Tachometer Reads almost 2x too high
#1
03-05-2017, 10:30 PM
Join Date: Mar 2017
Location: Colorado
Posts: 4
Likes: 0
Received 0 Likes
on
0 Posts
Tachometer Reads almost 2x too high
Hi,
I'm new to the forum, but I've had a 1969 FHC for a couple of years now. I have a strange problem, wherein the tachometer consistently reads about 2x too high. I had a Pertronix setup, and was told by an expert that could mess up the tachometer reading, so I took it out and put in points. The tachometer still reads the same... I thought perhaps it was a V-12 tach but that only lasted until I read the faceplate which clearly states "6-Cyl". It is very consistent and the needle never wavers, but I think redline happens around 3500 actual rpm. (this is judging by speed and gear ratio information). My speedometer is also quite strange, only reading accurately under heavy throttle. The moment I let up, the needle drops to just 20-30mph even if I am going 60 or more. The angle drive is not loose. Could the cable cause this? Thank you for your help!
Jonathan
I'm new to the forum, but I've had a 1969 FHC for a couple of years now. I have a strange problem, wherein the tachometer consistently reads about 2x too high. I had a Pertronix setup, and was told by an expert that could mess up the tachometer reading, so I took it out and put in points. The tachometer still reads the same... I thought perhaps it was a V-12 tach but that only lasted until I read the faceplate which clearly states "6-Cyl". It is very consistent and the needle never wavers, but I think redline happens around 3500 actual rpm. (this is judging by speed and gear ratio information). My speedometer is also quite strange, only reading accurately under heavy throttle. The moment I let up, the needle drops to just 20-30mph even if I am going 60 or more. The angle drive is not loose. Could the cable cause this? Thank you for your help!
Jonathan
#2
03-06-2017, 11:56 AM
#3
03-07-2017, 08:31 AM
#4
03-17-2017, 09:52 AM
Smith's Electronic Tachometer
From my Crane electronic ignition instructions:
INOPERATIVE SMITHSTACH
There are three types of Smiths tachs: external currentloop, internal current loop, and electronic pulse sensing (late models).Current sensing tachs have a wire loop with two turns that passes the coilcurrent through a pickup at the tach. This loop can be external orinternal. Electronic tachs have atrigger wire that connects to the COIL- terminal.
MODIFICATION OF SMITHSCURRENT SENSING TACHS
Some older British vehicles use Smiths current sensingtachs. The wire from the coil positiveterminal to the ignition switch passes through a current pickup at thetach. Installation of a Crane electronicignition may cause erratic operation of a current sensing tach, due to thehigher coil current. Modification of the current pickup to reduce the signallevel will usually eliminate the problem.
1. Remove the Smiths tach from the instrument panel. Thetach has two threaded studs that are retained from the rear of the panel. Labelall wires to avoid errors upon reinstallation.
WARNING: Improper connection may damage the tach.
2. Locate the external current pickup on the rear of thetach. Refer to Figure 27. If your tach does not resemble this figure but hascoil and ignition key wires going to a plug, it may have an internal currentpickup. In this case, it will benecessary to disassemble the tach. Once the tach is disassembled, you can usethe same general procedure as explained in step 3.
3. Modify the current pickup by removing one loop of wire asshown in Figure 28. Note the directionthat the wire passes through the pickup. If this direction is reversed, thetach will not function.
4. Recalibrate the tach for best accuracy. Connect a testtach and have a helper rev the engine. Hold the tach in the same position it ismounted (orientation may affect calibration). Adjust the calibration screw onthe back of Smiths tach until the reading matches the test tach. 4,000 RPM is areasonable engine RPM to use for calibration. Please note that older Smithstachs may vary as much as 500 RPM throughout the RPM range. This variation isnot the fault of the ignition system.
5. Reinstall tach in instrument panel. Check all wireconnections.
TROUBLESHOOTING SMITHSELECTRONIC TACHS
In some cases the tach will not read the correct RPM afterinstallation of a Crane electronic ignition. A calibration screw on the back ofthe tach can usually be adjusted to give correct readings. If the tach stillreads high, put a resistor in the tach wire to reduce the signal level. Startwith a 10K ohm ½ watt. You can go as low as 1K ohm 1/2 watt. You can buy theresistors from Radio Shack or other electronic suppliers. Solder into tach wireand wrap with electrical tape for protection.
INOPERATIVE SMITHSTACH
There are three types of Smiths tachs: external currentloop, internal current loop, and electronic pulse sensing (late models).Current sensing tachs have a wire loop with two turns that passes the coilcurrent through a pickup at the tach. This loop can be external orinternal. Electronic tachs have atrigger wire that connects to the COIL- terminal.
MODIFICATION OF SMITHSCURRENT SENSING TACHS
Some older British vehicles use Smiths current sensingtachs. The wire from the coil positiveterminal to the ignition switch passes through a current pickup at thetach. Installation of a Crane electronicignition may cause erratic operation of a current sensing tach, due to thehigher coil current. Modification of the current pickup to reduce the signallevel will usually eliminate the problem.
1. Remove the Smiths tach from the instrument panel. Thetach has two threaded studs that are retained from the rear of the panel. Labelall wires to avoid errors upon reinstallation.
WARNING: Improper connection may damage the tach.
2. Locate the external current pickup on the rear of thetach. Refer to Figure 27. If your tach does not resemble this figure but hascoil and ignition key wires going to a plug, it may have an internal currentpickup. In this case, it will benecessary to disassemble the tach. Once the tach is disassembled, you can usethe same general procedure as explained in step 3.
3. Modify the current pickup by removing one loop of wire asshown in Figure 28. Note the directionthat the wire passes through the pickup. If this direction is reversed, thetach will not function.
4. Recalibrate the tach for best accuracy. Connect a testtach and have a helper rev the engine. Hold the tach in the same position it ismounted (orientation may affect calibration). Adjust the calibration screw onthe back of Smiths tach until the reading matches the test tach. 4,000 RPM is areasonable engine RPM to use for calibration. Please note that older Smithstachs may vary as much as 500 RPM throughout the RPM range. This variation isnot the fault of the ignition system.
5. Reinstall tach in instrument panel. Check all wireconnections.
TROUBLESHOOTING SMITHSELECTRONIC TACHS
In some cases the tach will not read the correct RPM afterinstallation of a Crane electronic ignition. A calibration screw on the back ofthe tach can usually be adjusted to give correct readings. If the tach stillreads high, put a resistor in the tach wire to reduce the signal level. Startwith a 10K ohm ½ watt. You can go as low as 1K ohm 1/2 watt. You can buy theresistors from Radio Shack or other electronic suppliers. Solder into tach wireand wrap with electrical tape for protection.
The following users liked this post:
ExcelsiorZ (09-29-2022)
