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Adding an actual Coolant Temp gauge to 79-85 Eldorado

Started by 79 Eldorado, December 11, 2021, 01:33:01 PM

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79 Eldorado

After seeing what forum member Willem Bostoen "Wbostoen" added to his '76 I thought it would be interesting to integrate a gauge within the instrument cluster of the 79-85 with analog gauges. For anyone who missed it what Willem did was pretty impressive and he shared what he did within the following thread:
https://forums.cadillaclasalleclub.org/index.php?topic=166662.0

I previously test fit a 1989-92 Buick Century temp gauge (first photo below). That fit fairly well but I decided against continuing.

When I saw what Willem did I thought I would consider reviving the topic of adding a gauge. This is more of a back-burner project but there were a few things I was able start without much effort. I started by creating some sample graphics (photo2) and with cut-paste I added a sample version to a 1979 dash photo (photo3).

Next I needed to learn something about Arduino programming. I was able to get it to work today using a spare 70's EFI air/coolant temp sensor as the variable input (measured using a reference resistor). I selected that specific sensor because I know them well enough to convert the reading to a temperature (doesn't hurt that I'm making them either). It's a start and could be applied to any vehicle just by creating a gauge which fits and looks like it belongs in the dash. Of course EVERY vehicle I have other than the Cadillac already has a real temp gauge. I guess the Cadillac management decided having an actual gauge was too complicated for us.

I described the idiot light set-up to a friend as an "oh crap" light followed a few seconds later by "I guess it's time to call a tow truck" red light. He asked me if I considered simply replacing the existing sensors with lower temp sensors. I had started looking into that once before and it would be an alternative if someone wanted a very quick improvement. I would suggest moving the ~260F red light sensor wire to the lower temp sending unit (Red light would then come on at ~240F). I would then replace the red 260F sensor with one which grounds at around 210-220F. The thermostat should be a 190F stat so if my car gets as hot as 210F I would certainly like to know; 240F is too late. If it hits 240F it's an issue where you will likely see 260F 30 seconds later if you're lucky.

Scott


bcroe

Many approaches will work.  Practice here for a long time has been
to install a compact 1.5 in 270 degree gas bulb gauge in the dash. 
Not much space taken, and works even after shutdown. 

On the 79 Eldo work is underway to install this gauge paired with a
fuel pressure gauge, which has been seen as a useful addition, esp
with fuel injection.  One problem in this case is finding enough coolant
ports for all the sensors.  Bruce Roe

79 Eldorado

#2
Bruce,
Yes a fuel pressure gauge would be great for diagnosing an issue as it occurs live.

On the coolant temp it would be great if I could read the resistance which the ECU is reacting to directly from the ECU (without altering the resistance). I could use any generated signal which is proportional to that resistance. Maybe the voltage could be read parallel to the sensor wires. If that were possible there would be no need for an additional sensor. I assume the ECU knows the resistance through a voltage divider set-up.

I was also thinking I could use the position currently occupied by one of the idiot light sending units. If the red light sending unit was removed we would still have the "lower" temp 240F light. I don't recall the thread size for those. I would likely place the wire for the red light on the 240F sending unit (red light would come on at 240F).

The Arduino version needs power and ideally a 5V source but it can tolerate a higher input; so a key turn without starting would display the temp.

Scott

bcroe

Quote from: 79 Eldorado. Bruce,
Yes a fuel pressure gauge would be great for diagnosing an issue as it occurs live.

On the coolant temp it would be great if I could read the resistance which the ECU is reacting to directly from the ECU (without altering the resistance). I could use any generated signal which is proportional to that resistance. Maybe the voltage could be read parallel to the sensor wires. If that were possible there would be no need for an additional sensor. I assume the ECU knows the resistance through a voltage divider set-up.

I was also thinking I could use the position currently occupied by one of the idiot light sending units. If the red light sending unit was removed we would still have the "lower" temp 240F light. I don't recall the thread size for those. I would likely place the wire for the red light on the 240F sending unit (red light would come on at 240F).

