Temperature gauge ideas

[Keith7481]

After a quick search of this forum, I thought I'd post this question, which is probably pretty simple for most on this forum, but I'm stuck. My temp gauge has only worked once, I was getting ready to winterize, a muff slid and I didn't have enough water running through the system and the gauge started to register warmer. Other than that time, it has never moved. I tested the gauge by grounding the wire that attached to the sender and the qauge pegs instantly. I've just put in a new sender and thermostat. With muffs on and engine running for 10 minutes, still no movement on the gauge. I have two suspicions, the wiring is corroded (its an 87, but looks ok to the eye) to the point it doesn't register the change in resistance from the sender. Or, since the old sender was wrapped in teflon, the new sender is not fully grounded because of old teflon on the threads. I cleaned them as best I could before putting in the new sender, but who knows. I'd like to test the new sender/wiring and saw on this forum a recommendation to heat the sender in hot water and see what the gauge reads. If I were to use this test, wouldn't I want the gauge to still be grounded as if it were screwed into the housing?

 

[rndn]

Re: Temperature gauge ideas

What motor are you talking about?

 

[Keith7481]

Re: Temperature gauge ideas

1987 3.0 mercruiser with a GM inline 4.
Keith

 

[Silvertip]

Re: Temperature gauge ideas

You don't need the gauge to test the sender. You do need an ohm meter (multi-tester that measures AC and DC voltage, current, and resistance). What you need to make sure of is that you actually have the right sender as the sender must match the gauge. To test the sender you connect one meter lead to the shell of the sender and the other lead to the terminal. Immerse the sender in a pan of cool water and measure the resistance. A candy thermometer will tell you the water temp. Now heat the water and every 10 degrees of water temp change, jot down the resistance. Do not allow the sender to touch the bottom of the pan - just suspend it in the water. Your service manual will tell you what the resistance should be at various temperatures. Here are some numbers that will get you into the ball park:

120 degrees = 240 ohms
180 degrees = 70 ohms
240 degrees = 30 ohms

Other senders may read as follows:

120 degrees = 120 ohms
180 degrees = 35 ohms
240 degrees = 15 ohms

 

[Keith7481]

Re: Temperature gauge ideas

Silvertip,
thanks very much for that suggestion. Seems pretty easy to do. Now the more difficult question. How to find the right sensor. When I bought the boat a couple of years ago, I'm pretty sure the sensor had been replaced (perhaps with the wrong one), it had teflon tape on the threads which is a no no and suggests replacement. So, if I have the wrong one (the new one I just purchased) how do I find one that matches the gauge. When I went to my local store (West Marine) and when I check on line sources (Overtons) they don't offer different senders by resistance, at least not that I've seen. Only differences I see are thread size??

Keith

 

[Silvertip]

Re: Temperature gauge ideas

The resistance values I gave you were for an outboard sender. I/O values are shown in the link to the Teleflex data sheet shown below.

http://ww2.tflx.com/pdf/tempsy~1.pdf

Check the gauge for the manufacturers name. It will very likely be Teleflex, Faria, or VDO. VDO definitely uses different senders as they are European. Teleflex and Faria use the same sender. Check their web site and then locate the sender part number. From there you can google that part number and pick from a gazillion sources. But before you do that, Make sure the sender wire is good from the engine to the gauge. Then make sure you have 12v and ground at the gauge. If those are good, you have a sender issue. Teflon tape is not as big an issue as its made out to be. The threads cut the tape and an electrical connection is made. The tape fills the gap between the threads to seal that area. The ground issue occurs when too many wraps of teflon tape are used.

 

[Keith7481]

Re: Temperature gauge ideas

Thanks very much. I was planning to try new wire, using alligator clips to connect the sender directly to the gauge to see if that might be the problem. As the gauge pegs when I touch the lead to sender at the block, I think I"m ok with power and ground at the gauge. If the new wire doesn't make any difference, I'll check the gauge/sender match. Thanks very much for the link.

Keith

 

[Keith7481]

Re: Temperature gauge ideas

Silvertip,
hit that one right on the head. The gauge and current sender had different resistance scales. I contacted Faria and they gave me the correct part number for the right sender. Thanks very much for the thoughtful idea.

Keith