. Better yet, if you could get the manufacturer name and any part numbers, take a cell phone photo, AND, go online to the manufacturer this would be your very bet solution. Their is no way that you need to add a resistor to the circuit, something is wrong with Guage, or sender, most likely the sender, replace the sender, they are not expensive, but first call mfg,
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Old 06-19-2021, 06:34 PM   #4
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I think the voltage regulators are internal to every gauge now.
I suppose a poor ground at those gauges could cause an issue.
A quick Google search did show resistor as an option for unavailable calibrated sender in VDO gauge/sender. In racing forum.
Other search showed miss matched gauges and senders.
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Old 06-19-2021, 07:32 PM   #5
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Quote:
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Other search showed miss matched gauges and senders.
That was my first thought. Maybe someone changed senders and didn't realize they're not all the same.
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Old 06-19-2021, 07:48 PM   #6
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Since the numbers you gave aren't linear if you graph them a resistor won't help your issue, it will only add X ohms of resistance to a non-linear circuit. You need to look for some other solution to your problem. I'm with the others I think it's a sensor issue but I really don't know. One other question I have is how did you determine that the gauges are wrong? Good luck.
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Old 06-19-2021, 11:41 PM   #7
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Why don't you check the voltage in and out of the gauges. Dropping the voltage drops the current. I honestly don't know what that does to the resistance of the sending unit. But google must know if you ask the question properly. After thinking a bit, a short on the sending unit side of the gauges is a possible culprit.
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Old 06-20-2021, 03:49 AM   #8
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Quote:
Originally Posted by arcaguy View Post
Since the numbers you gave aren't linear if you graph them a resistor won't help your issue, it will only add X ohms of resistance to a non-linear circuit. You need to look for some other solution to your problem. I'm with the others I think it's a sensor issue but I really don't know. One other question I have is how did you determine that the gauges are wrong? Good luck.



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Old 06-20-2021, 05:33 AM   #9
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Thanks for the replies. I'll try to respond the questions/comments in order.

1) There is no external "instrument cluster voltage regulator" used (like my old for truck has on the back of the cluster). Possibly something internal to the gauges. The wire labeled "IGN" (12V power with key On) wire is direct to the gauges and other items behind the dash.

2) The dash gauges are made by Faria-Beede. They were "custom" for the Beaver. There are no replacement gauges available (and it would not be worth the cost anyways since I have the Aladdin unit for actual temp readings.

3) I replaced the Sender Unit first with a Faria 90404 (top photo) which was said on the forum to be correct, then later I found this one (see bottom photo) which is said to be the "correct sender for Monaco" but it didn't help.
Both senders read the same when the actual temp is 190 degrees - it reads around 240. Oddly the Water and Trans temp gauges both read around the same 240, so it doesn't seem that both senders would be bad. Of course both senders could have failed.

4) I have not measured voltage at the gauges yet. I know the IGN source wire has proper voltage at other locations (but did not specifically test at the gauge). I'll do that later today. For now I'm gathering ideas.

5) If "adding resistance" will get the gauge to read in the correct range at an operating temp of 180 - 195 degrees, I would be fine.
I don't care if it reads wrong at cold temps. I just would like to be able to glance at the gauges when driving for a quick confirmation. I usually keep the Aladdin screen on showing engine data, but sometimes want to set Aladdin to show camera views.

6) I know the dash gauges is wrong because I can monitor the engine and trans temps through the Aladdin display - which uses the OEM senders that come on the engine and trans from the manufactures (CAT and Allison). So the Aladdin shows (normal) temperatures around 185 - 190 degrees while driving and the dash gauges will show around 240 degrees.
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Old 06-20-2021, 08:26 AM   #10
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I just did some more testing. Running engine till warm and recording ohms at the sender unit. This is what I'm getting:

100 degrees = 400 ohms
130 degrees = 300 ohms
140 degrees = 225 ohms
150 degrees = 175 ohms
170 degrees = 120 ohms
180 degrees = 100 ohms
(+/- 5 ohms)

When engine is 180 degrees F, this is the gauge display. This is about what the Faria spec sheet says it should be (i.e. 99 ohms for 175 degrees F) so the Sender is good.
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Old 06-20-2021, 09:07 AM   #11
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Is it a sealed unit, or can the gages be removed individually?
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Old 06-20-2021, 09:28 AM   #12
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Some where the voltage is regulated to gauge I believe. otherwise when your alt is charging at 14 volts the gauge would read higher. I seem to remember around 5 volts or so.



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Old 06-20-2021, 10:16 AM   #13
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Any idea how the fuel gauge reads? I wonder if the regulator is built into the gauge?
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Old 06-20-2021, 10:54 AM   #14
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Country B - solution depends on wether your gauges are Ammeters or Voltmeters, but IIRC, they are usually ammeters.

So you have your battery in series with the sender in series with the meter. Heat changes the resistance, voltage stays the same, current changes, gauge measures the current.

Most likely culprit is the battery voltage is higher than what the gauge was calibrated for, leading to more current and a higher reading.

If the coach was designed properly there should be a single voltage supply reference to all gauges that isn't feeding anything else (like dash lighting). If it is feeding something else the trick is to branch off just the gauge reference and put the regulator in series in that leg.

Looks like the operating range of the gauge is 30-200 mA or so @ 12 V. Voltage changes will result in a linear offset e.g 13.2V will equal a 10% increase in gauge reading. 180 would then be 198.

I would guess the gauges were calibrated for less than what your system is putting out, and may not have an internal regulator. Do you notice a change in readings when there are big changes in system voltage? You might try putting the sender in a cup of boiling water and then turning the key on. When the grid heats you'll see one reading as the line power drops, another as the heater relay cycles off, and perhaps another as the voltage stabilizes. Then turn on the headlights and finally adjust dash lights from high to low. If you see the temp moving around you will have your answer.
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