RPM indicator needle drop/jump….. (1 Viewer)

[duece71]

All,
1990 HDJ81, 1-HDT, dual battery (24v starter) OEM alternator.
I noticed today that the rpm needle jumps considerably at lower rpm. It will drop to zero if I turn on the headlights, only to indicate again at higher rpm (highway speeds). If if turn on either left or right turn indicator, the rpm needle jumps rhythmically with the on/off blinking of the turn signal. If I switch on the fog lamps, the rpm needle drops to zero and stays at zero.
I checked the alternator belts, no squealing or loss of tension. Could there be a loose connection somewhere? I checked both battery connections and are both are tight. This involves only the needle, engine is running normally and smoothly, no surging or hesitation.

 

[duece71]

 

[duece71]

 

[TheBussman]

could defiantly be a connection issue but start with the basics regardless: alternator test, disconnect batteries and test each separately. Make sure there isn't a big voltage dip just for turning on the lights.

 

[duece71]

Ok, sounds good. No fuses blown.

 

[quasi]

your tach gets its signal from the injection pump. mine reads low to no idle at times but works when reved past 2krpm. maybe the cluster circuit sheet or a bad connection, or a tach going bad. we are talking about 34 year old parts.

 

[duece71]

your tach gets its signal from the injection pump. mine reads low to no idle at times but works when reved past 2krpm. maybe the cluster circuit sheet or a bad connection, or a tach going bad. we are talking about 34 year old parts.

Yes, this is what is happening to mine as well. Once past 2k, it seems to work fine. Curious when I turn on lights and turn signal though.

 

[roaminglost]

Mine does this, too. I think it's a grounding issue. but I have no idea where

 

[duece71]

Welp, while Fluid Film spraying the under body, I found what I think is a temperature sensor (small metal piece surrounded by a cylinder open at both ends with some wires) dangling precariously by the wires. I used a “mystery bolt” for the bench and resecured the sensor back in front of the condenser just behind the front bumper and VOILA! Rpm needle jump no more! The temperature sensor must have needed to be grounded to metal, I checked a bunch of other grounding points and all were secure.