‘Literally the Worst Fault to Chase in a Vehicle:' He Turns on the Turn Signal. Then the RPMs Start Yo-Yoing
A
viral video
from automotive-inclined creator Andywhyisthat (@andywhyisthat) shows the mystifying footage of a Honda idling normally at just over 1,000 rpm. However, when the left blinker is engaged, the tachometer needle starts bobbing in rhythm with the blinker, between 0 and 2,000 rpm, with no change in engine speed.
What's Causing the RPM Yo-Yo Effect?
The most common explanation? A faulty or corroded ground.
Many vehicles rely on
shared grounding
points for multiple systems. If a ground connection is
loose or rusty
, electrical signals can 'bleed' between circuits. That's how a blinking turn signal might send a pulse to your gauge cluster, causing your tach needle to bounce like it's trying to keep time with the blinker.
There are plenty of forum discussions about this problem. 'When the turn signal is blinking, the tach needle jumps like 50-100 rpm in time and rhythm of blinker,' one user wrote on
CamaroZ28.com
. 'The engine idle is not fluctuating.'
According to
Speedway
, You Should Check:
Negative battery cable (tight and corrosion-free)
Engine ground strap
Ground connections under the dash
Even a small voltage drop can create weird symptoms, especially in older vehicles.
A Weak Battery or Alternator Could Make It Worse
If your battery or
alternator is struggling
, even the small electrical load from a turn signal flasher can cause voltage fluctuations. That ripple can interfere with sensitive systems, such as gauges.
One
Toyota 4Runner forum
user noted, 'My thinking is the battery is not fully charged and the extra load causes the alternator to put out more juice, which increases the rpm.'
That could mean the tachometer is reflecting a real (but small) change in engine rpm, or just reacting to low voltage and sending false signals to the gauge.
On
, multiple users suggested testing the charging system first. A failing voltage regulator, loose battery terminals, or corroded posts can all contribute to the issue.
According to
Firestone
, You Should Check:
Battery health and charge level
Alternator output (especially at idle)
Voltage stability while using turn signals, lights, or A/C
It Could Be a Real RPM Change (But Probably Not)
There's also a chance the bounce on the gauge reflects a real fluctuation in engine speed, caused by a dirty sensor, failing idle
air control valve
, or glitchy
throttle body
.
That's especially plausible in
drive-by-wire vehicles
, where the electronic control unit handles all throttle inputs electronically.
A thread on
JustAnswer
notes that dirty or aging sensors can cause momentary RPM dips or spikes, especially at idle. But more often, these cases turn out to be electrical quirks, not engine issues.
How to Tell the Difference, According to JustAnswer:
Plug in an OBD-II scan tool and monitor actual rpm readings
If the scanner shows steady rpm while the dash needle bounces, it's likely a gauge issue, not a real engine change.
Commenters on Facebook were quick to offer their amateur diagnoses, along with some humor.
'Likely a corroded ground. Water takes the path of least resistance. When a ground wire is corroded, it has plenty of resistance, pushing the electricity to take different paths to ground. Literally the worst fault to chase in a car,' one sympathetic gearhead wrote.
Another shared their similar experience: 'had a loose fuse cause the speedometer to do that. Only did it while in gear.'
And another couldn't hold back with a dad joke: 'Low blinker fluid.... It happens.'
Motor1
reached out to Andy via email.
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