The Arduino version needs power and ideally a 5V source but it can tolerate a higher input; so a key turn without starting would display the temp.  Scott

I created a T from a block of brass threaded for a couple clamp
fittings to fit into a section removed from the fuel rail.  The third
port is pipe threaded for the electronic pressure sensor on the 79. 
The mechanical sensor in this old pic is not the compact solid state
sensor I am now promoting. 

There is a near linear voltage in the ECU coolant temp sensor ckt. 
I actually wired this to a spare op amp section of the LM124, and
the output went thru an extra connector to a triple gauge unit I
had.  There is a trick to help cancel slight non linearity.  This worked
fine.  HOWEVER I was into swapping ECUs, which killed my temp
gauge, it quit with ignition off, and the extra connector was more
wiring to deal with.  I eventually decided to go to the gas bulb gauge. 

I am not much a fan of pure digital readouts. I like moving needles
and moving bar graphs.  Bruce Roe

79 Eldorado

Quote from: bcroe on December 15, 2021, 12:32:40 AM
I created a T from a block of brass threaded for a couple clamp fittings to fit into a section removed from the fuel rail.  The third port is pipe threaded for the electronic pressure sensor on the 79.  The mechanical sensor in this old pic is not the compact solid state sensor I am now promoting. 
Nice idea. It reminds me of the AN blocks designed for a gauge.

Quote from: bcroe on December 15, 2021, 12:32:40 AM
There is a near linear voltage in the ECU coolant temp sensor ckt. I actually wired this to a spare op amp section of the LM124, and the output went thru an extra connector to a triple gauge unit I had.  There is a trick to help cancel slight non linearity.  This worked fine.  HOWEVER I was into swapping ECUs, which killed my temp gauge, it quit with ignition off, and the extra connector was more wiring to deal with.  I eventually decided to go to the gas bulb gauge. 

I am not much a fan of pure digital readouts. I like moving needles and moving bar graphs.  Bruce Roe
I was thinking of the reference 9.5V you mentioned in another post where you explained the standard for devices like this had not been yet established when the ECU was designed (later 5V and 3.3V became the standard). I assumed that possibly the ECU has a voltage divider set-up to know the resistance of the sensor. I mean if the ECU has a reference resistor, in series, the voltage across the sensor and the reference would be proportional to each other and add to 9.5V. I was thinking the ECU would "react" to that variable voltage to know essentially read the temperature. If that is true I was thinking I could read the voltage in parallel to sensor. Since the sensor resistance is very close linear, in the range we need to read, the voltage in that range would be as well. If I know the value of the reference resistance, and the input voltage (9.5V?) I could calculate the expected voltage at sensor resistance which we have he reference temp for. Assuming that is how it works I could also measure the voltage at a known temp and that should allow me to determine the reference resistor value.

I guess maybe it's not that simple but if it is the connections could be made on the external plug going to the ECU which would mean you could change ECU's anytime you wanted without the need to re-wire.

I agree I like moving needles more than digital output for things like this. I also want it to fit the look of the instrument cluster.

Scott

bcroe

Likely a better arrangement than my first attempt, would be to
build the gauge ckt entirely outside the ECU, any ECU would
work unmodified.  I would suggest a meter circuit box which
ties into the temp coolant sensor wire on RED conn pin D, and
the sensor dedicated ground on RED conn pin G.  Pick up 12V
ignition power to run the meter on on BLACK conn pin F.  All
this from my drawings, could forward the latest 79 version. 

The ECU would supply the regulated voltage divider, you just
measure the voltage across the sensor pins and translate that
to your readout deg F.  Bruce Roe

79 Eldorado

Bruce,
If you do have a latest circuit diagram it would be great. If the circuit is similar to what I described the Arduino could likley read the reference directly but it normally deals with a 5V reference. It has the ability to read the reference (analog in). If the ECU has a ref resistor in series I guess what I would be reading with the Arduino would be roughly half of 9.5V though. I could always use a diode to divert over a value which could be too high.

