Speedometer drop with blinkers

[steffan]

i was a little miffed when this started happening. at first i'd catch it out of the corner of my eye - almost like it wasn't happening: the speedometer needle would drop randomly. then, every now and then, i would signal a turn and the needle would drop... but not all the time. lately, it would happen more often, almost consistently but not entirely. i was thinking it was a bad blinker switch or maybe a crack in the speedometer circuit board or possibly a bad ground.
a little searching found several others with similar situations. turns out its primary cause is a bad flasher. i wanted to go OEM but even at the cheapest, it was over $40, i settled for parts store model for $12.
for the uninitiated, the flasher is under the speedometer, mounted on the dash cross bar, between the left dash vent and the steering column. i was hoping i could swap it without taking the cover off but no such luck. even with taking the cover, the entire job took 15 minutes.
just wanted to post here as someone will inevitably see the same issue

 

[pappy]

Did anybody else here read this and have a WTF reaction? So, I think we are about to have an educational moment. Why, because some of us have speedometers run off a cable from the transfer case. The flasher has no effect.

Let us to know what you are working on. It might help us understand why something like a flasher could impact your speedometer..

 

[steffan]

Agreed, if the speedometer was a cable driven unit there would be some serious voodoo going on. My ‘92 pickup has the fancy dash (with a tach), it has an electronic speedo. I’m still not sure what the relationship is other than they share a common ground somewhere along the line. I read every thread I could find and even the folks who spent serious time with a multimeter couldn’t exactly determine the exact cause.

 

[Hilife]

Super old thread but I thought maybe I would get lucky here.

I have a 94 pick up. I am assuming I have the electronic speedometer with the tachometer. Was driving it today and saw something similar to what Steffan saw.

Left turn signal would drop the Speedo to 0 on the first flash and then recover. Would do it all the time between 20 and 75 mph. Right turn signal it would do it occasionally.

Coincidentally if I would go 80 or so, I couldn’t get it to happen. I tested this out a bunch in the commute home. Must have driven the people behind me crazy with turning my blinker off and on in rush hour traffic.

So rather then driving over 80 mph all the time or never using my turn signals ( the later being an entirely reasonable option in the greater Boston area) I figured I would try and figure out what the issue was.

So my issue occurs on the first flash of the turn signal. Subsequent flashes are not issues. The speedometer stays where it should.

So my questions are
1) did the flasher fix your issue? Is there a way to test the flasher?
2) wouldn’t I expect to see the speedometer go up and down with the flashing of the turn signal. ( assuming the turn signal turning on and off is drawing the voltage down the same amount on the first flash as it does the second and the third).
3) any other thoughts if you don’t think the flasher is the issue?

I know this thread is a few years old but figured I would give it a shot.

Thanks for any help you can provide.