78 Burb - Original Tach Issues

[bd]

Hey bd,

Thanks for sticking with me, I really appreciate it. The starter comes in today and will be installed Friday morning. This 4/10 work schedule has me waking up when it's dark, and leaving work when it's dark.

Been there and done that - don't miss it one bit.  I've had a few backcountry epics hat started and ended that way.  I learned to carry a flashlight!   

im still with you also keep at it. but bd's knowledge is much more vast and complex than mine. so im still lurking around learning off the two of you

Boy!  Have I got you buffaloed.   

[BlackTomC]

I appreciate your help as well Irish, I've already learned quite a bit from both of you here on the forums

[bd]

We all learn from each other.  It's part of the fun of our little community.

[BlackTomC]

So before Friday rolls around, can anyone speculate as to why the tachometer drops substantially more after replacing the horn relay?

* Tach needle drops anytime the horn is pressed *

[bd]

Four scenarios come to mind:  (1) a floating common ground, (2) excessive circuit resistance resulting in a voltage loss/shift under load, (3) the voltage spike generated from the current surge when energizing the horns is interfering with/mistriggering the tach, or (4) a faulty component on the tach head circuit board.  Each scenario has its own correction.

[BlackTomC]

Made a video this time to show the problem.

https://www.youtube.com/watch?v=p6rwVK3qN4E

Tach used todo this everytime the horn was pressed, now it does this every time the fan speed selector is changed on the HVAC controls.

Any ideas?

[bd]

Needle jump was most pronounced when switching high blower.  Does the symptom persist if you unplug the horn(s) and the blower motor?

[BlackTomC]

This is from memory, I'm almost positive it did. Tonight when I get home I will disconnect the horn relay and unplug the blower motor, test it out again.

A few months ago, I hosed the engine bay down real good to clean everything, right after I did that, I drove the burb, and interestingly enough the tach worked just fine, for about an hour till it started reading slowly, doing this whole needle jump thing.

If it does/doesn't, what are you thinking is the issue?

[bd]

The tach has electrical noise filters built into its circuitry.  The filters depend on electrolytic capacitors.  Electrolytic capacitors contain liquid electrolyte.  If when the electrolyte dries the capacitors become ineffective, defeating the associated filters.  Without functional noise filters, spurious voltage surges can mis-trigger the tach, causing its needle to jump.  The tach's circuit board capacitors are typically surface mount devices that are not practical (easy) to replace.  The question about whether the symptom occurs even with the horn(s) and blower motor unplugged is related to whether the voltage spikes affecting the tach are generated by the loads (horn and blower) or the relays controlling the loads.

Based on your response I recommend installing a 680 Ω, 1/2 watt resistor across the control coils of the horn relay and the high blower relay.  In addition, install a 35 - 50 µF, 50+ volt electrolytic capacitor across the tach power leads (cathode to black wire, anode to pink wire).  Adding these components (the two resistors and capacitor) is an attempt to suppress the voltage spikes generated by the relay coils and any other spurious electrical noise that may spike the power bus.

[BlackTomC]

Would these noise filters cause the tach to respond slowly, and not read correctly ALL the time? (Unless I hose down the engine bay?)

I read somewhere that these tach's have a resistor that goes bad and that's what causes the issue.

[bd]

Four scenarios come to mind:  (1) a floating common ground, (2) excessive circuit resistance resulting in a voltage loss/shift under load, (3) the voltage spike generated from the current surge when energizing the horns is interfering with/mistriggering the tach, or (4) a faulty component on the tach head circuit board.  Each scenario has its own correction.

Did you ever follow up on any of these suggestions from one year ago?

[BlackTomC]

I did, my apologies for not addressing this sooner

1) grounded straight to battery
2) ran new wiring straight from the distributor to the tach
3) Horn issue was resolved, now the tach drops when the AC switch is switched to the fast blower speed
4) Not sure how to test this

[BlackTomC]

Made this vid to show the issue

https://www.youtube.com/watch?v=p6rwVK3qN4E

[bd]

Okay.  Since you established a dedicated ground from the tach head directly to battery, you effectively eliminated a floating ground.  Next, monitor the voltage on the B+ power lead into the tach head.  The best tool for this is an oscilloscope, because of the brief duration of the effect.   Alternatively, a good voltmeter may suffice, although a sensitive analog meter (i.e., VTVM) might outperform a digital meter under the circumstances.  Watch for a momentary drop in B+ voltage that's coincident with de-energizing the high blower.  A momentary dip in voltage suggests a reverse voltage spike generated off of the high blower relay coil.  Under this scenario, adding a 680 Ω, 1/2-watt resistor across the relay coil leads may suppress the voltage spikes sufficiently to relieve symptoms.  Otherwise, a high-value electrolytic capacitor inserted between the tach head B+ and ground leads may be indicated.

Before attempting to create the supplemental noise filter, if you wish, visually inspect the tach head circuit board for obviously damaged components.  If you're unsure of what to look for, post or link a pic of the tach's PC board that clearly shows all of the discrete components in as high a resolution as you can provide.

[BlackTomC]

There are 2 issues, with the voltage spike being the lesser issue.

The issue I am trying to fix is the incorrect tach reading.

Would the voltage spike also cause the low RPM reading the tach is producing? Also, the board looks good.