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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

Here's the first 3D test print of the gauge holder. It wasn't easy to measure everything relatively from the deeper area to the face. There are locating pins at the bottom. Those control the location but they need to be closely aligned with the screw hole locations on the face. It looks like I got most things spot-on but I need to move the face down, relative to the body, to get the screws to align properly. As you can see the lower right corner is close to the screw boss. The lower right corner of the OE part is very similar. I will alter the geometry slightly on that corner before moving the face, and holes, down.

I was very happy with the first test print. I still need to determine where to add a hole in the gauge holder for the gauge wire. That gauge wire will also need to go through the instrument panel.

Something I realized while doing this was the OE "gauge holder" was needed to only block any light from illuminating the amber idiot light at night when the normal dash lights were on. The normal light which illuminates that part of the dash is above the gauge holder "cup". The OE design has a rounded triangle as part of the face plate. I assume that reduces light over the idiot light area so I eliminated the obstruction.

Here's a photo of the test fit.
2023_01_22 10_37 AM Office Lens.jpg



Am waiting to see what mechanism you will put in there.  With
some electronics, a single temp sensor could drive it, with the
Red signal coming from the same electronics.  We did something
like that for the 57 Eldo gauge fix. 

While your Olds intake has spots for a lot of coolant sensors, they
are not unlimited.  All might be used on my car, a pipe thread drill
and tap on the intake manifold done.  I typically have a switch for
a HOT light and a resistive sensor or gas tube for a dial gauge. 
But the 79 E also wants one for the EFI, and I had a second for
an after market ECU experiment.  That was too many, the after
market sensor is now gone. 

One possibility here is connect to the wires of the EFI coolant
sensor, the operating voltage can be directly translated to temp. 
That then used to drive a gauge.  One less sensor. 

A problem here with my cars having so many miles, driven so
much at night, is the incandescent dash illumination bulbs get
hot and eventually distort the plastic mounts.  Contact may be
lost.  And of course they get black/burn out.  My conclusion is
that white LEDs might be the cure.  But LEDs do not have an
omniudirectional light pattern, so some experimentation is
needed to get it right.  That might be part of this project. 
Bruce Roe

79 Eldorado

Before I started designing the holder I found the aftermarket gauge which fit (guessed and it worked). The holder is designed for that gauge specifically and I will as well create a custom face plate. I also found the sending unit for the same gauge. It's very small diameter and single wire. Take a look at the photo attached below. It was one I had from when I was working on the intake. There is a large brass plug partially blocked by the oil fill cap in the photo. That position is not used on the EFI cars. I plan to design a brass adapter bushing for the sending unit and use that position. I think most Olds intakes have that position.

One other thing I thought about was using the amber warning light position. I thought possibly an adapter could be plugged into the dash light socket and the same wire could be used. The sender for the amber light would be replaced by the sender I found. I don't think they're the same size though. I don't recall which one is used for the amber light. One of the ones used is the position between the EFI coolant sensor and the thermostat housing; not certain if that's the red or the amber one though.

When I changed the lights in my Eldorado dash I thought about LEDs but my experience with them is some do not dim properly with a dimmer, seems to take a specialized version, and second the life isn't always up to the advertising. I had a 2010 BMW 3 series in Europe and the rear taillights were sealed because who would ever need to change a bulb if LEDs are used (sarcastic)? Well a single LED went bad in one and the entire light was garbage. It then happened on the other side a year later. The original BMW replacement, single side tail lamp, was around $700. An electrical shop tried to repair one of the failed LED assemblies by hot knife cutting part of the rear of the assembly out but the repair lasted about 6 months. Finally an aftermarket company created replacements for something like $100 per side. I do have some Phillips home LED bulbs which work well on a dimmer but prior to those I tried a different brand and while they were advertised as dimmer compatible they were not. That said if someone had an LED which they know works with a dimmer I could certainly try them.

Intake350z FelPro1356.JPG



Yes every Olds intake here has the 3 positions, just use an
adapter (or tap it out) to get the right size.  And you have solved
the gauge issue.  Will the original bulb function also be
included in the design?

The LEDs have a different voltage response curve, may only
agree with incandescent at max.  If you have 1 LED in series
with a resistor, it will not be so bad.  Stack more LEDS in series
and it is more efficient, but the curve gets worse as it is
dominated more by the LEDs and less by the series resistance. 

I never dimmed those lights much, so I have just been using
LEDS set as I like and putting them on an undimmed tail light

As for BMW, lets just say I would never own one.  Bruce Roe

79 Eldorado

The plan is to keep the idiot light functioning with the gauge. I plan to add a window in the face plate, in the area of the temp bulb graphic, and then hopefully I can get some orange/amber material possibly as the sticker/decal stock. I know the bulb/window will work but I need to see what's available regarding labels.

The LEDs just seem like more trouble than they're worth.


79 Eldorado

Do you happen to have a photo of the dash with the instrument panel removed? The gauge wire needs to go through the light blue gauge holder and I'm wondering which area has sufficient clearance for the wire. I was thinking through the back of the blue holder (front relative to the car) or the top. I see locating features on the back and I know the dash slides in but I don't have a reference for the clearance around that part.

