Adding an actual Coolant Temp gauge to 79-85 Eldorado

[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]

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]

. 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]

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.

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?

[bcroe]

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

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]

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.

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