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1965 Cadillac temperature gauge pegged to hot

Started by Scot Minesinger, March 24, 2020, 08:48:54 PM

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Scot Minesinger

Working a 1965 Cadillac Fleetwood with a rebuilt original 429 engine.  The gauge pegs to H even when it is just started up, and sometimes falls to what might be a true reading.  This is not a pressurized system, like a 1969 or newer.  It is understood that the gauge reads about a quarter under normal circumstances - tonight it was 55'F and ac is off, so should have been around 1/4.  I measured radiator water temp and it was 180'F, which seems normal.  The temperature in and out of the radiator was only 15'F, which seems too low, since it is around 30'F on a 472, but higher flow water pump may cause this - do not have much experience with this engine.  Just started to get into it. 

Maybe thermostat is stuck closed?

Where is the temperature sender for the gauge - want to make sure there is not a wiring issue (engine was of course removed for rebuild)?
Does anyone know what the radiator temperature drop should be, as 15'F seems low?

Just started to get into it, are there any places to look first that are problematic?

Fairfax Station, VA  22039 (Washington DC Sub)
1970 Cadillac DeVille Convertible
1970 Cadillac Sedan DeVille
1970 four door Convertible w/Cadillac Warranty

The Tassie Devil(le)

Quote from: Scot Minesinger on March 24, 2020, 08:48:54 PM
This is not a pressurized system, like a 1969 or newer. 
I thought all later model Cadillacs were pressurised in the cooling department.   I know my '60 CDV was.   But, they weren't the Recovery System.

Bruce. >:D
'72 Eldorado Convertible (LHD)
'70 Ranchero Squire (RHD)
'74 Chris Craft Gull Wing (SH)
'02 VX Series II Holden Commodore SS Sedan
(Past President Modified Chapter)

Past Cars of significance - to me
1935 Ford 3 Window Coupe
1936 Ford 5 Window Coupe
1937 Chevrolet Sports Coupe
1955 Chevrolet Convertible
1959 Ford Fairlane Ranch Wagon
1960 Cadillac CDV
1972 Cadillac Eldorado Coupe

35-709

Right, that is a pressurized system but not a recovery system.  Pressurized systems go back to at least the early '50s and probably earlier.
1935 Cadillac Sedan resto-mod "Big Red"
1973 Cadillac Caribou - Sold - but still in the family
1950 Jaguar Mark V Saloon resto-mod - Sold
1942 Cadillac 6269 - Sold
1968 Pontiac Bonneville Convertible - Sold
1950 Packard 2dr. Club Sedan
1935 Glenn Pray - Auburn Boattail Speedster, Gen. 2

cadillacmike68

Quote from: The Tassie Devil(le) on March 24, 2020, 09:28:42 PM
I thought all later model Cadillacs were pressurised in the cooling department.   I know my '60 CDV was.   But, they weren't the Recovery System.

Bruce. >:D

I think Scot meant that it is not an overflow capture (sealed) system. The pressure relief fitting on the RC-15 cap just connects to a tube that empties on the ground. I retrofitted a 1969-70 tank and the RC-27 cap to make my 1968  sealed.
Regards,
"Cadillac" Mike

chrisntam

Temp sender is either in the heads or intake, should be easy to find...
1970 Deville Convertible 
Dallas, Texas

bcroe

I put overflow tanks and pressure caps on my 60s cars (when I
was driving those year), was well worth it.  Find the wiring diag
on the gauge circuit, should not be too hard to sort out while cold. 
Bruce Roe

The Tassie Devil(le)

On a side note, when you fitted the Overflow tanks to the earlier cars, did you add the feature of being a Recovery Tank?   Or, just to stop the expelled coolant from simply falling on the ground?

When converting to Recovery system, one has to modify the radiator top of the tank to change the cap fitting from a simple pressure cap, to a deeper cap, to add the recovery availability.

Bruce. >:D
'72 Eldorado Convertible (LHD)
'70 Ranchero Squire (RHD)
'74 Chris Craft Gull Wing (SH)
'02 VX Series II Holden Commodore SS Sedan
(Past President Modified Chapter)

Past Cars of significance - to me
1935 Ford 3 Window Coupe
1936 Ford 5 Window Coupe
1937 Chevrolet Sports Coupe
1955 Chevrolet Convertible
1959 Ford Fairlane Ranch Wagon
1960 Cadillac CDV
1972 Cadillac Eldorado Coupe

bcroe

The overflow kits I used had the required cap, which I saw working in
extreme heat.  Parked by Boulder dam it filled a quart container, then
sucked it all back in, a little later.  Bruce Roe

fishnjim

Couple things.
There could be an air pocket in the block, if you changed coolant, and that would cause a "hot spot and peg the gauge.   ie, Needs "burped".
The coolant will change temperature as the car moves, so if it's 180 sitting still with a 165-180 stat, it's probably not stuck.   Use test a stat in a pan of water from cold to boiling.   Should be 100% open at the nominal temperature.
The cheap and easy thing is to replace the sending unit.   It'll have a single electrical wire with covering leading to a connector a top a pipe plug.   Unscrew and replace put a dab of pipe thread sealant.  And recheck.  If no air pocket, same reading, then it's the gauge or wire.
You could take sending unit and gauge out and test with water bath and instruments, but that's the advanced course.
These work on resistance change due to temperature, resistance increases with temperature, so varies the voltage.   The gauge is reading volts but the dial is indicating temperature.   Those crafty engineers!

