I have run in to an issue with my 70 Mustang Factory tach

[Rich Ackermann 174]

I have run in to an issue with my Factory tach in my 70 Mustang. I have a 1970 Mustang Mach 1 with a Cleveland stroked to 408 with a factory tach. I have installed a Holley Sniper EFI with the Holley Hyperspark ignition system (Distributor, Coil and Ignition Box). Its running well. I am using the wire that Holley provides for to drive an aftermarket tachometer (Pin G Dark Brown wire is for aftermarket Tach Output). The Cleveland idles between 950  and 1000 RPMs according to the Sniper hand held monitor. When I look at the factory tach it reads a steady 5,000 rpms with engine hot. What’s really strange is when I increase engine rpms (give it gas) the RPMs on the Holley hand held increase accordingly while the RPMs drop on the factory tach. A couple of video links of this below.

 Has anyone heard of this before? Any thoughts on what might be wrong and can this be fixed?

 I am trying to get the Holley tech guys to explain this, but so far no luck.

 Thanks,

[Midlife 840]

Rich:

The factory tach is a current-sensing device, and all aftermarket tachs are voltage-sensing devices.  The ignition system you're using has multiple sparks per cylinder, and the factory tach is expecting only one spark per cylinder, which is why it is readings are crazy.  You need to contact Holly and see if they offer an adapter for the Ford factory tach, or *gasp* change your factory tach to an aftermarket tach.

[Rich Ackermann 174]

8 hours ago, Midlife said:

Rich:

The factory tach is a current-sensing device, and all aftermarket tachs are voltage-sensing devices.  The ignition system you're using has multiple sparks per cylinder, and the factory tach is expecting only one spark per cylinder, which is why it is readings are crazy.  You need to contact Holly and see if they offer an adapter for the Ford factory tach, or *gasp* change your factory tach to an aftermarket tach.

Thanks Randy. I have also contacted Rocketman's to have my original converted to voltage sensing... www.RCCInnovations.com

Randy, Another question... My fuel gauge is reading low with an aftermarket Holley EFI sender. Is there a way to adjust the CVR? I have a new aftermarket CVR, Does it have a some sort of adjustable potentiometer? I am only using it for the temp and fuel gauges.

Thanks

 

 

[Mach1 Driver 632]

1 hour ago, Rich Ackermann said:

Thanks Randy. I have also contacted Rocketman's to have my original converted to voltage sensing... www.RCCInnovations.com

Randy, Another question... My fuel gauge is reading low with an aftermarket Holley EFI sender. Is there a way to adjust the CVR? I have a new aftermarket CVR, Does it have a some sort of adjustable potentiometer? I am only using it for the temp and fuel gauges.

Thanks

 

 

The aftermarket IVRs are all electronic and are not adjustable. The OEM is adjustable, but since oil, water and fuel are connected to it, they should all read high if you adjust it up. In order to operate correctly the resistance should be from 10 to 73 ohms- what is the Holley?

[aslanefe 356]

4 hours ago, Rich Ackermann said:

Thanks Randy. I have also contacted Rocketman's to have my original converted to voltage sensing... www.RCCInnovations.com

Randy, Another question... My fuel gauge is reading low with an aftermarket Holley EFI sender. Is there a way to adjust the CVR? I have a new aftermarket CVR, Does it have a some sort of adjustable potentiometer? I am only using it for the temp and fuel gauges.

Thanks

 

 

Which sender are you using? Holley 12-305?

[Midlife 840]

The above posts are on track: your gauge and sending unit need to match one another for sensitivity.

[Rich Ackermann 174]

On 8/13/2021 at 10:48 AM, aslanefe said:

Which sender are you using? Holley 12-305?

Yes. That's the one.

HLY-12-305 Fuel Pump, Electric In-Tank, OEM Style EFI, 67 gph/255 lph, 58 Max psi, Ford, Mercury, Kit (Mfr. #: 12-305)

[aslanefe 356]

On 8/15/2021 at 8:01 AM, Rich Ackermann said:

Yes. That's the one.

