Coolant Temp Gauge - What makes it work?

[BigBlue86K30]

So I have had an issue with my factory temp gauge for the past 2 or so years now. I originally thought it was my old tired 454 (that was pretty tired and worn out) just getting old and time to be replaced.

So this spring I dropped in a new ZZ454 crate engine, new aluminum radiator, all new hoses, new clutch fan, new belts, new water pump came with the engine, new temp sending unit, new 160 thermostat, new radiator cap and of course all new 50/50 green coolant. I also used a SnapOn coolant tool when we filled the system to ensure that all the air was out of the system and coolant was properly filled.

So on my first test drive, I noticed that the temp gauge would fluctuate between the first hash mark between the 100 and 210 marks, seemed normal. Then on another drive later on, the gauge started staying around 210 and a tiny bit above. Now when I stop at lights, the gauge start climbing over 210 and has gotten as high as the mark between 210 and the 260 mark. I immediately shut it down when it got to that point. Again, double checked everything and nothing is visibly wrong. Radiator is totally full, no air pockets that I can find, all parts are new. So I again let it idle and get hotter than normal on the gauge I checked the radiator hoses with an laser style temperature gun. Just as I suspected, even when the gauge is reading that it is getting hot, the top (hot) radiator hose is only about 180 degrees, the bottom hose coming out of the radiator is about 140.

I am thinking I have a bad gauge. I cant figure it is anything else. Anyone else have any ideas? I did note that when I got a new temp sending unit at autozone, the counter parts guy asked if I wanted the smaller or larger one. The sending unit in my old 454 block was the larger one. Is there a difference? Anyone know what ohms the gauge is supposed to take to show a reading? I am curious as to what ohms would show 100, 210 and 260 on the gauge, if I knew that I could use my test tool to check the gauge.

[rich weyand]

Aftermarket temp sending units do not have the correct resistance curve for our gauges. The GM #513321 sending unit hasn't been made in 20 years, and the cross-reference replacement doesn't have the right resistance curve either.

The #513321 sending unit is the correct one for our trucks for all engines. The sending unit has to be matched to the gauge to get the right reading; what engine it is in doesn't matter. It is the resistance curve of the sending unit together with the gauge that have to be matched.

When you previously saw the needle at 210, the temp was actually lower, probably because you were moving and the radiator had a lot of ram air through it. When you saw it much higher, it was probably running at full operating temp. With the wrong sender resistance curve, though, it will read incorrectly all the time.

So, what do you do? If you got a couple hundred bucks laying around you don't need, you can get a NOS #513321 on ebay. Or you can order a reproduction unit with the correct resistance curve from these guys. They even have the right connector. I put them in my truck and now my gauge reads dead on with my laser style temp gun.

http://www.americanautowire.com/shop...ding-unit-5028
http://www.americanautowire.com/shop...ure-sender-con

[BigBlue86K30]

Well this is interesting. I been struggling with this for a while now. Is there anyway to test the new sending unit before I install it? I am going to loose a lot of coolant swapping out the sending unit and going to have to rebleed the whole system. I hate to order the new sending unit and still have it turn out to be my gauge or something else.

[rich weyand]

I changed mine on an SBC, where the sending unit is between #1 and #3 on the underside of the head, and I hardly lost any coolant. Maybe a quarter of a cup, if that. More like an eighth. If yours is on the top of the manifold, you won't lose anything.

Here's what I did on my sbc. Put a pan on the floor under the sending unit. Have the new one standing by with just a bit of teflon paste on the threads. Yeah, yeah, I know, some people will tell you not to use any because it will insulate the unit electrically. Nonsense. Back the old one out until it is just about to come off. Now, holding the new unit in your right hand, spin the old one out with your left hand, and then with your right hand spin the new one in. If it is sticking out sideways like on the sbc, it will go glug-glug-glug-glug when you pull the old one out, but as long as the system is cold (not under pressure) it leaks out pretty slowly and it's only unplugged for a second or two.

I don't think you even have to burp it. I didn't. The coolant system is designed to clear the occasional air bubble. It will blow it out into the reservoir when it heats up and suck coolant back to replace it when the system cools.

But your gauge will never read right without the correct sending unit. And the Autowire #01513321 unit is the only one that I know is correct.

[ray_mcavoy]

I agree with what Rich posted about the GM #1513321 sending units.

However, GM only used the #1513321 up through 1978. They changed to a different temperature vs. resistance curve for the gauges & sending units starting in 1979. And based on your BigBlue86K30 name, I assume your truck is a 1986, correct?

The 1979 & newer temperature gauges are designed to work with either a GM #8993106 or #8993146 sending unit. The #8993106 has 1/2" NPT threads while the #8993146 has 3/8" NPT threads. They're both approximately 1425Ω at 100°F and decrease to around 147Ω at around 220°F. Those are nominal ratings and can vary ± 10% or so.

The #1513321 sending unit (for 78 & older) has lower resistances across the whole temperature range and will therefore make the 79 & newer gauges read too high.

Originally Posted by BigBlue86K30

I am curious as to what ohms would show 100, 210 and 260 on the gauge, if I knew that I could use my test tool to check the gauge.

Based on testing some sample gauges, here's what I've found for the later 80's gauges that have the 100, 210, and 260 markings:

Anything in the 1500 to 1200Ω range will make the gauge read close to the 100°F mark.
Around 105 to 110Ω will make the gauge read close to the 210°F mark.
Around 58 to 60Ω will bring the gauge up to the 260°F mark.

