Difference between Temp Sending Unit of Idiot Lights vs. Gauges?

[5150]

Is there a difference between the Temp Sending Units on Idiot Light clusters and Gauge Clusters? I'm swapping mine over to gauges and can't seem to find out. The style that is on there now is on the drivers side of the block and has a spade connector.

To get more confusing it's my 87 with a 95 small block in it.

Thanks

[Corts60]

Idiot light senders are just a switch (on/off) whereas gauge senders have variable resistance.

[ray_mcavoy]

Yes, there is a difference.

The unit used with the warning light is simply a switch that closes once it reaches a certain temperature, completing the light's circuit to ground.

The unit used with the gauge provides a variable resistance to ground based on the temperature.

As far as physical appearance goes, they both look quite similar and some even have the same type of terminal.

[5150]

Anybody got pics of the two or part numbers?

[ray_mcavoy]

Here is a pic of some of the most common gauge type sending units along with the GM part numbers:



1973 - 1978 temperature gauges use sender #1513321. It has 1/2" NPT threads. The equivalent replacement is Standard Motor Products TS-6.

1979 & newer temp gauges require a different resistance range and use one of the following sending units:

#8993106 with 1/2" NPT threads. The equivalent replacement is Standard TS-71.

#8993146 with smaller 3/8" NPT threads. The equivalent replacement is Standard TS-76.

[5150]

Awesome Ray, thank you very much.

[Ski-me]

Interesting....

Ray, I have the top sensor in my 89' K5, however the block itself is an older rebuilt one. I thought it was an early 80's block. You mentioned that there is a resistance difference but my gauge reads just under 210 (around 195 or so). Maybe the person that installed it spliced it with a different connector?!?

[ray_mcavoy]

Originally Posted by Ski-me

Interesting....

Ray, I have the top sensor in my 89' K5, however the block itself is an older rebuilt one. I thought it was an early 80's block. You mentioned that there is a resistance difference but my gauge reads just under 210 (around 195 or so). Maybe the person that installed it spliced it with a different connector?!?

According to the info I have been able to find in the GM service manuals, parts catalogs, & such, both the temperature sending units and gauges were changed for 1979. And looking up the specifications on the sending units shows that the old vs. newer ones have a different temperature vs. resistance curve.

A few years go I did some experimental testing on some spare gauges I had laying around and found there is about a 40Ω difference at the mid-scale reading on the old vs. newer gauges. That generally causes the newer gauges to read a bit higher than normal when connected to an older type sending unit. However, aftermarket sending units don't always match the OEM specifications and just going by the visual appearance alone isn't always a sure fire way to id the sending units. So if your sending unit & gauge combination are working correctly, I wouldn't worry about it. If you suspected a problem, you could check the calibration as shown here --> http://www.madelectrical.com/worksho...mp-gauge.shtml

As for the connectors, all 3 of the sending units I pictured accept a female Packard/Delphi 56 series terminal. With the "button head" type sending unit, the terminal just slides on sideways (and therefore requires a plastic connector shell with a notch in the side).

[Axle]

I'm going to go on the limb and say that the connector type is a guide for replacement guages only. My '83 w/guages has a button head sender and so does my '81.

Alex.

[ray_mcavoy]

Originally Posted by Axle

I'm going to go on the limb and say that the connector type is a guide for replacement guages only. My '83 w/guages has a button head sender and so does my '81.

Alex.

Yes, I probably should have stated this earlier, but out of the 3 sending units I pictured, only the top #1513321 (for 73-78) is an original GM part. The other two pictured are aftermarket replacements for 79+. They should ideally have the same temp/resistance curve as the original equipment units but do not necessarily have the same visual appearance or terminal style.

[5150]

Well, I installed the 87 3/8 thread one and now my gauge sits at 250 degrees. That's pretty much 30 seconds after I started it.

[ray_mcavoy]

Originally Posted by 5150

Well, I installed the 87 3/8 thread one and now my gauge sits at 250 degrees. That's pretty much 30 seconds after I started it.

Unfortunately, I've had that happen with some aftermarket sending units as well.

As shown here ( http://www.gmpartswiki.com/getpage?pageid=56477 ) in the GM parts catalogs, #8993146 is the correct sending unit to be using with the factory gauge cluster.
And according to this listing in an old Standard Motor Products catalog I have, the TS-76 is supposed to be an equivalent replacement:


I'd recommend using an ohmmeter to check the resistance of your sending unit at various temperatures. You can put it in a pot of hot water as shown in the link I posted earlier and compare it to the specs shown in the catalog listing above. Obviously you won't be able to get a pot of water up to 250°F but the resistance readings at the lower temps should give you a good idea if it is working correctly or not.

Here is a link to more info about the temperature gauge --> http://www.lectriclimited.com/freque...stions.htm#E02 It covers the common problems and shows how to add resistors to re-calibrate the gauge if you have too much trouble finding a sending unit that will work correctly. Note that they sell sending units that are guaranteed to meet the original GM specifications ... but they only have them for the earlier years.

[5150]

Thanks Ray, you've been a big help.

Now onto the oil pressure sending unit

[Just call me Sean]

If I might jump in here. Parts store gave me a TU66 for my '83 and my gauge pegs to hot. I guess they gave me the wrong one?

[Just call me Sean]

No comment?

[ray_mcavoy]

Originally Posted by Just call me Sean

If I might jump in here. Parts store gave me a TU66 for my '83 and my gauge pegs to hot. I guess they gave me the wrong one?

Originally Posted by Just call me Sean

No comment?

Sorry for the delayed response ... I'm subscribed to this thread but for some reason I didn't get an e-mail notification until your 2nd post.


Anyway, the TU66 appears to be a Wells part number that (according to the info I could find on the net) is also supposed to be a replacement for the GM #8993146 ... same as the Standard TS-76. So it looks like they gave you the correct unit ... at least as far as the catalog listings show.

To rule out the new sending unit being defective or mis-matched, you can place it in a pan of hot water and measure it's resistance at various temperatures and compare to the specs in the catalog.

Another problem that can cause the temp gauge to peg hot is a defective (open circuited) resistor on the back of the gauge as described in this link I posted earlier --> http://www.lectriclimited.com/freque...stions.htm#E02

And finally, if your temp gauge pegs hot immediately when you turn the key on, it could be a short to ground somewhere in the wire connecting the sending unit to the gauge.

[Just call me Sean]

Thank you. The gauge doesn't move until I plug in the wire, then it pegs so bad I have to remove the lens to unstick it from the max.

[ray_mcavoy]

Originally Posted by Just call me Sean

Thank you. The gauge doesn't move until I plug in the wire, then it pegs so bad I have to remove the lens to unstick it from the max.

You're welcome.

The gauge not moving with the wire unplugged from the sending unit means you can rule out a short to ground in the wire.

But if the gauge pegs immediately when you plug in the sending unit (regardless of engine temp), that sounds more like an issue with the ceramic resistor on the back of the gauge. That resistor is on the ground leg of the temp gauge so a bad connection between the gauge & the cluster ground would cause the same thing.

There are 2 electromagnetic coils inside the temp gauge that pull the needle in opposite directions. Both share a common power connection. The coil that pulls the needle toward the cold side is grounded through the resistor to the cluster ground. The coil that pulls the needle toward the hot side is grounded through the sending unit. So a bad ground at the gauge (or open circuited resistor) will prevent any current from flowing through the coil that pulls the needle to the cold side. And with no magnetic field pulling the needle back toward the cold side, even a tiny amount of current flowing in the other coil (that's grounded through the sending unit) will peg the needle to the hot side.

[Just call me Sean]

Thanks again. At least now I know what my next step is.