Thanks,
Scott

smokuspollutus

This is a cool idea Scott. I actually did a rendering of what that would look like years ago. It's buried on my Flickr somewhere. I like the Buick Century gauge approach. This is all a moot point for me since my car has the digital display and that slot is occupied by the fuel range display. But most of my Eldos have had the analog unit and I couldn't help but feel that the coolant temp idiot light was a last minute addition considering the ample space in the info centers for another light and that the hole is a perfect mirror of the fuel gauge. Maybe a fuel gauge could be repurposed for the task? Or swap the Buick century guts to a spare Eldo fuel gauge body so it will mount up cleanly in the cluster?
1984 Eldorado Biarritz Coupe
1983 Sedan deVille

bcroe

Quote from: 79 EldoradoBruce,
If you do have a latest circuit diagram it would be great. If the circuit is similar to what I described the Arduino could likley read the reference directly but it normally deals with a 5V reference. It has the ability to read the reference (analog in). If the ECU has a ref resistor in series I guess what I would be reading with the Arduino would be roughly half of 9.5V though. I could always use a diode to divert over a value which could be too high. Thanks, Scott

Easy enough to find a voltage which translates directly to a coolant temp
when the ignition is on.  Tell me again just how you want to drive a display
of this temp?  Bruce Roe

79 Eldorado

#9
Quote from: bcroe on December 20, 2021, 02:44:09 PM
Easy enough to find a voltage which translates directly to a coolant temp when the ignition is on.  Tell me again just how you want to drive a display of this temp?  Bruce Roe
Bruce,
The Arduino can move a servo with a PWM (Pulse Width Modulated) signal. So far I've been using the voltage divider concept to measure the resistance (through voltage measurement) in one of my sensors versus a fixed known 1000 Ohm resistor in series. Once the voltage is read (input pin) a simple program relates measured voltage to resistance and temperature. I then map the voltage accordingly to the servo to move to the proper temp.

Quote from: smokuspollutus on December 20, 2021, 02:29:31 PM
This is a cool idea Scott. I actually did a rendering of what that would look like years ago. It's buried on my Flickr somewhere. I like the Buick Century gauge approach. This is all a moot point for me since my car has the digital display and that slot is occupied by the fuel range display. But most of my Eldos have had the analog unit and I couldn't help but feel that the coolant temp idiot light was a last minute addition considering the ample space in the info centers for another light and that the hole is a perfect mirror of the fuel gauge. Maybe a fuel gauge could be repurposed for the task? Or swap the Buick century guts to a spare Eldo fuel gauge body so it will mount up cleanly in the cluster?
Anthony,
I thought the Century gauge looked close but in the end it would have been a fair amount of work for something I wasn't certain I would be happy with. I felt I would have likely tried to disassemble it and create a new custom face plate. That would be a lot to do for a single car and nothing easily offered to others as the Century parts aren't that readily available either.

I'm not a gauge expert but they need to be tailored to the particular resistance range. The fuel gauges are something like 0 to 90 Ohms by memory. I don't know for sure but I don't think that's a range which is easily found in temp sensing devices which vary resistance. The EFI sensors, as an example, are around 1000-2000 Ohms in the temp range we need. So the fuel gauge might work but we would need additional circuitry to translate what the sending unit is reading to what the gauge would see as equivalent to 0-90 Ohms (or the resulting voltage). Spare fuel gauges for our cars I suspect are not easy to find either.

The project Willem worked on was digital. If you could find a place where it fit the look of the digital dash I'm sure his concept could be used on your car. He even customized his graphic. He's the one who turned me on to the Arduino idea. As long as there's a way to fit the parts in the space I think I could get a local sign/graphics place to create a custom overlay. I may paint one myself to start though.

I would like to retain at least one of the idiot lights but I think I could set things up so it would be visible despite the gauge. I'm not certain yet.

