[Howie Sparks]

I have an '88 graduate s3 with a tach that most often reads half speed, I also have a volt meter that reads 12.5 V while at the battery I have 14 V. I've looked & cleaned the grounds at driver's kick panel. I wonder if anyone has had this issue & what the fix would be.

 

[pantera928]

Have you tried cleaning the connectors on the instrument cluster.
FYI, the volt meter usually never is that accurate in my experience. As long as it is charging and not draining, I don't worry about it.

 

[Andrew]

The electric tach gets its message from the coil. Check all connections, as he says above, and especially at the coil. And grounds. Are any other elec gauges screwy?
Andrew

 

[spiderserie4]

as above check the coil connection first

99% of the time a slow reading tach will be an internal fault
see here: (oops! link now corrected)
How to fix your slow reading speedometer or tachometer...

not sure if yours will be a Jaeger or Veglia monopod, but these 3 screws need to be making good contact
(this is a Jaeger unit)

 

[Howie Sparks]

The electric tach gets its message from the coil. Check all connections, as he says above, and especially at the coil. And grounds. Are any other elec gauges screwy?
Andrew

Yeah, the voltmeter reads 2 volts too low

 

[spiderserie4]

Yeah, the voltmeter reads 2 volts too low

EDIT
ah ok, I see you checked alternator output in your first post, and it is good.

 

[Gubi]

I also have a volt meter that reads 12.5 V while at the battery I have 14 V.

Very often that's a voltage drop across the ignition switch. You can verify this with a voltmeter by going around and seeing what the voltages are in various places.

Easiest way to measure ignition switch voltage drop is probably at the fusebox. Measure voltage between the input from the alternator (big red wires) and the input side of the switched fuses (6-8) with the car running (carefully!).

 

[Howie Sparks]

Very often that's a voltage drop across the ignition switch. You can verify this with a voltmeter by going around and seeing what the voltages are in various places.

Easiest way to measure ignition switch voltage drop is probably at the fusebox. Measure voltage between the input from the alternator (big red wires) and the input side of the switched fuses (6-8) with the car running (carefully!).

Very often that's a voltage drop across the ignition switch. You can verify this with a voltmeter by going around and seeing what the voltages are in various places.

Easiest way to measure ignition switch voltage drop is probably at the fusebox. Measure voltage between the input from the alternator (big red wires) and the input side of the switched fuses (6-8) with the car running (carefully!).

as above check the coil connection first

99% of the time a slow reading tach will be an internal fault
see here: (oops! link now corrected)
How to fix your slow reading speedometer or tachometer...

not sure if yours will be a Jaeger or Veglia monopod, but these 3 screws need to be making good contact
(this is a Jaeger unit)
View attachment 1675665

Well, I finally got to fixing my periodic slow reading tach. Took the cluster out & apart found the calibration screw, removed the glue from the screw. I began with moving the screw through about 300 degrees & then increased the position clockwise by roughly 15 degrees. Put the cluster back in & the reading was now 500 revs from 700 revs, the external tach was reading 1100 revs. From this I discovered that clockwise rotation decreases the speed ( not increases the speed ) My car is a Canadian sold 1988 Graduate in Kilometres, series 3. So, I took the cluster back out & turned the screw 15 degrees counter clockwise from where it was originally was, then the revs read 1200. So the end result was back in the original position & I've determined that rotating the adjustment screw corrected a bad connection. So, I would suggest that the first try should be to just rotate the screw & return it to original position.

 

[Gubi]

So the end result was back in the original position & I've determined that rotating the adjustment screw corrected a bad connection. So, I would suggest that the first try should be to just rotate the screw & return it to original position.

That works for a little while as it basically cleans out the potentiometer. But it'll likely start to read low again over time. The longer-term fix is to replace the pot.

 

[sbl]

Use the "tuner cleaner" of your choice on the pot, and turn the pot through full rotation lots of times. Crud builds uo within the pot and give intermittent false signals as the car jostles around. For this I would use Deoxit (Gold?) G5 which may clean, lube. and prevent future crud.As an aside, I have been working on an outboard pot to eliminate the need to open up the pod, and, the outboard pot is fully sealed so should be set once and forget but available for adjustments. Its a silly little project, I know.

 

[spiderserie4]

I have been working on an outboard pot to eliminate the need to open up the pod, and, the outboard pot is fully sealed so should be set once and forget but available for adjustments. Its a silly little project, I know.

Shakey has done that too.
Gauges still dead need advice