Author Topic: Tach Conversion Kit Install  (Read 386 times)

Offline 78 K5 Cheyenne

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Tach Conversion Kit Install
« on: April 15, 2019, 11:02:50 AM »
Hello all,

I am very new, both to this community, and to these trucks. So, please bare with me as I try to explain my issue.

I picked up a tachometer conversion kit from... a vendor. If you aren't familiar with this, it replaces the big gas gauge to the right of the speed-o with a tachometer, and moves the fuel gauge to the small spot to the bottom left spot, where I believe there would have either been a warning light, or a clock (mine was equipped with a clock). It's all pretty straight forward, and the kit is nice because it comes with the two gauges, tach wiring harness, a new cluster 'chassis', new lens, new backing plate/spacer, circuit and clips. Everything one would need.

Since I'm such a newb, I haven't really ever done any sort of wiring under a dash or for instruments in a cluster like this, but this tachometer wiring seemed pretty easy, as it is really only three wires. Power (ignition-ed 12V), signal (harness came with the correct clip which plugs into the 'Tach' terminal on the HEI Distributor) and ground (self-explanatory).

I got everything taken apart, got all new bulbs, dielectric grease, put everything back together, wired everything up, tested that I had a good 12V ignition-ed source, good clean ground, ran the signal clip through the fire wall, to the HEI, ,clipped it in there, went to turn the ignition to 'Run' position, and... nothing. The needle on the tach didn't even move to zero when I turned on the ignition. Just stayed floating at like 1,100 rpm, where it had been the whole time.

I took my handy-dandy multi-meter, checked to make sure it is getting the required 12V at the clip on the back of the tach, and it is. I guess I'm not sure what else could be the issue, unless it's just a faulty tach that was DOA? Is it possible that I am not getting a signal from that tach terminal on the distributor? I would imagine it is getting signal, if the truck starts, but who knows. As I said, I'm no expert.

Any thoughts, suggestions, ideas would be most helpful.

-Mike
Mike - 1978 K-5

Offline rayjos_1

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Re: Tach Conversion Kit Install
« Reply #1 on: April 15, 2019, 11:15:33 AM »
I know that when I installed an Edelbrock E-street TBI, I had to install an MSD unit for the tach to hook up to the ECU on my 1976 Silverado. I hear the Tach connection on the HEI sucks and does not give a good signal.

Online JohnnyPopper

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Re: Tach Conversion Kit Install
« Reply #2 on: April 16, 2019, 01:37:03 PM »
Sounds like DOA, the needle should 0 out when 12v is applied. Does your gas gauge work? 

Offline 78 K5 Cheyenne

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Re: Tach Conversion Kit Install
« Reply #3 on: April 17, 2019, 01:02:54 PM »
Sounds like DOA, the needle should 0 out when 12v is applied. Does your gas gauge work?

I actually don't think it is... it is maxed to full. It was pretty full when I did the conversion, but it seems to be pegged past full atm... I drove it a few miles yesterday, and it didn't move at all.

Beginning to think it isn't working either.

I did get confirmation from the vendor that they are willing to send me another tach, so that's good. I'll see what happens when it arrives.
Mike - 1978 K-5

Offline bd

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Re: Tach Conversion Kit Install
« Reply #4 on: April 17, 2019, 02:05:47 PM »
Regarding the tach:  verify I+, ground and tach signal from the distributor using your meter or test light.  If all three circuits function properly, the tach head is faulty.

Regarding the fuel Gauge:  repair the open circuit between the gauge and the fuel tank sending unit.
Rich
It's difficult to know just how much you don't know, until you know it....
In other words... if people learn by making mistakes, by now I should know just about everything!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline 78 K5 Cheyenne

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Re: Tach Conversion Kit Install
« Reply #5 on: April 18, 2019, 05:31:36 PM »
Regarding the tach:  verify I+, ground and tach signal from the distributor using your meter or test light.  If all three circuits function properly, the tach head is faulty.

Regarding the fuel Gauge:  repair the open circuit between the gauge and the fuel tank sending unit.

Thanks for the tips, bd. As mentioned, I am a super noob, so I need some clarification. How does one "Verify I+"? I don't know what I+ refers to. I have a multi-meter, but no timing light. Can you give me a little bit more detail as to how I can verify the tach signal using my multi-meter?

Also, for the fuel gauge, I wouldn't know the first thing about repairing an open circuit between the new fuel gauge, and the sending unit. As far as I know, nothing other than the new gauge, and the new circuit (with re-routed pathways to accommodate the fuel gauge being moved to where the clock used to be) has changed. The sending unit signal was reaching the original large fuel gauge no problem previously. Are there any additional details, or suggestions you can give for this?

Again, thanks so much for taking the time to help me out!

-Mike
Mike - 1978 K-5

Offline bd

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Re: Tach Conversion Kit Install
« Reply #6 on: April 18, 2019, 08:00:02 PM »
Bookmark Common Automotive (& Shorthand) Acronyms for future reference.

I+ is ignition power (or 12 volts with key on). 

Set your voltmeter to the 20 DC Volts scale (or substitute a 12-volt incandescent test light), clip the black probe of the meter (or test light) to a shiny clean, bare metal ground and check for ignition power (key must be on) to the tach head via the pink harness wire.  Good I+ = meter will measure ~12.6 volts (or test light will illuminate).  To verify ground to the tach head, clip the red voltmeter probe to a 12-volt hot spot on the fuse box and use the black probe of the meter to probe the black wire that connects to the tach head.  Good ground = meter will measure ~12.6 volts (or test light will illuminate).

To check the tach signal from the distributor, clip the black lead of the voltmeter (or 12-volt test light) to a shiny clean, bare metal ground, set the meter to the 20 DC Volts scale, start the engine and probe the brown wire that connects to the tach head using the red meter probe.  The tach signal is pulsed DC voltage.  Good tach signal = voltmeter will measure >6 volts (or test light will illuminate) and increase (grow brighter) with increasing engine RPM.  Be careful since the tach lead can produce quite a jolt.

An open circuit means the circuit has lost continuity and is essentially disconnected.  A fuel gauge sweeping past full to ~3:00 o'clock indicates that the fuel gauge terminal that normally connects to the fuel tank sender has lost its connection to the sender.  So look for poor connections or a wire that needs to be relocated in the instrument cluster connector that plugs into the back of the cluster.
Rich
It's difficult to know just how much you don't know, until you know it....
In other words... if people learn by making mistakes, by now I should know just about everything!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline 78 K5 Cheyenne

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Re: Tach Conversion Kit Install
« Reply #7 on: April 19, 2019, 09:01:53 AM »
Wow, bd, this is awesome! I really, truly appreciate you taking the time to help me out! I will do all of this, and try to get it sorted, hopefully this weekend between getting ready for Easter etc.

Again, many, many thanks!

-Mike
Mike - 1978 K-5