I wouldn't necessarily care now but I need to add a grommet/hole in my holder and where the wire comes in can make one location better than another. If it enters the rear it could come through the extreme right side of the speedometer area. If it comes from the top it could come in above the gauge. I guess I can also try to locate my clearance hole so it works ok with either location and hopefully there will be dash clearance in one of those areas.



Scott, have a look at my speedometer swap page, there are a few pics from different angles with the speedo cluster removed. The area where the connector passes through for the digital speedo head looks like a decent spot on the analog cars.

79 Eldorado

Thanks Cory!

When I looked at those photos I noticed that the back of the dash must have close clearance because those push-in ribbon connectors would require close proximity to what they plug into. It was interesting to see the green wire tucked-in. The green wire appears to go to the idiot light position and sending units often have green wires. I was even planning to use a green wire.

I placed the grommet in a position which would be good for a wire coming in any of the 3 places I assumed it would need to; left side of my holder facing the side of the speedometer. That will allow the wire to have pretty direct access if it comes through the speedometer area or in the idiot light area as well as either from the rear or the top of the blue instrument cluster holder.


79 Eldorado

I think I'm satisfied with the graphics now. There were issues with some of the characters in the font I was using so I selected another one which looks a lot like the fuel gauge font for "Coolant Temperature". I added a warning triangle with an exclamation mark. The idea is that area will be transparent red and so if the light comes on the triangular outline will light and as well the exclamation mark. I thought red would look nicer than amber and the warnings will still look different from one another.

Here's a photo with a paper copy printed out in very close to the correct size and position.
2023_01_27 8_16 PM Office Lens.jpg


79 Eldorado

I had a revelation this morning and not a good one. I was wondering how a single wire could possibly function. Well it seems it cannot or does not in the case of this gauge. The single wire is just the only easily recognizable connection. The gauge still needs normal power and ground. I think I must have had hysterical blindness. The gauge has 2 receptacles for what are likely 2.5mm pins. Those pins go into the back of the gauge. The center threaded stub, coming from the back, holds the gauge to one of the flexible copper/plastic circuit backings and creates the third connection.

That probably means that gauge holder I spent so much time on needs to change and I need a connector for what should be three wires. Kicking myself because I questioned it when I saw it and I should have questioned it again when Bruce asked a question which should have given me another chance to evaluate what I thought I was seeing.
2023_01_28 12_28 PM Office Lens.jpg


If I understand correctly, your printed housing won't accommodate the additional connectors? Since this is going to be custom and "permanent", I'd probably remove the gauge backshell and solder the +12 and Grd wires directly to the contact areas, reinstall the shell, then run those wires over to the fuel gauge for pow/grd. No connector protrusions to worry about.

79 Eldorado

I was trying to maintain the idiot light warning function else I could have totally eliminated the holder. That seems to be the only real reason for the gauge holder. In the OE application the "gauge holder" just blocks the normal dash light, always on with lights, from back lighting the amber idiot light. The idiot light specific bulb is boxed inside of the "gauge holder" which isolates that area. My part was a redesigned to be an actual gauge holder, still only needed to isolate the warning light, but for the gauge to fit in that location I needed to move the bottom wall down so that the gauge face would be properly positioned in the window. Moving the gauge down avoids the need to trim the black plastic, likely fragile, instrument panel cover panel.

I think I really only need to make one area deeper in the gauge holder. I was taking a closer look after I posted and it looks like the way I have it now it would be close; another 2 or 3 millimeters deeper in the area I bumped-out for the the gauge and it would likely work if there isn't already enough room now.

That style receptacle probably has an advantage because the part of the terminal sticking out can be flat like a nail head which would be easy to solder a wire to. I cannot find a loose connector but I saw a photo of the terminal holder and it has 2 pins outboard of a center hole. The connector goes over the stud and the nut keeps the pins from pulling out.

I do think it would probably be easiest and acceptable to solder the power/ground to the existing circuit board because every lamp in the cluster needs the same.

Here's a capture of my current version of the gauge holder. That barrel bump-out area might need to be deeper.
GaugeHolderCapture 28JA2023.JPG


Ah, didn't realize you were keeping the warning light with the gauge. Should things not work out, you could always populate the other Coolant Warning light location in the left Info Center panel.


Quote from: eldofever58Scott, have a look at my speedometer swap page, there are a few pics from different angles with the speedo cluster removed. The area where the connector passes through for the digital speedo head looks like a decent spot on the analog cars.

Nice guide to the digital vs analog change.  I was
well aware that the tank sending unit typically
reads full for about the first 1/3 tank, than drops
like a rock.  Not much use for calulating stop
points.  One of the things I always do is pull the
sender and modify the arm, so the float will just
touch the top and the bottom of the tank.  That
makes it quite accurate, can say within a gallon
what a refill will need.  Probably makes it equal
to the digital pickup.  Speedo gets callibrated
a lot better, and a few other things.  Bruce Roe