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Two temp sending units on my engine. Why?

steven_p

Member
Sorry if this is a rudimentary question but I'm not finding a whole lot of info online and I'm pretty green when it comes to gauges and sending units.

I'm trying to figure out why neither of the temp gauges work in my 'new to me' boat. The engines are mid 80's Crusader 270hp, raw water cooled. There's a temp sending unit in the tstat casting and another in the intake manifold. The one in the tstat casting seems to affect the alarm buzzer when I mess with it, so I'm guessing this is an overheat alarm? The one in the intake manifold pegs the gauge when I ground it which should mean it's associated with the temp gauge, and that the temp gauge is functioning properly, right?

Picture attached showing both sending units.

Any insight you guys can offer as to how these two sending units interact would be greatly appreciated.
 

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One might be an alarm SWITCH (ON/OFF) vs. a temp SENDER (reads in degrees).

Jeff

Makes sense. I can measure ohms on the senders on the left. At room temp both read approx 600 and gradually fall to around 240 in 125*F water.

The ones on the right do appear to be switches because I cannot measure any ohms or continuity. Not sure what temp they're supposed to activate but there's no response in continuity when the probe is submerged in boiling water.

From what I see so far the gauges are functioning properly and the senders are functioning properly. The switches don't appear to be reacting but maybe I'm not getting them hot enough :confused:
 

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I also tried disconnecting the sender wire from the gauge and the sending unit, and jumping the path directly with some spare wire. Made no difference.
 
The two on the left appear to be switches; the other two sender units.

With the alarm on, ground the wire to the switch and the alarm should sound. (If the oil pressure switch is going off, you'll need to temporarily disconnect it.)

Jeff
 
Jeff's original post was going down the right path...

On the pics, I'd be more inclined to call the pair on the left the "sender units".

The overheat warning switch 'closing temp' will vary a bit but most are around boiling...

As far as the gauges, the OEM design uses the 'US Standard'. This means that the sender unit will provide ~ 240 ohms for minimal deflection on a matching gauge and ~ 33 ohms for full scale, for single helm units. The quick test is to use an open or short (to ground) for the gauge...use resistors for a more refined test. you can measure the resistance of a sender (to its case) and compare it with a reference chart (teleflex has several on their web page)..
 
The unit that your finger is pointing towards, will be a temperature guage sending unit. This will be in the coolant path prior to the thermostat.
The other unit is likely for an over-heat alarm device.

When testing a temp guage circuit, DO NOT take it directly to a Negative source.
Instead, take the circuit to Negative using a test light, or something with resistance to it.


BTW, Mark touched on something that you may want to know.
Temp guage senders are specific to either single station or dual stations.


.
 
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Appreciate all the comments. I'm going to do some troubleshooting today.

I have dual stations. I'm wondering if the sending units are single station units. Hypothetically, how would the gauges react if they were connected to a single station sending unit?
 
I understand they won't work properly. I'm just wondering if the previous owner installed the wrong ones. The sending units function properly when I bench test them with a single gauge. The gauges on the boat are getting power. The sending wire to both stations tests ok. So, the type of sending unit is suspect IMO.

There is a number stamped on one of the sending units but I cannot cross reference it with anything online, so I have no idea if they're dual station or single station units. All I know is they function properly when I bench test them with a single gauge.

<edit>
The ohms readings I'm getting above seem to coincide with dual station sending units. Hmm. I will check some things out and report back. Again, thanks for the help.
 
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Well, after some thorough evaluation I've determined that the gauges and sending units are working fine. My motors are just running cold.
 
The dual station senders have to sink the current from TWO gauges vs only ONE...so, to keep the gauges happy, the dual station senders should show 1/2 the resistance, at any given value, of a single station sender.

Using a single station sender in a dual station application will result in lower than spec current whick will cause the gauge to read "low"...in the case of temp gauges, this means cold....and the amount will vary based on the temp of the coolant.
 
Mark,
Thanks for the explanation. So, let's say hypothetically that you had dual stations connected to a single station sending unit. If you disconnected/bypassed the "S" terminal on one of the gauges, then the remaining gauge would read accurately, correct?
 
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Mark is dead on!

Dual helm harnesses are typically in parallel with one another, which causes both upper and lower station instruments to all become powered when only one helm is excited by the key switch, of which powers both temperature guages simultanesouly.

So, let's say hypothetically that you had dual stations connected to a single station sending unit.
If you disconnected/bypassed the "S" terminal on one of the gauges, then the remaining gauge would read accurately, correct?
In theory... I believe that would be correct!


.
 
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