Tachometers and diodes

Printed From: the12volt.com
Forum Name: General Discussion
Forum Discription: General Mobile Electronics Questions and Answers
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=71780
Printed Date: May 09, 2024 at 4:59 AM

Topic: Tachometers and diodes

Posted By: volksnut
Subject: Tachometers and diodes
Date Posted: January 29, 2006 at 9:11 PM

I just installed a new VDO tachometer which I must say does not read right....I've tested it against a engine analyzer, when my engine is idling at 800 the tach reads 1200, yes the ground is good, and set at the right cylinders...When driving it flucuates up and down about 300 rpm....I've been told I need to install a diode inline, so I bought a diode from Radio Shack (IN4004) and installed it, (the package says forward voltage drop is 1.6V and forward current is 1A) it now reads right at idle and up to about 2000 rpm, above that it doesn't follow the engine revs (doesn't want to go above 3000rpm)....did I install to big of a diode...thanks for the help

Replies:

Posted By: fingaz22
Date Posted: January 29, 2006 at 9:16 PM

to small i would say. what kind of car?

-------------
JUST ONE MORE AMP!!!
hu,alpine cva 1005/dva 5205
sound processor,symmetry(first one).
sub amp,power 1000 the terminator.(1992).
subs,spl comp dual 1 ohms.
punch 150hd on a 10" ev.
alotofhighs

Posted By: volksnut
Date Posted: January 29, 2006 at 9:28 PM

Volkswagen Bus

Posted By: fingaz22
Date Posted: January 29, 2006 at 9:32 PM

sorry i need the year and engine also and i will be able to look it up and tell you what the voltage range is for your tach.

-------------
JUST ONE MORE AMP!!!
hu,alpine cva 1005/dva 5205
sound processor,symmetry(first one).
sub amp,power 1000 the terminator.(1992).
subs,spl comp dual 1 ohms.
punch 150hd on a 10" ev.
alotofhighs

Posted By: volksnut
Date Posted: January 29, 2006 at 10:08 PM

well it's a 4 cylinder 1600cc based motor, vehicle is a 68 , the tach is an aftermarket VDO unit

Posted By: fingaz22
Date Posted: January 29, 2006 at 10:27 PM

sorry i figured that is was older. my data base don't go back that far.

but it sounds like your restricting the voltage to the tach. use a dmm and confirm voltage before and after the diode. it should flucsuate with the rpm of the engine.

see what the voltage is before the diode when the engine is above the point the tach stops. and compare with the voltage reading after the diode to the tach. if different(lower voltage after diode) then you will need to upgrade diode. to allow more flow of voltage. if the same and no change in voltage you will need to find out why you are losing voltage.

-------------
JUST ONE MORE AMP!!!
hu,alpine cva 1005/dva 5205
sound processor,symmetry(first one).
sub amp,power 1000 the terminator.(1992).
subs,spl comp dual 1 ohms.
punch 150hd on a 10" ev.
alotofhighs