Temp sensors and LS swaps

[Nick_R_23]

Boy there is sure a lot of confusion in this area! Been searching for two days and still can't find a solid answer. My question is really in two parts here.

I have a 1978 GMC 2500 that is a very base model truck other than it had gauges instead of idiot lights. I'm planning on a carb'd LS swap soon with this truck, so I really don't need much as far as electronics go, other than I'd like the factory gauges to function (not willing to swap to aftermarket PERIOD, I want to retain the complete stock look of the dash).

The oil pressure gauge isn't a huge concern, as there are adapters to put a '78 oil sensor into the stock LS position at the top rear of the block. Easy install there. However, there seems to be little in the way of information on the temp sensor. Obviously there is a massive difference in the thread size of the '78 and the LS sensors, and no adapters are available for this.

I've done this nearly identical swap before on an '86 C10, and my solution then was to take the temp sensor out of the original engine, turn it down to match the LS thread size, and install it like a normal LS temp sensor. The difference was that was a previously running vehicle, and I knew that the sensor functioned correctly. This '78 however, was overheated to the point of completely cooking the old 350, so I really don't trust the original sensor to work anymore.

This leads me to basically two options:

1. Take my chances on a parts store temp sensor and hope that the Ohm values are within the same solar system as the original sensors spec. I have not heard of too many of these sensors reading correctly, which makes the gauge basically worthless. I would have to turn this down on the lathe as the previous sensor which is not a big deal, but would obviously make it nonreturnable. I've also noticed that the parts stores show both 1/2-14NPT and 1/2-18NPT for the thread size on the OEM sensor. I cannot find any information that determines which is accurate or why/when a change would have occurred.

2. Source a GM 3 wire temp sensor (I've been told 98 Camaro among others uses this) and only use the 'Green' wire, which is used to feed the gauge, just like older vehicles. This would be a thread on deal, but the Ohm scale wouldn't match the '78 gauge. HOWEVER, according to some internet information I found (http://forum.73-87chevytrucks.com/sm...?topic=25339.0), in 1979 GM went to the standardized 1365-55 Ohm scale on the temp gauges, which is apparently what is still used today, so the 3 wire sensor should be exactly compatible in theory. So, I would be able to use a bolt in sensor with a change to the '79-80 gauge to match the older design (or '81+ if you need to match the newer design). I've seen a little bit of talk on the subject of using the 3 wire sensor, but nobody I've found has reported that's what they ended up using.

I know there are plenty of people on here that have gone through a similar dilemma, so I would appreciate some input and opinions on what route you'd take.

[mongocanfly]

ive never seen or heard of a 1/2-18npt....all npt ive seen are 1/2-14...
if you got a replacement sender for a 78 it should work with your gauge...you could test it before you do any machine work though...put it together and drop it in a pan of boiling water with a temp probe in it and compare that to what your gauge is showing...you could test your old sender as well

[hatzie]

There are several temp senders used between 73 & 91.
78 is right on the edge of the end of the obsolete 1/2 NPT Nailhead sensor. 1/2 NPT is tapered 1/2" x 14tpi.

Some 79-91 motors used 1/2NPT and some used 3/8NPT. The 79 & later engines with a 1/2NPT sender have a 1/4" disconnect tang rather than the older nailhead connection and the thermistor resistance curve is different from the older sensors. There are at least two 3/8NPT sensors. One with a simple 1/4" tang and one with a sealed Metripak 150 connector.

The LS motors use an M12x1.5 metric thread for the sensor. You can buy a 3/8NPT to M12x1.5 adapter for LS swaps.

You have to match the temp gauge to the temp sender or it will not read as accurately as they were originally setup to read. Different may not be that bad. Honestly these gauges are a matter of getting used to the "normal" reading rather than any real accuracy. As long as the gauge reads near the middle when the engine temp is somewhere between 190°F and 210°F it'll work fine. I'd try the 1979-1980 3/8NPT sender in an LS adapter and see if that makes you happy.
If you just can't stand having a slight mis-match I'd get a temp gauge from a 1979-1980 to go along with the sender. The fonts and colors should match 1978.

[Nick_R_23]

I've never heard of the 1/2-18 sensors either, but they are listed on both Napa and Oreilly's website so I didn't think it was a misprint.

