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Old 09-06-2006, 08:17 PM
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Temperature gauge behaving badly

All,
I've recently rewired my '31 Chevy 5w using an It's a Snap wiring harness and TPI gauges (the six gauge set with programmable speedometer). Almost everything works great.
The problem is with the temperature gauge. I have a 350 SBC and with my old gauge the temperature ran about 180 degrees (with a 180 degree thermostat).
With my new temperature gauge, the reading is about 225 with the engine running and 180 with the engine off (immediately, not after the engine has cooled down).
I'm confident the 180 is corrrect and the 225 is wrong.
Any ideas on what is causing the difference and what to do about it?
Thanks in advance.

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Old 09-06-2006, 08:49 PM
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Doc here,

Are you using the matching resistance sender for the gauge?

If not it will read in error..

Check the new gauge instruction sheet for the recommended sender resistance value and be sure the engine sender is proper.

Doc
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Old 09-06-2006, 09:01 PM
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Sender

Doc,
Yep, the gauges are all new and came with the proper (I assume) sending units. It's just the temperature gauge that's behaving badly.
PA-Farmer
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Old 09-07-2006, 12:23 AM
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Doc here,

Have you eliminated the mechanical first..? Burped the system of air..?

a sender can not read air , and a bubble will travel to it's highest point and lodge..you may want to do that first if not already..

Also is the sender bourden tube INTO the water stream or perhaps slightly shorter than the one that came out..? that will give errors also..

let me know, if good, we can try a calibration test..

Doc
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Old 09-07-2006, 08:30 AM
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Here is something you might check.

If you have used a thread sealer or Teflon tape on the threaded portion of the temperature sending unit; you may have a poor ground connection and this might be the reason for the erratic readings.

What I do is apply thread sealant (Clear RTV) only to the upper portion of the threads on the sending unit. This allows for good contact with the lower portion of the threads while still sealing the upper section.

I am very interested with your results using the TPI gauges. They are fairly new on the market and I am considering purchasing a set.
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Old 09-07-2006, 02:32 PM
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"you may have a poor ground connection and this might be the reason for the erratic readings" Maybe you check for a good ground by getting someone to watch your gauge while you temporally connect (or hold) a piece of wire from the sending unit to a good ground and see if the gauge temp drops to 180
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Old 09-07-2006, 05:50 PM
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Doc here,

That was going to be my next idea..grounding..

The last few sender body's I installed were dry..no sealer..and worked well with no leaks.

Doc
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Old 09-07-2006, 07:53 PM
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All,
Thanks for the ideas on grounding and air. For the record, I've checked the grounding with a meter but not in the way lamoth1 suggests. I'll give that a try and see what happens. I'll also loosen the fitting to bleed the air as Docvette suggests. I did use teflon tape so I guess I should reinstall without it just to eliminate that possibility raised by Frisco.
While I'm game for all of the above, to me, the important data is that after the engine is fully warmed to operating temperature, the gauge reads exactly correct with the key on but the power off and then reads about 45 degrees higher with the engine running. The gauge does bounce around at all, it just reads one temperature with the engine running and another immediately after I shut off the engine.
I searched previous posts and saw some discussion where someone else had a similar problem but the thread never resolved the problem.
Firsco, I'll get a picture this weekend and you can see how the TPI gauges look in my '31. Overall, I'm happy and think the programmable speedometer is worth the extra.
I'm going to call TPI tomorrow and I'll let you know what they say.
Thanks for the ideas.
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Old 09-07-2006, 09:23 PM
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For thread sealant, try using some anit-seize on the threads. It does not need much to help with the seal and not affect the ground.

Good luck, Ed
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Old 09-08-2006, 08:35 AM
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Quote:
Originally Posted by PA-Farmer
I did use teflon tape so I guess I should reinstall without it just to eliminate that possibility

Overall, I'm happy and think the programmable speedometer is worth the extra.
I'm going to call TPI tomorrow and I'll let you know what they say.
Thanks for the ideas.
Removing the Teflon tape should help (if not solve the problem). I use Teflon tape on many connections with excellent results. Unfortunately, when used correctly the tape insulates as well as seals. When an electrical continuity is needed (such as you need for the temperature sending unit to function correctly) the tape can interfere with this. Keep in mind that the intake is also somewhat insulated from the engine block by the intake manifold gaskets. If the intake manifold bolts are also coated with a sealer (such as the ones that may protrude into the water jacket areas) , these also will insulate the intake from the block.

