I am not sure if this has been covered elsewhere, but when I used my horn today, I noticed that my tachometer pegged. Has anyone else had this issue? Do they share a common circuit? I am lost here.

The tach is frightened by the loud noise. Stuff earplugs into the instrument cluster.

Seriously, it sounds like a ground problem. The tach's normal ground may be faulty so the tach grounds through the horn until you use the horn.

Tell us what model and year Honda you have so we can check the right circuit diagram for the exact problem.

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

1982 300td - apparently I assumed you all would somehow know my make and model via telepathy or something. I'm an idiot.

The TDC sensor is very sensitive, so it could be that the electromagnetic "noise" from the horns is causing interference.

Anyway,
I'll have to check out the gound situation tomorrow and see if that is the problem.

__________________
1983 300td - Velvet 265,000 mi
1975 Toyota Landcruiser FJ40 160,000 mi
2004 Toyota 4runner 103,000 mi

If I have seen farther, it is because I have stood on the shoulders of giants.
- sir isaac newton

The left side horn does share a ground with the TDC/Tach.

fantastic. That hopefully will solve the problem once i sort out the ground issues. Much thanks for the help!

__________________
1983 300td - Velvet 265,000 mi
1975 Toyota Landcruiser FJ40 160,000 mi
2004 Toyota 4runner 103,000 mi

If I have seen farther, it is because I have stood on the shoulders of giants.
- sir isaac newton

1981 300SD - my tach does not work but it pegs when I turn the fan on high. It also pulses to the blinker. I finally gave up.

Most likely a bad amp. I bought one from techguy512; it works great.

OM617A Tach Amp

Mine does the exact same thing. Pegs when I use the horn. Don't know why though....i'll look into the ground issue.

BTW I Also Have the new digital tach amp, not the old garbage can one (Mine caught fire )

The tachometer reacts to electrical pulses from the tach amp, which amplifies what it gets from the tach sensor. If you have a lousy ground somewhere in the circuit, the noise from the brushes in the fan motor or the contacts inside the horn will get into the tach. This noise will simulate the pulse train from the tach amp and you have deflection.

At least you know your tach is OK. Fix the ground issues and maybe replace or resolder the bad connections inside the tach amp and you'll be back in business.

__________________
1984 300TD

My SD,with all paper work when new,had 3 amps. installed.I guess the owners gave up.My solution was to unscrew the amp.,take small screwdriver and slide the cover off amp.Then insert 2 water hose washers in cap,reassemble and screw back on.It like helps pins go deeper in socket.No problems 2 years.
I've heard some take a metallic paint and paints the contacts,or drive up socket pins.

The tachs in these cars are actually sound pressure level meters. The faster the engine spins the more noise it makes. Move the microphone away from the horn.



Sounds like a classic symptom of a grounding problem to me.

__________________
Dale

http://i52.photobucket.com/albums/g1...MG_2277sig.jpg

1990 300D 2.5 Turbo -155k
2000 E430 - 103k
2014 Jeep Grand Cherokee Ecodiesel 4x4 - 11k
2014 VW Passat TDI SE - 7k Bro's Diesel
2006 E320 CDI - 128k Pop's Benz

Pre-glow - A moment of silence in honor of Rudolph Diesel

I also purchased one of the tach amps. Did not make the tach work although it moved through its self test process fine. Not sure where to look for ground wires but if it involves removing the dash, not sure I have the guts.

I added a tach to my 240d as per funola's instructions on this site through the EGR computer. I have the same problem now. Everytime i honk the horn it pegs the tach. Also its a little erradic. I'm sure its a ground problem but i dont know where to start as its not a factory setup.

__________________