Retaining both lights would be possible if the voltage signal comes directly from the ECU; at least 1979. That said we don't need 2 lights which are 20F apart. I could fairly easily design a sensor to go in place of one of those existing sensors. If they're already 1/4 NPT the 1976-79 EFI coolant sensor would work without modification. One problem with using the ECU signal is the gauge only works for the 1979 then. If I replace an idiot light sensor it can work on anything from 1979-1985 (I guess?).

Scott




bcroe

All the 70s Cad EFIs use about an 1100 ohm resistor from 9.5V to the temp
sensor, current flowing to the ECU ground.  My notes say the sensor voltage
is about 3.96V at 0F, 5.38V at 200F.  You could zero that in a bit on your setup
and find other values for a gauge.  It is not perfectly linear, but most temp
gauges on close examination are not either.  Sometime an extra bend can be
added to get what is wanted, pretty common practice from way back. 

I gave the pin numbers, you want to use that ECU ground, as random body
grounds will shift some with things like alternator current to headlights or blowers. 

Use a fuel gauge with recalibrated scale?  Once you have curves for the output
of the sensor and input of the gauge, all you need is an amp with that transfer
function, not difficult.  The amp may run a bit of power for that gauge. 

You can do the same with any engine, but supply your own voltage, resistor, and
sensor to pick up the temp.  A current source might give a bit more linear output
than a resistor.  If you want it to work without ignition power, you will
need to find another way to turn it off and not run down the battery. 

Bruce Roe

79 Eldorado

#11
After touching on this under a topic Cory started it seems like using a traditional gauge shouldn't have been dropped as a potential idea without more investigating...
Link to Cory's topic:
https://forums.cadillaclasalleclub.org/index.php?topic=170475.0

I found a gauge which will fit in the position to the right of the speedometer and the gauge face doesn't even look that bad. It's a single wire gauge. I can come-up with a holder to mount it securely but without changing the gauge face, easier to leave it the way it is, you will very likely be able to see the edge of the round face. The needle can be painted to match the other needles.

I was concerned about the lighting in that area of the dash but I found the photo I took after changing all of my dash lights and there is a light source there.

A gauge face could be made in the same shape as the "Coolant Temperature" panel graphic already there. A decal with lines, maintaining "Coolant Temperature" could even overlay the original panel. The material of the original looks brittle though and holes need to be added to put the pointer and the screws through. The original panel as well doesn't have much structure.

Conclusion:
Working traditional gauge doesn't look that difficult... there are a lot of different ways to go and some are easier than others.

Any thoughts? I initially thought a sticker/decal would make the black backing appear too gloss like but I'm not certain that's true. Can the gauge face be swapped without risk of damage?

Scott
OE Dash lights at night:
IMG_20190518_222737.jpg
Replacement gauge without changing the face just sitting in place for now:
2023_01_15 3_32 PM Office Lens.jpg


TJ Hopland

That doesn't look bad.  Mask off the roundness and it would look better.  What's it gonna look like in the dark with it lit tho?   That may be where it falls apart.
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason

bcroe

That looks pretty good.  It occurs to me, another
gas gauge might fit well over there, needing new
scale.  But they are not very sensitive, would
need a proper drive ckt.  Bruce Roe

79 Eldorado

TJ,
When I had it apart I didn't realize there was "another" light source, other than the coolant bulb, until I saw my old photo in the dark. I suspect though it would at least be as bright as the photo. I can probably make the mask hide it a little better but I didn't want the mask to give the appearance of shrinking it too much.

Bruce,
Cory mentioned using the fuel gauge as well but I don't love that idea. I would still need to change the face plate, come-up with a different sending unit and I also realized GM error proofed the mounting screws to seemingly prevent someone from mounting the gauge on the wrong side. On top of that there's a limited supply of those old fuel gauges.

