Tach not registering

[jimmy78]

I purchased a truck with a aftermarket tach jammed in the bin and I am in the process of removing it.

The after market tach was tied into the factory harness so i disconnected it and plugged it back into the factory gauges.

When I turn the key the gauge pins down to the bottom and doesn’t move with any rpm.

Is the gauge faulty or could it just be a bad wiring job?

 

[AuroraGirl]

I purchased a truck with a aftermarket tach jammed in the bin and I am in the process of removing it.

The after market tach was tied into the factory harness so i disconnected it and plugged it back into the factory gauges.

When I turn the key the gauge pins down to the bottom and doesn’t move with any rpm.

Is the gauge faulty or could it just be a bad wiring job?

you have an oldsmobile diesel? The oldsmobile diesel, im not sure how they got a Tach signal off it but it wasnt the same way as gaser. if you have a 5.7 350 then i can almost guarantee the factory tach wiring couldnt work for that because they probably used a different source (The distributor on a HEI ignition)

The tach you have is for a 5.7 oldsmobile diesel which had no ignition system

 

[AuroraGirl]

They did come with a tach as an option even on the 350 Olds Diesel. The diesel tachs drive off pre-diode AC from the alternator. If it's a diesel tach it wont have the right calibration for V8 or I6.

@bucket you know much about this?

 

[jimmy78]

you have an oldsmobile diesel? The oldsmobile diesel, im not sure how they got a Tach signal off it but it wasnt the same way as gaser. if you have a 5.7 350 then i can almost guarantee the factory tach wiring couldnt work for that because they probably used a different source (The distributor on a HEI ignition)

The tach you have is for a 5.7 oldsmobile diesel which had no ignition

@bucket you know much about this?

All the info I have found for 1978 jimmys this is the tach that it came with. With a stock 5.7 350.

 

[AuroraGirl]

All the info I have found for 1978 jimmys this is the tach that it came with. With a stock 5.7 350.

5,7 350 sbc..?
There is also 5.7 350 oldsmobile diesel. Looking at the twach.. lkay maybe it is gas. but its usually shortened to 5 k vs 6k like you would expect.

 

[jimmy78]

5,7 350 sbc..?
There is also 5.7 350 oldsmobile diesel. Looking at the twach.. lkay maybe it is gas. but its usually shortened to 5 k vs 6k like you would expect.

 

[AuroraGirl]

tbh that doesnt clarify at all since 1978 is the first year of the dieselm but kets just assume its gas for a moment. cna you take a picture of the HEI distributor cap from the engine bay>

 

[AuroraGirl]

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see that salmoncolored wire going to that HEI cap? if you have the TACH wire on your cap it will look like that
That wire needs to be good from the cap to the dash cluster, you could bypass it temporarily with a spsade terminal and a new wire to test the wire

 

[jimmy78]

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see that salmoncolored wire going to that HEI cap? if you have the TACH wire on your cap it will look like that
That wire needs to be good from the cap to the dash cluster, you could bypass it temporarily with a spsade terminal and a new wire to test the wire

I traced the wire from the gauge to the distributor and it has definitely been altered.

The terminal has a aftermarket female terminal on it that is very loosely plugged into the tach port.

I’m going to replace the terminal and see if it fixes the issue.

 

[mtbadbob]

tbh that doesnt clarify at all since 1978 is the first year of the dieselm but kets just assume its gas for a moment. cna you take a picture of the HEI distributor cap from the engine bay>

I pulled this cluster from a '78 gas Blazer, looks the same as pic from jimmy78. Sold it to a member on this forum.

 

[mtbadbob]

I'm wondering if those require or have a tach filter?

 

[jimmy78]

I pulled this cluster from a '78 gas Blazer, looks the same as pic from jimmy78. Sold it to a member on this forum.

I with mine was so nice. After giggling the wire the gauge actually moves further backwards instead of registering the rpm. I’m starting to thing the gauge is fubar.

 

[jimmy78]

I'm wondering if those require or have a tach filter?

Tach filter?

 

[AuroraGirl]

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Tach filter?

Tachometer signal filter GM HEI, MSD, Holley, Speedmaster

Tachometer signal Filter Camaro Firebird Fiero GM GMC Truck Monte Carlo Corvette other GM models. Removes HEI caused spikes, voltage surges, electrical noise. Fast and checkout including Google Pay, PayPal, Apple Pay, Amazon Pay.

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If it’s wired like an OEM tach it has a three wire pigtail and they are completely separate from the other cluster wiring. Its just power, ground and tach signal from the HEI.

For the main cluster it’s really important to verify you have a good ground, usually a single black wire grounded to the emergency brake bracket, or to a ground block on the firewall above the driver’s feet.

OEM tachs usually had a filter to prevent HEI interference with the signal. You can still buy them new (https://www.ecklers.com/hei-ignition-tachometer-filter-66-74728-1.html) but can also build one using a few resistors and capacitors (https://www.elcaminocentral.com/threads/tach-filter-schematic.133330/). I do not have a filter and my tach seems to work fine.

 

[jimmy78]

You must be registered for see images attach

Tachometer signal filter GM HEI, MSD, Holley, Speedmaster

Tachometer signal Filter Camaro Firebird Fiero GM GMC Truck Monte Carlo Corvette other GM models. Removes HEI caused spikes, voltage surges, electrical noise. Fast and checkout including Google Pay, PayPal, Apple Pay, Amazon Pay.

www.wc-autoparts.com

The pink black brown pig tail would be what you’re referring to. I will have to dig a bit deeper. Thank you for the help. Hopefully I can figure this out.