Originally Posted by hatzie

You have to match the temp gauge to the temp sender or it will not read as accurately as they were originally setup to read. Different may not be that bad. Honestly these gauges are a matter of getting used to the "normal" reading rather than any real accuracy. As long as the gauge reads near the middle when the engine temp is somewhere between 190°F and 210°F it'll work fine. I'd try the 1979-1980 3/8NPT sender in an LS adapter and see if that makes you happy.
If you just can't stand having a slight mis-match I'd get a temp gauge from a 1979-1980 to go along with the sender. The fonts and colors should match 1978.

It's been a while since I've looked under the hood, but I believe this truck uses the 1/2" nailhead sensor.

I know the gauges are not super accurate by any means but I'd rather not make the problem worse by installing junk aftermarket sensors. I read that most people with those have the gauges barely move or end up in the first 1/4" of range. If I could see the gauge stay in the middle at operating temp like you mentioned above, that would be the ideal scenario for me, as that would be the closest to how a modern gauge reads.

I found that the Lectric Limited sensor seems to be favored for the highest accuracy...which I may try first. Any personal experience with these?

[hatzie]

No personal experience with them. They're supposed to be exact duplicates of the originals.

You may be able to drill and tap one of the LS heads for the nailhead sensor. That's an adventure I'll leave to you to decide.

Put a thread gauge on a nailhead sensor. I believe its 1/2" NPT which is 14tpi according to Machinery's Handbook... darn handy tome to have in the shop or your office bookcase.

It wouldn't shock me to find that GM used a proprietary thread pitch... not unheard of to have parts store specs be incorrect either.

[Nick_R_23]

I think I'll go ahead and give the Lectric sensor a try. It seems to have the best reviews and I'd like to keep the gauges as functional as possible.

The discrepancy on the thread is strange, but I guess the thread pitch in the end doesn't really matter, as I will chuck this new sensor up in the lathe and turn it down to LS specs (M12x1.5) to make it fit directly in the LS head. It worked well on the previous build and since it now uses a sealing washer, it keeps the ground path cleaner and eliminates the potential leaks and need for PTFE paste.

[hatzie]

Is there enough meat that you're not going to run into the thermistor when you turn it down and cut the threads?

The LS sensors have shoulder with a copper crush washer so the M12 threads are likely straight and not tapered like NPT.

I think I'd try it with a cheapie or a dead soldier before I did it with one that mattered.

Post the carnage either way...

[Nick_R_23]

I didn't run into that issue on the sensor from the '86 truck I did an LS swap on. The threaded portion of the body is quite a bit larger than the stepped down portion of the body where the thermistor resides. Even at M12 threads it was still larger.

Here's the pictures from that specific sensor on the '86 truck. This is with the threads already turned down to M12:



And installed in the LS head:

[hatzie]

I'd say that works.

If you have the technology use it... No reason not to.

An 86 didn't come with a nailhead sensor so the gauge was likely off in the lower end of the range, from what GM originally intended, but it's not that far off IMHO.

[Nick_R_23]

I may as well, seems like the best shot I have at having a closely functioning gauge.

That sensor is actually off a 4.3L V6, which is what that truck had before it was swapped over to a 5.3L. I'm not sure if those nailhead sensors carried over longer in the smaller displacement engines? The truck was LOW mileage and seemed very, very original.

[hatzie]

The V6 was an odd duck. They didn't carry over on the V8 engines.

I might be tempted to make up a spare.

[Nick_R_23]

Lectric Limited sensor is on its way!

[MalibuSSwagon]

I did exactly what Nick_R_23 did for my swap. Except I did not have a lathe so I chucked up the temp sensor in my drill, clamped a flat file in the vise and went at it. It's brass so it cuts easy. When done used a copper washer and screwed it into the passenger head. Gauge reads perfect.

I also used an Autometer adapter for the stock oil pressure sender. Part # 2268 if I recall.

[kipps]

Originally Posted by MalibuSSwagon

...I did not have a lathe so I chucked up the temp sensor in my drill, clamped a flat file in the vise and went at it. It's brass so it cuts easy. ...