The ease of programming the speedometer is the feature I am looking for that and the initial cost are the reason for my interest in the TPI gauges.

Look forward to your results.
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Old 09-08-2006, 05:35 PM
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Something doesn't sound right electrically. You're saying that with the key in the on position and the sending unit getting only battery voltage it reads correctly. Then, with the motor running and the alternator most likely putting out something like 14 volts or so the gauge reads 45 deg. higher.
I think that either the sending unit OR the dash gauge isn't grounded properly and forcing more of the voltage to sending unit. Power should come from a positive source then thru the dash gauge to the sending unit then to ground.
P.S. Is the motor grounded? I've seen some weird things happen when the engine isn't grounded properly, that's why GM runs a grounding strap from the intake manifold to the firewall.
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Old 09-08-2006, 05:50 PM
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Doc here,

Remove the sender..and plug the sender hole.

Get a pan of water, And a hot plate , As well as an accurate Lab thermometer..

ATTACH the sender to the car sender wire..Strip ANOTHER about a foot back and wrap it around the sender body tightly..Ground the other end to the body/Engine

BOIL the water to 212 degrees. CAREFULLY immerse the sender in the water and turn the car on..(running) ...

Turn the heat off, and note the readings of the LAB thermometer against the dash gauge..write down any errors or non linerarities..on a note pad.

Repeat the test with the engine dead stopped..Readings should be + / - 5 % through the working point of the gauge..Non~linearities (poor spread throughout the gauge) may require upgrade to a better gauge.

Keep your notes for future reference so you will know where the errors live..

When you re~install the sender, use a light coat of liquid pipe compound in place of tape or TV or permetx..you could also try Aluminum wire bonding paste if you have any..

Doc
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Old 09-08-2006, 07:41 PM
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Thanks for a couple more good ideas; this weekend I'll get rid of the teflon tape, do the grounding checks and pretty much try each of the suggestions in the thread. If all else fails, Doc's test should provide some more data.
Your help was more than I got at TPI. They did mention, in a general way, grounding. They also said that the sender location on the side of the block below the exhaust manifold was the cause of the high readings. They said GM purposely changes the actual temp by 25 degrees to compensate for the location. They suggested I drill a hole in the neck where the top radiator hose connnects to the block and install the sender there for more accurate readings. As I said, the ideas from you guys far exceeded any help they provided.
Just in case you're wondering, I've attached two pictures of the gauges installed in my coupe (at least I think I have attached them).
Saturday is our anniversary so the wife and I are off to the big tractor show at Penns Cave so it'll be Sunday before I get to work on the car again. I'll post the results when I know something more.
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Old 09-09-2006, 08:07 AM
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Quote:
Originally Posted by PA-Farmer
They also said that the sender location on the side of the block below the exhaust manifold was the cause of the high readings.
Having the sending unit installed in the head will show temp readings higher than actual coolant temps due to the close proximity to the exhaust port. These figures will be approximately 25 degrees higher than actual coolant temps.

There should be a threaded opening in the intake manifold near the top radiator hose for the sending unit. This is the preferable location for the sending unit. Chevy engines also have a top radiator hose outlet that bolts to the intake with a tapped hole in the rounded section that the sending unit can fit in. Originally this was for a temp sensitive vacuum switch but has the correct threads for the temp sending unit.

NOTE:

The above does not have anything to do with why you are getting two different temp readings with the engine running or off. Even if you leave the sending unit installed in the head the readings should be more constant (just higher than you expected).

Was the original gauge sending unit also installed in the head? You wrote that it worked correctly is why I ask.
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Old 09-09-2006, 08:48 AM
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Frisco,
You're right, where the sending unit is installed has nothing to do with the different readings. That is in part why I found the TPI guy of little help.
The sender of the previous gauge is exactly the same location as the sender of the new TPI gauge.
Off to the tractor show....
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