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1989 Chevy Silverado 5.7L / 305 Engine: Problem with Temperature Gauge


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patrickclaytonworking
New User

May 10, 2019, 5:35 PM

Post #1 of 4 (246 views)
1989 Chevy Silverado 5.7L / 305 Engine: Problem with Temperature Gauge Sign In

Hello all, this is my first post and I am very grateful for anyone who can lend me their brain and experience.

I have a 1989 Silverado (1989 Chevy Silverado 5.7L / 305) that I have been slowly restoring. One of the items on the list is the temperature gauge. When it failed back in the mid 2000's, my dad opted to put an after market mechanical/gas gauge in, and I am doing the work to restore the one in the "moonie" cluster.

My first try at this was replacing the sending unit with an AC Delco single wire sensor. I also resoldered a fresh spade connector to the wire. This didn't seem to work, and after doing some research I found that the sending unit is just a thermistor to ground, and that 'grounding out' the wire should peg out the temp gauge. I attempted this, and the gauge did not move.

I've since taken out the cluster and replaced the gauge with one from LMC truck. Now, here is where I am - I have a new sending unit, and a new temp gauge, and when I ground out the line the new unit thankfully pegs out to the "car fire" side of the gauge. Unfortunately, even after plugging the wire back into the sending unit I could not get a reading, that is, the gauge stays at the lowest reading.

Aside from the AC Delco sending unit, I have also tried swapping to a Duralast brand from Auto Zone, and it didn't make any difference.

I did a little investigating, I know that the sending unit works, as I can meter the ohms between the lead and another part of the block. The sending unit starts around 1.7kohm at ambient temperature, and once the engine gets hot it will drop the resistance down to to about 700 ohms.

I decided to see what resistance actually makes the temperature gauge dance...Using some resistors, I was able to make a couple reference tests and found that the gauge will follow this action:
Resistance - Gauge Reading
218ohm - ~110f
150 ohm - 150f
75 ohm - 230f
50ohm - 260+f

So here is my frustration - is my gauge moving in accordance with proper specs? Are the after market sending units simply not calibrated for my vehicle? Could there be another electrical issue in the cluster or leading from the sending unit that could be causing these off the shelf sending units to not play nice with my cluster?

Thank you for any help you can provide, I have been beating my head against this problem for far too long and hopefully someone can point me in the right direction.

TL;DR:
Replaced sending unit and gauge - gauge works, sending unit works, but seem to operate on different/incompatible ohm sweeps - bad gauge / electrical? - poorly spec'ed aftermarket sending units?


(This post was edited by patrickclaytonworking on May 10, 2019, 5:41 PM)


Hammer Time
Ultimate Carjunky / Moderator
Hammer Time profile image

May 10, 2019, 7:59 PM

Post #2 of 4 (214 views)
Re: 1989 Chevy Silverado 5.7L / 305 Engine: Problem with Temperature Gauge Sign In





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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.



patrickclaytonworking
New User

May 13, 2019, 10:58 AM

Post #3 of 4 (167 views)
Re: 1989 Chevy Silverado 5.7L / 305 Engine: Problem with Temperature Gauge Sign In

Thanks for the help and repair guide. I've settled on the conclusion that either my refurb gauge is responding to an ohm sweep standard more like the C10's from 1970-ish. I'm going to try a brass 3/8ths male to 1/2 female npt adapter and install the sending unit that was typical of the previous generation trucks. I'm not sure if it is the gauge, my cluster or my truck, but I have a feeling this will work. I will post the outcome when I get the part in the mail and get a minute to install .


Tom Greenleaf
Ultimate Carjunky / Moderator
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May 13, 2019, 11:29 AM

Post #4 of 4 (160 views)
Re: 1989 Chevy Silverado 5.7L / 305 Engine: Problem with Temperature Gauge Sign In

Yes to brass when at all possible. Just asking but subject line says both 5.7 then 305 CID either were possible but are not totally the same. Do pay attention it should be the 350CID is 5.7 the 305 is/was called a 5.0. IF stickers still on this it says on radiator shroud's stickers or VIN code if known original.
Just that, good luck with the fix,


T







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