81AB-C2A

[labvja]

Hi, I've been tasked with repairing an old analogue/mechanical instrument cluster from a Ford Transit van. The fuel and temp gauge instrument was not working. I determined that the voltage regulator wasn't working. It has labels of '950A' and '81AB-C2A'.

Trying to research '81AB-C2A' I get very few results, a post in a Ford Transit forum where someone apparently successfully replaced one with a 9v 'TS 7809 CZ', and a listing of some old stock in Poland that seems to have the '81AB-C2A' part number and also has the part number 'L2610CV'. I found a datasheet for L2610CV which is a 10v output model, but I'm not 100% certain the association of the two part numbers is correct.

I ended up ordering a small set of 'L7810CV' instead which seem to have very similar specs to 'L2610CV', but supporting up to 1.5A instead of 0.5A.

Having fitted it and the owner having tested the unit back in the vehicle, the instrument is working, except the fuel gauge is pegged at maximum despite the owner having driven about ~70 miles since last refuelling. I'm not certain yet how the temp side is functioning. It's possible that the issue could be a faulty fuel level sensor, but I'm worried that maybe the replacement voltage regulator wasn't the correct one after all, maybe I needed a 'L2605CV' (5v output) instead? (I've seen some discussion of old mechanical units from other manufacturers having a 5v design).

Could someone please confirm what would be a suitable replacement for an '81AB-C2A'.

[labvja]

Sorry, just to clarify, the listing of old stock shows a photo of the part which has '81AB-C2A' on it, the listing also says 'L2610CV' in the title and states that it's an ST product, there's no L2610CV or ST marking on the part in that listing.

[Peter BENSCH]

Welcome @labvja, to the community!

The 81AB-C2A appears to be a Ford part number, which means it was a custom device and no information was or is publicly known. I assume it was the L2685, which together with the L2605 and L2610 (all long obsolete) first appeared in the SGS Linear Integrated Circuits of 1982. The three variants had identical designs and differed only in the typical output voltage: 5V, 8.5V and 10V.

You can use a pin-compatible 7809 instead resulting in almost negligible +500mV, but then you no longer have the reverse voltage protection that was included in the L2685. As a shirt-sleeved DIY solution, the reverse voltage protection could be replaced by a 500mA or 1A diode, which you connect between the input pin 1 (cathode of the diode) and the battery voltage (anode), thus preventing a higher output voltage from passing through to the input (the battery). And while you're soldering, place an electrolytic capacitor (~10...22µF, 35...50V) directly between the INPUT and GROUND pins of the L7809 so that the regulator can regulate properly - please observe the polarity of the cap.

Hope that helps?

Good luck!
/Peter

[labvja]

Thank you, that's really helpful.

I had noticed the lack of mention of reverse voltage protection in comparing data sheets but being pressed for time and lacking some knowledge in this area I'd just cross my fingers hoping that the 'SOA protection' was something broader that encapsulated it, but apparently not, so good to know that I need to add a diode if I stick to L7810CV/L7809CV.

While I'm here, I've attached a picture of the gauge instrument. As you can see, it's a bi-metallic strip based design. Do you happen to have any idea what the red substance is that has been used to attach the ends of the nichrome heating elements to the steel studs? The one bottom left broke off, I tried supergluing it but had no continuity, I then tried soldering it and encountered the unexpected difficulty of soldering nichrome with ordinary solder, and ended up having it stuck to the surface of the solder with superglue on top to hold it in place. I'm worried that this may not last. I may try silver soldering or trapping the nichrome wire in copper that can be easily soldered, but I'm really curious what this red stuff is and maybe it would be worth sourcing some instead.

[Peter BENSCH]

Unfortunately, I can't help with the red substance.

Also, soldering such materials is not easy, but maybe you can find a way to contact it with spot welding?