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Fixing P1800S fuel gauge
So I pulled apart (and fixed) my P1800 fuel gauge the other night, so I thought I'd share some things I learned, as the sw-em data focuses on the 122 fuel gauge. There are some similarities, but enough differences that I hope this write-up helps others.
For starters, the P1800 gauge works similarly to the 122 gauge. A broken sender unit will cause the gauge to read full. A simple test can be performed with the car on (but not running) and disconnecting the sender (in the trunk under the rubber mat) - this should make the gauge read full. Then ground out the wire that was going to the sender - this should make it read empty.
My gauge read empty during both tests, indicating an issue with the gauge. I was able to perform a lot of tests without removing the gauge. Before I go in to those tests, let's look at the wires on the gauge.
There are 4 connections in the back:
Red wire: power for the light (may be doubled up)
Black wire: ground
Green wire: power for the needle (may be doubled up)
Green with black stripe: signal from sender.
The bulb is inserted at the top of the gauge. The ground is a round connector attached to one of the two posts on the left and right of the gauge - these posts hold the little 'shovels' that hold the gauge in place. The power and signal are both spade connectors. The 'double' one (marked B) is power, the single (marked T) is signal.
With a multimeter you can check voltage across B and a known good ground (I got 12v) and then check voltage across B and the ground wire on the fuel gauge (I got 12v again). This confirms both power and ground are good. I got +3V while checking between the signal wire and the ground. My tank was 1/4 full at the time. All signals were pointing to an internal gauge failure.
I pulled the gauge: Remove two retaining finger nuts. The ground wire will come off here. Watch for the little washers that will come off too. Pull the bulb out, and remove the two spade connections. The gauge now comes out from the front. No force should be needed - if you have resistance, something is wrong.
I retested the gauge on the bench with the same test results. My gauge stayed on E.
Separating the gauge in half is an exercise in patience. Find the smallest flathead screwdriver you have and insert between the case and the chrome surround. Twist until the chrome starts to move outward. This first "twist" will be the hardest. Expect several slipped attempts. Once you have some separation, move the screwdriver along about 1mm, and repeat. Keep repeating until you've gone all the way around. Then take a larger flathead and repeat. Then, find a mini (but flat) pliers to fully straighten the collar. This gradual approach keeps the chrome from deforming as much as possible. If you try to straighten the collar edge fully right away, you could crease the visible portion of the chrome.
I pulled my gauge apart because I assumed that it was damaged beyond repair. My goal was to take some inside dimensions, see if I could find a white-needled gauge that registered similar ohms, and place those innards in my housing. It seemed like a reasonable alternative to spending $130 on a 50-year-old gauge. So, all that is to say, I wasn't gentle separating the two pieces because I thought it was dead anyway.
What I noticed is that inside there are two rectangular pieces with crescent shaped notches in them, attached to two wire-wound posts. Those posts can be re-positioned by loosening two nuts on the back. There was also a circle at the base of the needle with a similar diameter of the crescents, but my crescents were no where near lined up with the base of the needle. It seemed that logically, the two crescents should frame the circle. So I moved the crescents until they did just that.
Then, I retested before re-assembling the gauge. It worked! Now, I don't know if my positioning of the crescents was correct. It just 'looked' right to me, and it did seem to make my gauge work. I have no idea if it's actually incorrect and maybe after an hour of use like this it's going to burn out. So copy at your own risk.
I re-assembled the two halves of the gauge, and before crimping the chrome back, I tested it again. It did not work. The gauge doesn't have much power. Even a slight touch against another surface seems to make it stick. I tried a few more times to re-assemble the two halves, only to find each time it did not work. On one occasion, I even smashed the needle. Its almost paper-like. It's bends (in directions you don't want it to bend) very easily.
What I did next (and should have done first) was take the gauge face fully apart and assemble each piece individually. This allows you to not damage the needle, which avoids it sticking on any other piece. When a final re-assembly test passed, I began the process of re-crimping the trim ring. I used a nut driver (as opposed to a screwdriver as I wanted more surface area) and rolled the edge around. This was a harder process than I thought.
In the end, it all went back together and the gauge reads about where I'd expect it to. I''ll need to fill up to be 100% sure.
Last edited by rickjames8; 11-05-2019 at 10:30 PM.
A good write-up of your experience!
Those pieces with the crescent shapes are the two magnetized coil assemblies (at right angles to each other, this is the basis of both the 122 and 1800 instruments. Note in the 122 instrument, the coils are part of the rotating armature, in the 1800 instrument, they are fixed to the instrument case. See: https://www.sw-em.com/Fuel%20Gauge%2...spulinstrument), which act upon the Needle (more specifically, what the Needle is mounted to)...it sounds like mechanical securing of those coils became dislodged, so the magnetism was out of place and because of this, ineffective to work on the Needle as intended. By moving the poles to where it seemed they should be, you have brought back the function, but I expect the calibration will have less than its original accuracy and should not to be highly trusted until you get a few tank-fulls of experience with it! Congrats on the repair!
I will certainly link to your presentation on sw-em and request permission to repost pictures (as a matter of fact, I see they are hosted by another party, so I request you e-mail them to me so that I can host them on my server, with no chance of loosing them in the future. Attribution of course as always.)
Last edited by Ron Kwas; 11-06-2019 at 08:12 AM.
Hey Ron - thanks! I consider that a complement from the king of write ups! I'll gladly send over my photos, or you can right-click-download them off imgbb. They're all taken by me. You're also free to modify any of the text if you feel something could be described better.
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