Hello,
There are 4 belts. The resistance between the steel fibers of 3 belts are 8.4 ohms. But 4th belt's one fiber is arround 0.7 ohm. So the device breaks the safety. Do we have to replace belts? Thank you very much for your support.

Check the cwt ends of the belts. Recut them if necessary (grinder with cutoffs wheel). The ends should look like a new pack of number 2 pencils not sharpened yet. If the ends are ragged and the rubber material is not tight it can let the cords touch. Check this first. I've seen cord pairs shorted together that were just bad ends that needed recut.

Also, its entirely possible the pulse unit is bad. I just had two blue ones we had to replace and both new ones are bad within a month. I think they had a bad batch because both were from the same batch/date code. What I've been seeing is one or maybe two belt slots start failing to accurately read the cord pair resistance.

You can easily identify if its a bad pulse unit by using your blue tool see what it's showing the resistance is for each cord pair. Then pull that belt off and check it with your meter. If your meter says the same thing and you've checked the ends lts the belt. If your meter says all the cord pairs are good and it's the same as the other belts, but when you put that belt back in the pulse unit and the pulse unit is reading the resistances wrong, It's a bad pulse unit. You can verify by swapping two belts from slot to slot, if the problem moves its the belt. If the problem is at the same slot you know for sure its the pulse unit.

The pulse units read/show live resistance measurements. So If you get on a cartop and ride it up to the unit and put your tool on it and it says 3 belts are all 8.4 ohms and one is .7 ohms, without moving the car you can expect to take that belt off and check it with your meter it will tell you what's really going on. The units don't show a low or high measurement, it shows a live measurement. Now if the belt is flashing a 5 or 6 and it shows a good resistance measurement when you read it with the tool, there's likely a bad spot somewhere because the unit will latch a fault even if the resistance goes back to within the good range. When a belt is removed it will show roughly 181 ohms on all cord pairs for that belt. Also when reinstalling the belt, if any pairs show 181 ohms, that pairs not making contact with the pins, need to tighten it down more.

Always leave the power to the pulse unit on when taking belts on/off, it will prevent static from the belt damaging the pulse unit.

Also it may be possible to use an unused slot. I'm not sure on the black units but on the blue units you can use slot 3 on a 4 belt car by simply reprogramming the unit to use slot 3 and whatever other slot will be empty.

Dear EElevator, you explained clearly. Thank you very much for your helps. I wll try it.