Instrument Cluster Temperature Gauge Goes Directly to Red

[ChaseY]

Hello all, this issue has burdened me for the past few weeks after I complete my complete top end rebuild on my 1986 BMW 528e after blowing a head gasket and sitting since Fall of 2020. Chassis with original engine and transmission has approximately 295k on the clock.

With the motor off, power on, the temperature gauge on the instrument cluster maxes out past the red indication. last time the car was parked, the temperature gauge was working as it alerted me the motor was overheating. After about 2 weeks or pondering, researching and tinkering, I have resulted to ordering a Black Star Manufacture coolant temperature gauge (https://blackstarmanufacture.com/produk ... apter-set/) . This is a temporary solution as I do not want to rely on an aftermarket solution for full time monitoring of my cooling temperatures but should allow me to bleed the cooling system properly and get the car back pm the road.

My thought process so far:
Bad sensor?
The temperature gauge shoots beyond red with the sensor both plugged in and unplugged leading me to believe it is not the sensor. I also get an ohms reading through the sensor. A sensor is $80 via FCP euro. (12631279719)

Bad ground?
Possible culprit. I removed the instrument cluster this past weekend and adjusted the ground in as many ways as possible looking for changes on the instrument cluster. I attempted adding my own good ground to both the frame and directly back to the sensor in case it was a closed loop. I saw the temp needle change some when moving the ground wire around but not reliably enough to say it was the wire and not me shifting the cluster around to trying and get the wires to sit correctly.

Bad sensor signal?
Gauge does straight to red with sensor connected or disconnected. I don’t believe the sensor to be bad.

Bad IS board/Cluster?
Main suspect but unconfirmed. This past weekend I pulled apart the entire instrument cluster as much as possible searching for anything suspicious. Electrical is not my forte so I could've missed an obvious issue. The cluster has the original NiCa batteries and look to be in decent condition though they could be toast.
The cluster has other issues but I chalk those up to my younger self tinkering with the cluster a few years ago. I really though I was onto something painting the needles red so here we are. In the process of doing that, I know I messed up the tachometer servo moving the needle by hand so now it reads a few hundred RPM low and the scaling is way off. Essentially useless. The MPG indicator has also never been the same. The odometer does not read due to broken odometer gear. Speedometer does not read currently due to broken speed sensor in differential (on the list). Planning on sending the cluster along with AFM in Bavarian restoration when the time is right. Like I said, electrical is not my forte so I rather send it to the experts.

Summary of work performed since 1/1/24.
Complete head restoration (decked, cleaned, pressure tested, valves ground, new cams, new rockers) by Aluminum Head Rebuilders in Portland, OR.
New Cooling system - Silicone hoses, thermostat, Ebay Chinese aluminum radiator
Silicone vacuum line kit
Ignition system – Spark plugs, wires, rotor, cap
New starter
Any and all gaskets
All filters and fluids of course
Motor mounts
Electric fan conversion with Davies Craig fan controller
Went through wiring checking for bad insulation. Added shrink wrap/electrical tape where possible.
Looking for all and any suggestions on finalizing this chapter of my project. Pending other issues that arise, this is hopefully the final step in being able to get this car back on the road. I know these engines can be finicky when bleeding so I won’t bleed it until I can see a reliable coolant temp.

Got a bunch of photos and videos but I am not familiar with to add those to my post here. I’ll check the FAQs when I have more time when I get home.

[Mike W.]

First off that's not the sending unit for the gauge, it's a single tab and inexpensive. Looks like this.



Now if you're checking with the wrong thing disconnected, check the single prong one and see what happens.

[Ju@n]

Hi,
If you already tried unplugging the sensor and the needle is still reading high, it's either a short to ground on that wire, or the cluster is bad (could be something else, but I would suspect those).
I know I wrote somewhere in the forum a procedure to bypass the "new style" si board, if you have the old one, the instructions are available easier.
The late style clusters get that issue if you plug them in with the key on ACC and the battery connected (don't ask me how I know).

[ChaseY]

Mike, Juan,

Greatly appreciate you input here. I’ve decided to wait for the aftermarket coolant temp sensor to arrive instead of going through the harness and or the cluster to find a fault. Definitely a project I’ll be taking on this coming winter when she gets put back in the garage but because I am so close to getting my old girl back on the road I’m electing to wait. Hopefully my 4 month project is coming to a close soon.

I researched conflicting information about which temp sensor is responsible for the temp gauge. I did replace 12621710512 hoping for a bad sensor to be the issue.

Again, thank you for your help with this issue.

[Mike W.]

I researched conflicting information about which temp sensor is responsible for the temp gauge. I did replace 12621710512 hoping for a bad sensor to be the issue.

Again, thank you for your help with this issue.

While not the "correct" replacement, it should be functionally the same.

[brownsmith]

OK