I'm working on a board that is meant to power both our Horn (HRN) and Telemetry system(TEL). This is because we want to reduce the load on the main pack as much as we can. However, this means we need to isolate the TEL output of the board, such that the TEL has a stable 12V rail, this is because the horn causes massive voltage spikes(Idk why and this is apart of the problem). The pervious designer of the board was slightly carless and didn't consider a lot of things when trying to isolate the TEL output, things such as routing over the isolated GND plane with the supplemental battery input(SUPP) and not reducing the size of the loop between the HRN and the SUPP. He did use a isolated BUCK BOOST and a pi filter(I don't know if he sent the values of the Cs and L correct) and a common mode choke but these didn't solve the problem. Now I am tasked with trying to solve this mess. I tried tests to see if routing over the ground plane was the issue by de-soldering components and using wires to avoid the route above the isolated GND plane, but it didn't help the problem. I tried using connecting the output of the buck boost to a bread board and using capacitors in parallel but that didn't help much either. The voltage spikes where still like +/- 7V and still very short in time (hence why I am calling them spikes). I researched and I found out about using flyback diodes but I don't really know how to implement them and my teams lead said that this solution isn't the best, I don't really trust him I just don't believe I voiced my idea the best because I don't even know how to implement it myself, all ik is I wanted to put a flyback diode from the 12V to GND when the horn was on. Other ideas I had where making the distance between the SUPP connector and the HRN connector smaller (reducing the loop size). I also said we should use bulk capacitors and smaller capacitors on the output near the horn connector and the Input and output of the Buck Boost, my lead liked this but idk because when we tried the bread board test added capacitors didn't improve much.
This is the horn we are using: https://www.jspecperformance.com/product/universal-hella-sharptone-horn-kit-pair-yellow
Overall, idk what exactly is causing the voltage spikes I am observing when the horn is on and I need help trying to find a solution to this problem. If you need more information please ask I tried to include everything I thought is important but Ik I missed something. Thank you so much in advance for any help.