Rpm counter and capacitor?
Posted: Tue Apr 22, 2025 7:38 pm
Email to Fres Winterburn who developed the CDI and rpm adaptors I our car.
Just drove for 6 hours from Berlin to Poland. CDI and the 356 is running well. However, when we now idle at light, no press on the accelerator, the rpm counter jumps to the right 5/6k. When driving it's goes back to the left and shows the correct rpm? Happen briefly a few days ago in Belgium also when the car was hot (after 5-6 hours). I did a quick check and no loose contacts (will check again when cooler). Any ideas?
From Fred
Neil, OK, Very likely then that the internal capacitor in your tach is failing. In STD mode, the ignition coil acts like a large filter choke so cars with a completely shot filter cap will behave normally. Similarly, turning on other loads such as headlights might also make the tach behave as those loads will often shunt some noise from the dynamo or other source of electrical noise. In CD mode, the coil cannot filter anything. Often, the newer style LED bulbs (120V here in north america) have a 10µF electrolytic cap inside. I rip them apart just to save the capacitor once they start to flicker or fail. I imagine that your 220V LED bulbs in the UK also have such a cap inside. Solder some short leads to one and then connect the + lead of the cap to the tach's power supply wire (+ connection) and the negative lead to earth. I'll bet that will solve the problem. If not, there is something more serious failing inside the tachometer. Fred
Not to prester Fred too much. We got back in the car today and didn't have the problem but only drove for 2.5 hours.
I found some 10uf Caps online but rated at 500v? I found some 6.3v but 47uf. I've no idea which is appropriate and why a cap filters the signal? I do understand that caps can wear out. Do they stop working after too much use?
Fred's adapter unit was quite hot at the end of drive so will try to isolate that.
Sent from my 22011119UY using Tapatalk
Just drove for 6 hours from Berlin to Poland. CDI and the 356 is running well. However, when we now idle at light, no press on the accelerator, the rpm counter jumps to the right 5/6k. When driving it's goes back to the left and shows the correct rpm? Happen briefly a few days ago in Belgium also when the car was hot (after 5-6 hours). I did a quick check and no loose contacts (will check again when cooler). Any ideas?
From Fred
Neil, OK, Very likely then that the internal capacitor in your tach is failing. In STD mode, the ignition coil acts like a large filter choke so cars with a completely shot filter cap will behave normally. Similarly, turning on other loads such as headlights might also make the tach behave as those loads will often shunt some noise from the dynamo or other source of electrical noise. In CD mode, the coil cannot filter anything. Often, the newer style LED bulbs (120V here in north america) have a 10µF electrolytic cap inside. I rip them apart just to save the capacitor once they start to flicker or fail. I imagine that your 220V LED bulbs in the UK also have such a cap inside. Solder some short leads to one and then connect the + lead of the cap to the tach's power supply wire (+ connection) and the negative lead to earth. I'll bet that will solve the problem. If not, there is something more serious failing inside the tachometer. Fred
Not to prester Fred too much. We got back in the car today and didn't have the problem but only drove for 2.5 hours.
I found some 10uf Caps online but rated at 500v? I found some 6.3v but 47uf. I've no idea which is appropriate and why a cap filters the signal? I do understand that caps can wear out. Do they stop working after too much use?
Fred's adapter unit was quite hot at the end of drive so will try to isolate that.
Sent from my 22011119UY using Tapatalk