HLY-12-305 Fuel Pump, Electric In-Tank, OEM Style EFI, 67 gph/255 lph, 58 Max psi, Ford, Mercury, Kit (Mfr. #: 12-305)

I have that one too. Assuming you used the correct float on it. I am happy with mine (gauge reading); quarter tank shows almost quarter, full tank shows a little above F. When needle is at E, I have about 2 gallons of fuel in tank. I do not care much about anything besides quarter full shows quarter on gauge.

I believe there are upper and lower limit adjustment on the back of the gauge. May be you can try the upper limit adjustment and make it read higher.

[danno 128]

Bending the steel rod that  connects the fuel float to the potentiometer part is the simplest way to adjust the fuel reading, but draining the fuel to get at the rod is a very difficult process.  I have done it on my factory fuel sender. If you adjust your CVR so the gas guage reads correct,  then your temperature gauge will be wrong.   I am not aware of any method to make adjustments to the needle or gauge that is in the instrument panel.  If you took it apart,  you might be able to bend the needle, but that is a very delicate process. 

[aslanefe 356]

8 minutes ago, danno said:

Bending the steel rod that  connects the fuel float to the potentiometer part is the simplest way to adjust the fuel reading, but draining the fuel to get at the rod is a very difficult process.  I have done it on my factory fuel sender. If you adjust your CVR so the gas guage reads correct,  then your temperature gauge will be wrong.   I am not aware of any method to make adjustments to the needle or gauge that is in the instrument panel.  If you took it apart,  you might be able to bend the needle, but that is a very delicate process. 

Danno, just FYI, Holley 12-305 he is using is for 65-70 mustangs and comes with 3 separate floats (with different rods) so you have to install the correct one for the tank you have (16, 20, 22 gal tank).

[Rich Ackermann 174]

45 minutes ago, aslanefe said:

Danno, just FYI, Holley 12-305 he is using is for 65-70 mustangs and comes with 3 separate floats (with different rods) so you have to install the correct one for the tank you have (16, 20, 22 gal tank).

aslanfe, You are correct. I have a 22 gallon tank and that is the float option I used. 

Danno, Thanks Its down to draining the tank and pulling the fuel pump/sender to make sure the float is working and then ohm test the ranges. I know the gauge will operate as it goes full when grounded, but it still make be off due to it's age. 

Thanks

[1969_Mach1 335]

If Holley doesn't offer a Tach adapter, MSD offers two different styles.  I don't recall the part number, but the more expensive version is what is needed to work with current triggered tachs.  Since the adapter connects to the Tach output terminal of an MSD ignition box, it might work with your Holley setup.

[Mach1 Driver 632]

First I would disconnect the fuel sender wire and short it to the chassis. Turn the key on and see if the fuel gauge rises to full. Try not to peg the meter. If it doesn't go to full you can calibrate the meters- see my post on this site: 

 

[Rich Ackermann 174]

On 8/16/2021 at 11:47 AM, 1969_Mach1 said:

If Holley doesn't offer a Tach adapter, MSD offers two different styles.  I don't recall the part number, but the more expensive version is what is needed to work with current triggered tachs.  Since the adapter connects to the Tach output terminal of an MSD ignition box, it might work with your Holley setup.

I ordered and just installed the MSD 8920 tach adapter as per the instruction connected the Holley Sniper brown aftermarket tach wire to the MSD 8920 wire for a current triggered tach, and early indication is it appears to be working beautifully.

Thanks all.

[Rich Ackermann 174]

On 8/16/2021 at 2:03 PM, Mach1 Driver said:

First I would disconnect the fuel sender wire and short it to the chassis. Turn the key on and see if the fuel gauge rises to full. Try not to peg the meter. If it doesn't go to full you can calibrate the meters- see my post on this site: 

 

An update on calibrating my fuel sender and factory fuel guage... I purchased a product called MeterMatch, which is designed to read the ohms from a sending unit (ex oil, temp, fuel) in my case the Holley fuel sender, and output the ohms you set to the gauge. So you can essentially take the ohms generated by the sending unit with the tank 1/2 full and make the gauge show 1/2 full by generating the ohms your gauge needs. (Ex. 20 ohms from sender can be converted to 10 ohms to the gauge to move the needle to E or visa versa.)

It also hase the capability to turn on a low and a high warning light indicators at designated ohms you set.

So far the device is working well.