New temperature gauge reading low

[Nick_85]

The old temp gauge was pegged past 260 and the previous owner had an aftermarket gauge installed. I recently got a new temp gauge but it seems to be reading very low. After letting the truck warm up the aftermarket gauge reads 150 but when I connect the new gauge it only reads 110. The aftermarket gauge is wired directly to the temp sensor so I feel like it would be accurate, right?

I'm not sure what it means, but if I have the wire for the aftermarket gauge and the stock green wire connected to the sensor at the same time, the aftermarket gauge will read zero and the new gauge will read 140.

Any thoughts on what could be going on? I'm not familiar with the circuits and wiring behind the instrument cluster, could there be an issue there?

 

[1987 GMC Jimmy]

Not necessarily. The two separate readings don’t hold much value if you don’t know what your actual temperature is. I think that should be your next stop going forward. An infrared temp gun from Harbor Freight or Home Depot pointed at the side of the head where the sender is will tell you that and hopefully one of the readings will be close. Is there a thermostat in the truck?

 

[Charlie]

 

[Obwonkonobe]

Just my .02, I usually point the gun at the front intake water jacket, and use the port on the intake for the sender. It gives a more accurate reading of the COOLANT temp, instead of head temp, I find I always ruin "hotter" when reading the head

 

[Ol Blue 2018]

I am pretty sure different gauges require different sending units. For example if you plug an aftermarket gauge into the original sending unit, it will not be accurate. Also, the aftermarket gauge most likely requires a 12v power source in addition to the signal wire.

 

[hatzie]

You don't want to use the intake manifold for an engine temp sender. If your coolant gets slightly low the temp sender will incorrectly report a low temp. The RH cylinder head has a bung where the temp sender is on the LH cylinder head where you can put an aftermarket gauge sender.

GM used at least four stock temp gauge senders between 1973 & 1991. Each one has a different temp to resistance curve. You want to be sure the temp sender in your engine matches the year of the temp gauge in your cluster.

The oldest is called a Nailhead sensor because the electrical terminal resembles a nail head. The GM 1513321 nailhead sensor is obsolete and replacements do not have the same temp curve... even the AC Delco replacement part doesn't have the same temperature curve as the old GM part. That drives the Corvette and F-Body restoration guys nuts. Letric Limited makes a sensor that's supposed to match the original nailhead sensor fairly well.


The earlier and later temp senders have a different Ohm range for the sender.
This is a graphic from Letric Limited that shows several open terminal senders that GM used on cars and light trucks in the 1970's... Ignore the part #s in the picture. I have some of the right ones listed further down.

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GM PN from the 79-84 parts books...
AC Delco Part # 8993106 1/2"-14 Thread; W Gauges (Packard 56 Tang Connector), AIRTEX / WELLS Part # 1T1049, STANDARD MOTOR PRODUCTS Part # TS71
AC Delco Part # 8993164 3/8"-18 Thread; W Gauges (Packard 56 Tang Connector), AIRTEX / WELLS Part # 1T1008, STANDARD MOTOR PRODUCTS Part # TS76

For those with earlier trucks that used the 1513321 nailhead sensor...
Delco PN from the 73-78 Parts Books
Obsolete AC Delco Part # 1513321 1/2"-14 Thread; All W Gauges (Nail Head Connector)

The Standard Motor Products TS6, Wells 1T1053, and even AC Delco G1852 "Drop In Replacements" don't have quite the same Ohm range as the original AC Delco 1513321 sender. The gauge will read slightly high or slightly low depending on who made your sender. This may be good enough for you. You decide.

I installed a 6.5L Diesel with a 3/8" sender hole in a 1976 chassis. My solution was to use the 1981+ temp gauge guts with the 76 face, a painted needle, and a later model 8993164 sender.

Here's a thread on another board about the obsolete Nailhead sender.
http://67-72chevytrucks.com/vboard/s...d.php?t=587190
Letric Limited has a replacement sender that's pretty accurate above 140F.
http://www.lectriclimited.com/electrical_devices.htm
The C1 & C2 Corvette guys have hashed the obsolete 1513321 sender over pretty good with graphs of the Ohms Vs Temp on OEM and aftermarket senders...
https://www.corvetteforum.com/forums/c1-and-c2-corvettes/2148833-correct-temp-sending-unit.html

 

[Nick_85]

An infrared temp gun from Harbor Freight or Home Depot pointed at the side of the head where the sender is will tell you that and hopefully one of the readings will be close. Is there a thermostat in the truck?

Just picked up a temp gun this morning and the area around the sender was showing ~170deg with the gauges showing 160/110. And unless a previous owner removed it there is a thermostat. I haven't actually removed the housing to check.

I am pretty sure different gauges require different sending units. For example if you plug an aftermarket gauge into the original sending unit, it will not be accurate. Also, the aftermarket gauge most likely requires a 12v power source in addition to the signal wire.

GM used at least four stock temp gauge senders between 1973 & 1991. Each one has a different temp to resistance curve. You want to be sure the temp sender in your engine matches the year of the temp gauge in your cluster.

This might be part of the problem. The current temp sensor has a post requiring a ring terminal and a nut. I'm pretty sure the stock green wire had a female plug which I cut and replaced with a ring terminal so I could hook it up and troubleshoot a while ago. I guess I should have done a little more research first. I'll start looking into a matching sensor.

Thanks!

 

[hatzie]

My guess is the sender you need is the 8993164... as long as you still have a 1985 temp gauge and the part didn't change after 1984...

Look it up in the 85-91 parts books to be sure.

I scanned my squarebody 73-86 CK & 87-91 RV and the 88-93 T400 CK light truck parts and illustration books along with the parts and illustration book for the 1984 D series Military CUCV squarebodies.
I posted a link to the searchable PDF files in the forum library...
http://www.gmsquarebody.com/threads/1973-91-gm-10-30-light-truck-parts-books.14496/

 

[Hatchet54]

Did this ever get fully resolved? I'm having some temp gauge issues and doing a little research into related problems.