[rich weyand]

That's very interesting. Before answering his question, I thought to check to make sure the gauges had not changed. Not thoroughly enough, I guess! The aftermarket gauge suppliers list one replacement gauge for 1973-1987, which led me to believe they were the same.

An interesting question now is which sending unit do the aftermarket gauges need? How can they sell one gauge for that range of years?

The question for the OP is, What sending unit did you put in, and does it match the gauge in your truck?

BTW, if you get those resistor values from the local electronics store, you can check your gauge by connecting the sending unit end of the sender wire to ground through each resistor size and see that it matches what Ray is saying. You can also put an ohmmeter on the sending unit to test it against your infrared temp gun readings. That will allow you to see for sure whether the gauge and the sending unit agree on what resistance means what temperature.

[BigBlue86K30]

Thanks to both of you for responding. My truck is an 86 K30.

I located a NOS unit on ebay: http://www.ebay.com/itm/NEW-ACDelco-...f7731b&vxp=mtr

My truck takes the 1/2", I am guessing since its a big block. Will this be the proper unit to order? Or does a reproduction company make a better unit since there was some debate previously about even the GM/Delco ones being more universal since they haven't been made for a long time.

Suggestions?

[BigBlue86K30]

I cross referenced this part number against the one I purchased from AutoZone. The Autozone part is TU65. I am guessing that even though this is the "correct" part, I need to get one that is made for this application vs a universal one as stated above. Anyone know where I can get one made for the correct ohm readings for our trucks?

[rich weyand]

Well, that Autozone part cross-references correct, but aftermarket temp senders are notorious for being the wrong resistance curve. I would probably get a different aftermarket sender that cross-references correctly and try it, like this one. They're cheap enough you can experiment a little.
http://www.summitracing.com/parts/sm...olet/model/k10

You could also try to find a NOS GM part 8993106.

BTW, any chance the temp gauge in your truck was replaced? You should probably get some resistors and check those numbers to verify that it is the stock gauge. Easy to do.

[BigBlue86K30]

How would I test the gauge in the truck? I would like to test this before ordering more sending units that may not be the problem. I would assume that the gauge is original to the truck but I have no way of knowing for sure, with the truck being so old its possible that it has been replaced at one point or another.

[BigBlue86K30]

I would assume that even if my gauge was replaced at some point, that it is the correct era gauge for the truck. When I did a search most of the pre-79 temp gauges they just show COLD - - - HOT on the gauge, they don't have an actual temperature printed on the gauge face. Whereas most of the 79+ trucks actually have 100 - 210 - 260 on the gauge face.

[rich weyand]

I mentioned it before. Get resistors from Radio Shack in the values noted above. Remove the connection to the temp sender on the block and jumper the resistors one at a time from the sender wire to ground. Turn the ignition on for each resistor and note the gauge reading. From that, you should be able to tell which gauge you have.

[rich weyand]

Originally Posted by BigBlue86K30

I would assume that even if my gauge was replaced at some point, that it is the correct era gauge for the truck. When I did a search most of the pre-79 temp gauges they just show COLD - - - HOT on the gauge, they don't have an actual temperature printed on the gauge face. Whereas most of the 79+ trucks actually have 100 - 210 - 260 on the gauge face.

Ah. The gauges are mechanically (size, mounting) the same, but the aftermarket gauges are of the C<->H variety, like mine (1978).

So you probably do have the original gauge. Checking the readings by temp though will also verify correct gauge operation.

[BigBlue86K30]

Of the choices available, which brand might be most accurate? I did find that NOS one on Ebay for $22. Would that be a better choice than a new aftermarket one?

One is GM #8993106. It has 1/2" NPT threads. The currently available AC Delco replacement is #2134792. Standard Motor Products equivalent is TS-71 and Wells is 1T1049.

[rich weyand]

I would get the NOS one. So it's like double a good aftermarket one, and maybe 3-4 times as much as a cheap one. It's still pocket change, and you can buy a lot of inaccurate ones before you finally stumble on one that has the original resistance curve. I paid $27 for a reproduction one that was guaranteed to match the original #513321. NOS on ebay was $200!

[BigBlue86K30]

Great! Well I ordered the NOS 8993106. Time for some other work on the truck this weekend anyway; carb tuning, u-joints, maintenance. I will try to get it in at the same time. I am tired of coolant all over my shop floor, so hopefully changing this one doesn't make too much of a mess

[OJ1988]

I've been trying to find a suitable one for my 88, I have a thread going in the 88-98 forum, can you guys help me out?

[rich weyand]

Originally Posted by OJ1988

I've been trying to find a suitable one for my 88, I have a thread going in the 88-98 forum, can you guys help me out?

They changed the sender units when they made major changes to the instrument panels and gauges. The sender unit has to be matched to the gauge -- engine doesn't matter. !973-1978 is 513321, for example, but it changed in 1979. I think it changed again by 1987.

This page says the correct unit for the 1988 is a GM #8993146:
http://gmpartswiki.com/getpage?pageid=56466

It looks like it has the blade connector.

You can find them on ebay.
http://www.ebay.com/sch/i.html?_trks...at=0&_from=R40

Hope that's the right one, but it is the one listed in the parts book on-line.