I worked on the CAD version of the graphics. I spent hours before I realized the file font I was using seems to have an issue with the "H" as well as at least one character in the words "Coolant" and/or "Temperature". So I have a perfect "C" and that's it. What a time wasting exercise. Powerpoint must have a separate font file or it simply doesn't have the issue which FreeCAD has. I did create the lower divided graphic and part of the temperature bulb.

Another thing I thought of was building on Cory's idea of using that blank panel on the 1979. we could mount a couple of gauges in that position.

Scott

fishnjim

I generally like gauges, but in this case I don't think it makes much difference to have a warning vs indication.  I doubt the muckitymucks had much say over the engineering/design depts.  Cost targets yes, but dictating instrument layouts, doubtful.
The coolant system is capable of mild pressurization so it'll see temps over 220F as normal and some have catch tanks so there's no issue of overflow.  Depends on the cap pressure when it'll overflow to protect the integrity of the system.
You have to look at likelihood when applying instruments and how often is this a problem?  Regular inspection of the fluid level is easy.  I think some people will not understand what's going on/how cooling system works and it they see the temperature go up or down, they'll head to the service window and drive them nuts.   It's not controlled like the thermostat in your house.

79 Eldorado

Jim,
I've had a few situations where a gauge alerted me that something strange was happening before I had catastrophic issues. There are optional or standard gauge packages for almost everything GM makes...except these old Cadillacs. The Toronado of the same year, also an E-Body, had a coolant temp gauge even without ordering the optional gauges which were available. I could see someone making the decision going through the same thought process that you mentioned but doesn't that say all other owners are capable of understanding a gauge but Cadillac owners are not?

There are 2 lights on the Eldorado. As I recall they're 20 degrees F apart but the lower one is already higher than I've seen the Toronado run assuming there was no issue.

Scott

bcroe

That gauge keeps me up to date on the capability
of the cooling system.  It will tell me when the
situation is getting too difficult.  And it will
tell me when it is time to make adjustments (take
it easy), and if the adjustments are sufficent
to get back into safer territory.  Yes I have
rolled the windows down and run the heater at
max on a very hot day.  But got home. Bruce Roe

eldofever58

Bruce, here's a pic I took the other day subbing the fuel gauge. From what I could tell, mounting would be pretty easy- Cut a rectangular backer out of thin gauge aluminum, punch a hole in the center, and drop the gauge in. Drill two holes to match the cluster bosses and you're set. But to Scott's point, they are getting tougher to find. I see digital clusters 2:1 at the yards. I also suggested using the low fuel light as an overtemp attention-getter, but a driver circuit for the works would need to be drawn up.


79 Eldorado

Cory,
Cool that you tried moving a fuel gauge over. I've been working on CAD modeling the parts. The following capture is the aftermarket gauge body I found placed behind the idiot light mask and positioned so it should fit properly within the cover mask and be visible.

Some things I noticed:
-The idiot light mask looks amber in color so I suspect that's the orange light which comes on before the red light. I couldn't recall which was which.
-The OE mask is very thin. That plus better location relative to the plastic holder might be why they added those bottom locating bayonets. It shares the same screws though. In the spare dash I bought those bayonets weren't even inserted as they should be.
-The plastic part for placing the idiot light mask seems really over complicated. I wonder if some of that was to seal the light of the warning bulb from "leaking" to other areas of the instrument panel when the warning light lights. I suspect it must have been for that reason. I was going to create some form of that part to cradle the gauge body but the entire part could possible be eliminated or greatly reduced.
-If I use a decal or transparent stock sticker I could likely cut a window in the gauge face so that the amber/orange warning light still works through the gauge face.

Once everything is modeled I don't think this would be terrible to implement. That aftermarket gauge has a low-limit stop pin for the gauge needle. Does anyone know if that's really necessary? I could probably braze a pin in, I mean literally a sewing pin, and cut it off but that's the closest thing I can think of unless those pins are available someplace.
TempGaugeConceptonOEMask 19JA2023.JPG

Scott