Click on image for the lowdown. 
====
| | | Forums66 Topics128,043 Posts1,055,605 Members48,426 | | Most Online14,466
Apr 26th, 2026 | | AD Chevy Trucks  Over 6,000 pictures Brad Allen has an awesome collection of Chevrolet factory pictures that he has set up from film strips. Check them out!
Lots of work on Brad's part ... pure enjoyment (and references) for you!
| | THANKS!
For helping keep this great
old truck web site
on the Internet for over 30 years! and not clogged down
with Ads!  Again, thanks!
| | | | Joined: Aug 2015 Posts: 19 New Guy | | New Guy Joined: Aug 2015 Posts: 19 | First off, I did a quick check on the internet and couldn’t find an answer, so if I missed it please point me in that direction.
I have a 250 in my 51’, the temp gauge (after market) stopped working so I replaced with a new one. Now it won’t register until it’s up to temp, shut off, and restarted. On initial start up it’ll register if I tap the sensor area with a screwdriver handle. Do I have an air pocket or something? I’ve checked the gauge/sending unit (mechanical type) and it registers. I’ve got a flush tool that uses compressed air, but I think that’s for modern closed systems?
I would appreciate any help, thanks in advance.
Ryan | | | | | Joined: Jun 2011 Posts: 5,096 Crusing in the Passing Lane | | Crusing in the Passing Lane Joined: Jun 2011 Posts: 5,096 | Did you use teflon tape that would electrically insulate the sender from the block and thus the rest of the truck?
Ed
'37 GMC T-18 w/ DD 4-53T, RTO-610, 6231 aux., '95 GMC running gear, full disc brakes, power steering, 22.5 wheels and tires.
'47 GMC 1 ton w/ 302, NP-540, 4wd, full width Blazer front axle.
'54 GMC 630 w/ 503 gasser, 5 speed, ex fire truck, shortened WB 4', install 8' bed.
'55 GMC 370 w/270, 420 4 speed, grain, dump bed truck from ND. Works OK.
| | | | | Joined: Aug 2015 Posts: 19 New Guy | | New Guy Joined: Aug 2015 Posts: 19 | Ed, thanks for the reply. Yes the probe was Teflon taped.
Ryan | | | | | Joined: Mar 2010 Posts: 10,272 Renaissance Man | | Renaissance Man Joined: Mar 2010 Posts: 10,272 | Use an OHM meter to check to see if, while installation, the Teflon got out of the way between the threads of the block and the sending unit. It should read the same as though you were touching the two leads together. No resistance.
1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission
| | | | | Joined: Mar 2007 Posts: 3,379 Crusty Old Sarge | | Crusty Old Sarge Joined: Mar 2007 Posts: 3,379 | The OP says the gauge is mechanical type, this should be just a sensing bulb and a gauge. It wouldn't have an electrical coennection. Did I miss something? It may have an air pocket or the sensing bulb may be defective. You can test with a pan of hot water and a kichen thermoeter, the bulb should read the same.
~ Craig 1958 Viking 4400"The Book of Thor"Read the story in the DITY1960 Chevrolet C10"A Family Heirloom"Follow the story in the DITY Gallery'59 Apache 31, 327 V8 (0.030 over), Muncie M20 4 Speed, GM 10 Bolt Rear... long term project (30 years and counting) If you don't like driving yourTruck, you won't drive yourTruck..| Living life in the SLOW lane | | | | | Joined: Aug 2015 Posts: 19 New Guy | | New Guy Joined: Aug 2015 Posts: 19 | Tuts 59, yes mechanical and I checked it with hot water. Gauge / bulb are good.
I did check with a meter, but as mentioned it shouldn’t matter with a mechanical gauge?
Still confused, but thanks for all the replies!
Ryan | | | | | Joined: Mar 2007 Posts: 3,379 Crusty Old Sarge | | Crusty Old Sarge Joined: Mar 2007 Posts: 3,379 | Ryan, It could be that the sensing bulb probe is contacting the outer shell and causing it not to read. You did say it would read when you tapped the housing with a screwdriver. It may be loose internally.
~ Craig 1958 Viking 4400"The Book of Thor"Read the story in the DITY1960 Chevrolet C10"A Family Heirloom"Follow the story in the DITY Gallery'59 Apache 31, 327 V8 (0.030 over), Muncie M20 4 Speed, GM 10 Bolt Rear... long term project (30 years and counting) If you don't like driving yourTruck, you won't drive yourTruck..| Living life in the SLOW lane | | | | | Joined: Aug 2015 Posts: 19 New Guy | | New Guy Joined: Aug 2015 Posts: 19 | Craig, I’ll check if it’s contacting/loose. Not sure how that would change anything though. Probe touching metal or coolant might have different readings, but shouldn’t produce nothing?
Thanks for the help!
Ryan | | | | | Joined: Nov 2014 Posts: 623 'Bolter | | 'Bolter Joined: Nov 2014 Posts: 623 | I have this problem sometimes (not very often), when I first start the truck, and I've never figured out what does it. It's only at start up and, like yours, if I stop, turn it off and back on, it fixes it. Other times I'll forget about it, and then 5 or 10 miles down the road, it's back to working again. I've always wondered what's going on with that.
"When I rest, I rust"
1951 3100 5 window w/ '56 235
| | | | | Joined: Dec 2006 Posts: 165 'Bolter | | 'Bolter Joined: Dec 2006 Posts: 165 | Ditto, mine does the same thing.
Kevin
| | |
|
|