Scot Minesinger

The radiator hose has no pressure in it when hot, you can squeeze it like the car is off and has not run in a day.  It does have a replacement radiator, maybe cap is bad?
Fairfax Station, VA  22039 (Washington DC Sub)
1970 Cadillac DeVille Convertible
1970 Cadillac Sedan DeVille
1970 four door Convertible w/Cadillac Warranty

35-709

Good or bad, the cap isn't the immediate problem here.  Air lock, bad thermostat, blockage someplace from the sound of it. 
1935 Cadillac Sedan resto-mod "Big Red"
1973 Cadillac Caribou - Sold - but still in the family
1950 Jaguar Mark V Saloon resto-mod - Sold
1942 Cadillac 6269 - Sold
1968 Pontiac Bonneville Convertible - Sold
1950 Packard 2dr. Club Sedan
1935 Glenn Pray - Auburn Boattail Speedster, Gen. 2

cadillactim

Could be an electrical issue. If it pegs when you first start up with cold engine, then it is likely electrical. Some circuit may be "finding ground" through that gauge. Run a jumper wire from the metal dash cluster to the body and see if it goes back to normal. Had a similar issue with my 67 and it was a bad ground between the dash and the body.

Tim
Tim Groves

Michael Petti

I had same issue on my 60 CDV. Wire was shorting intermittently, giving me a Hot reading.  I taped up the worn spot and encased the wire in a split plastic casing and was good to go. No more problem.

Jason Edge

Most of the new replacement temp sending units on the 1956 to 1964 Cadillacs (+ 1965 Series 75) are junk in that they are calibrated incorrectly and cause Hot readings at normal operating temperature range (i.e. 180 to 195 degrees). I am not sure if the 1965 and later replacement temp sending units have this problem. They sure do for 1964 and prior!
If the temp sending unit was replaced by a new/aftermarket unit, you might try a good used unit instead. I have a few 64 and prior units for sale but none of the 65 and later but bet some of the other parts suppliers on the forums will.
Jason Edge
Lifetime Member
Executive Vice President
CLC 1963/64 Cadillac Chapter Director - www.6364Cadillac.com
CLC Carolina Region Webmaster - www.CRCLC.org
CLC MRC Benefactor
email - jasonedge64@outlook.com
1964 Coupe DeVille - Sierra Gold - http://bit.ly/1WnOQRX
2002 Escalade EXT - Black
2013 Escalade EXT Premium Edition - Xenon Blue
2022 XT5 Luxury Premium - Dark Moon Blue Metallic

cadillacmike68

Quote from: The Tassie Devil(le) on March 24, 2020, 10:02:08 PM
On a side note, when you fitted the Overflow tanks to the earlier cars, did you add the feature of being a Recovery Tank?   Or, just to stop the expelled coolant from simply falling on the ground?

When converting to Recovery system, one has to modify the radiator top of the tank to change the cap fitting from a simple pressure cap, to a deeper cap, to add the recovery availability.

Bruce. >:D

Mine is set up just like a 1969-70, overflow with recovery. It uses the 1969-70 tank with all the original tubes to catch radiator overflow and recover it when cooled off. That tank itself has a dump tube, I guess for when things get Really Hot! I've never had the car dump coolant since I put that tank on.
Regards,
"Cadillac" Mike

Dan LeBlanc

Lectric Limited offers the temperature sending unit in the correct resistance to overcome the issue Jason mentioned.
Dan LeBlanc
1977 Lincoln Continental Town Car

savemy67

Hello Scot,

If the gauge pegs hot when the car is started, and the coolant is cold, I would agree with Cadillac Tim that the problem is most likely electrical.

On my '67's 429 (most likely the same as your '65), the wire for the temp sensor runs from the sensor in the cylinder head near the distributor back to the firewall.  If the wire has become exposed, it may deteriorate rapidly due to exposure to the high temp portion of the intake manifold.  Check to see of the wire is intact and in OK condition.  If all is OK, I would proceed to Cadillac Tim's ground check at the dash.

The temp gauge and the fuel gauge are both the same type of gauge.  Many apparent fuel gauge problems (pegging empty or full) are a result of a bad ground in the fuel gauge circuit at the sender ground at the back of the car.

Respectfully submitted,

Christopher Winter
Christopher Winter
1967 Sedan DeVille hardtop

Scot Minesinger

To all those who advised blockage/stuck thermostat - it was a stuck thermostat.  Easy problem solve - thanks for making it quick for me.

It pegged hot quick, not as soon as car started, but within a few minutes - my bad on communicating this. 
Fairfax Station, VA  22039 (Washington DC Sub)
1970 Cadillac DeVille Convertible
1970 Cadillac Sedan DeVille
1970 four door Convertible w/Cadillac Warranty

chrisntam

Thanks for letting us know, it's always nice to know the rest of the story!
1970 Deville Convertible 
Dallas, Texas

MikeLawson

Quote from: Dan LeBlanc on March 26, 2020, 07:12:45 AM
Lectric Limited offers the temperature sending unit in the correct resistance to overcome the issue Jason mentioned.

Thanks for posting this information. I've got one on order for my '67. Hopefully, this will cure my inaccurate (too-high) gauge reading.