I tried the same thing, but when cutting the threads, the brass just fell off in little chunks, and I was left with no threads. What did I do wrong?

It was an old sender, if that makes a difference.

[Nick_R_23]

Just a small update to this thread, and to the post above, it appears that only 79+ sensors are machinable, at least the 3/8” ones. I tried turning down a 78 Lectric sensor and it fell apart too, the bodies are hollow. Picked up a 79+ 3/8” sensor and turned it perfectly, I will just need to find a 79-80 dash gauge to match.

[hatzie]

Originally Posted by Nick_R_23

Just a small update to this thread, and to the post above, it appears that only 79+ sensors are machinable, at least the 3/8” ones. I tried turning down a 78 Lectric sensor and it fell apart too, the bodies are hollow. Picked up a 79+ 3/8” sensor and turned it perfectly, I will just need to find a 79-80 dash gauge to match.

Owch.
Good to know.
Darn decent of you letting everyone else learn from your mistake.

[Nick_R_23]

That’s how we all learn, right?

[rodstored-72]

curious about this... I am in the wiring portion of my '86 & '13 ls swap. I would like to know how/what to do with the temp wire from the '86 harness relate to the '13 sensor and factory harness wire connector? I have heard (as mentioned above) to put a '86 sensor in the pass head & run the OG wire, but then what to do with the LS sensor/setup?
hhmmmm...

[Nick_R_23]

It depends on what type of fuel/ignition setup you will be running. For example, if you will be running EFI, either GM OEM or an aftermarket setup like Holley or Fitech, all of those will require use of the modern 2 pin sensor to operate, so you’d use the LS sensor like stock to operate the EFI. Then, you could either pull the temp output from the ECM to display on aftermarket gauges, or use a secondary stock sensor that matches the truck year to drive the original gauges.

I’ve heard that you can also use a 3 pin F body temp sensor that will work as a combo sensor. The black and yellow wires will be used for EFI, the green wire will drive a 1979 or newer stock gauge. I have not personally verified this will work but I have been told that it does.

In my case, I have a 1978 Chevy that will be getting a carburetor LS swap. I will be using the latest MSD ignition controller, which says it has an optional temp sensor input for more accurate timing. I haven’t opened it yet to verify, but I’m pretty confident that it uses a modern 2 pin temp sensor. I could attempt to use an F body temp sensor and make a jumper harness, or use the OEM LS sensor for the MSD box and use a stock 1979 sensor turned down in the passenger head, and use that to drive the stock dash gauge (with a change to a 1979 temp gauge to match the ohm sweep). Or, since the MSD box claims that the input is optional, just skip the use of a 2 pin sensor entirely and run a turned down 79+ sensor to drive the stock gauges in the drivers side head.

[hatzie]

AFAIK the Left and Right hand heads are the same. Just like a regular smallblock or big block. Temp sensor in one and a plug in the other.
One cylinder head temp sensor hole of the two available is not used by the PCM or anything else.
Put a stock squarebody sensor in the un-used hole.

[rodstored-72]

Originally Posted by hatzie

AFAIK the Left and Right hand heads are the same. Just like a regular smallblock or big block. Temp sensor in one and a plug in the other.
One cylinder head temp sensor hole of the two available is not used by the PCM or anything else.
Put a stock squarebody sensor in the un-used hole.

I am under the same info: the heads are not "side based" there is a "open hole" to insert a sensor.. as long as threads match or an adapter is used.

I wonder how to intergrate the LS sensor and wires into the truck?.... ....hhmmmm

[hatzie]

The LS sensors feed the PCM. That's engine control. Not integrated into the truck at all.
The late model gauges use serial digital data from the PCM.
The analog squarebody chassis gauges can be completely separate.

[rodstored-72]

hatzie,
I am finding that out... all the "GMLAN" stuff... I am going to go with dakota digital for my gauges, but I am trying ( thinking) that it would be cool (well different) to try to use most of the factory wiring (LS motor) as possible.

[hatzie]

It's going to be a hybrid.
Engine/transmission controls are the LS PCM.
The stock or stockish gauges are analog.
I'd probably use the AC compressor controls and radiator fan controls and AD series alternator controls built into the PCM. The PCM can cut the AC compressor when the proper load conditions are met.