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85 SVO cluster swap into 84 LX!

 
littleNGNthatcould
New User
Posts: 23
Joined: 08/12
Posted: 11/21/12 08:17 AM

ok guys, so i just got a complete SVO cluster for an incredible price. has some slight rust issues, i intend to sand it off with 3000 grit sandpaper. then buy new overlays from whitegauge.net. but thats a different story, my issue is this, when i plug in my original cluster the idle is about 900rpm, when i plug in the the SVO its 1600rpm, no actual change in my idle, just the tach. what can i do to solve this? is there a real solution? also i haven't tested to see if the speedo is accurate, but can i expect it to be?

P.S. the difference in the tachs is my LX is 7000 and the SVO is 8000 i didnt expect this to be an issue, as i thought it got its signal from the coil.  

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waynep71222
User
Posts: 166
Joined: 04/12
Posted: 11/21/12 04:23 PM

if you look at the back of the TACH... there usually is a knob or some way to change between 4, 6 and 8 cylinders...

the tach signal counts the pulses of the ignition system

4 cylinders create a spark every 1/2 turn
6 cylinders create a spark every 1/3 turn
8 cylinders create a spark every 1/4 turn..

so if your V8 is idling at 600 rpms... and you have a 4 cylinder tach.. its going to show 300 rpms..

if you have a V6 and your engine is idling at 600 RPMs... its going to show 450 RPMs..

why is it showing 1600 RPMs at idle???  do you have a MULTI SPARK Ignition ????  

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littleNGNthatcould
New User
Posts: 23
Joined: 08/12
Posted: 11/21/12 04:44 PM

no, it is factory ignition.  

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waynep71222
User
Posts: 166
Joined: 04/12
Posted: 11/21/12 08:07 PM

did your old cluster have a tach????

examine the back for a switch.. might be just a screw head through a hole... but it will have 4 6 8 markings...


lets take a look through this fox body tach replacement article from a sister magazine..

http://www.mustangmonthly.com/techarticles/mump_0703_fox_body_mustang_tachometer/viewall.html

there are 4 mustang and ford specific magazines on this site..  

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waynep71222
User
Posts: 166
Joined: 04/12
Posted: 11/21/12 08:20 PM

well that did not work...

but i have had a fox body tach in my hands and reset it.. as the owner had swapped out the puny 4 for a 5.0.   its just been too many years since i did it...  

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littleNGNthatcould
New User
Posts: 23
Joined: 08/12
Posted: 11/22/12 07:48 AM

there was nothing on the back of the cluster, maybe i have to pull the tach out of the cluster? also the fuel and oil pressure both work perfectly, and the speedo is almost dead on, (reads 2mph faster) but the tach is def. out of wack, i would like to keep it as it does have 8k vs my 7k. and yes my original does have a tach. if you'd like, i could do a quick youtube clip of swaping them out displaying the diff.  

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waynep71222
User
Posts: 166
Joined: 04/12
Posted: 11/22/12 05:03 PM

well .. now that i have a clear head and a belly full of turkey and stuffing.... i did some more research.. found this on another site..


Tach: Most Ford Fox body cars have a switch on the tach to go from 4, 6, or 8 cylinder cars. Some such as Mustang Turbo or Thunderbird Turbo models will need to convert theirs to work or use an aftermarket tach. For the conversion see the "Using a 79-82 Turbo 4 Tach"


does the original tach mount in your SVO cluster????  

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waynep71222
User
Posts: 166
Joined: 04/12
Posted: 11/22/12 05:19 PM

when i googled using the 79-82 turbo 4 tach i found this ...

by the way.. if you intend on doing something like this.. you might like to know that every radio shack has 2 different 10 turn trimmer pots in stock in the drawers...  flat things like a tiny domino.. with 3 wires and a screw head in one end..  you can use the first and second or second and third..  not the first and third... this gives you adjustability..   if you have an electric drill and a second distributer..   you could put a oil pump drive in the drill and spin the distributer at max drill speed..  if you have a jumper cable hooked to the distributer housing and the negative battery post.. so the distributer has a proper ground..  this also prevents you from getting shocked.. yep..   you might want to TAP the inertia switch so the fuel pump is NOT spinning as rpm input from the distributer will not only kick the fuel pump on. but fire the injectors filling your engine with massive amounts of fuel as the engine thinks the engine is running but cannot understand why the other sensors are not responding...

if you have one of the 38 buck harbor freight automotive mulitmeters with a clamp over a spark plug wire tachometer clamp.. you could check the RPM calibration that way while the dash is laying forward...

i don't know if the info on for the earlier version will help you with the 84...  but you will always have the V8 tach to reinstall if all goes badly...


Using a 79-82 Turbo 4 Tach
Repair and Manual


Introduction:

I always find it cool when you can use a factory component and some ingenuity for racing instead of an aftermarket part. My stock 4-cylinder tach only went to 6-grand. My combination needed to be shifted above that point, but I didn't want to alter the inside appearance of my gauges. I found an 8-grand tach that matched all my other instrumentation in a turbo 4 Mustang. This tach was not adjustable between 4, 6, and 8 cylinder cars like the 6-grand tach, so it was reading twice as high as it should when used in an 8-cylinder car. After some research I figured out how to modify it to work in my application. I even calibrated my tach to make it very accurate. So follow along and you can perform the same modification.

Tools Required:

Philips #2 screwdriver
1/4" nutdriver
Soldering iron and solder
Ohm meter, preferably digital
100K ohm resistor
Step-by-Step Procedure

1) Set engine RPM using the idle screw to 3,000 RPM using another tach if possible. Or using a 4-cylinder tach set to 6,000 RPM.
2) Remove instrument cluster from dash. Remove tach from instrument cluster.

3) Remove back of tach to reveal the circuit board.

4) Locate rpm calibration resistor on circuit board.

5) Measure resistance across resistor with an ohmmeter.

6) Solder 100K ohm resistor on to circuit board at the same terminals that the adjustable resistor is soldered.

7) Place the ohmmeter back on the terminals and adjust the adjustable resistor to 1/2 the previous measured value.

8) Hook tach back up to factory wiring and check it, the reading should be 3,000 rpm with the engine running.

9) Put tach back together, reinstall in instrument cluster, and put dash back together.

10) Reset idle to preferred setting.

Alternative Calibration Procedures:

1) Check with another reliable tach - I had an aftermarket tach that was on my Granada that I hooked to check the rpm readings. Check at idle through 4,000 rpm and assume rest of the scale is correct.
2) Non-Contact Optical Tachometer - A piece of equipment like this has an accuracy of +/- 5 rpm, therefore this is the most accurate calibration method that is necessary. This instrument uses a reflective strip and a light to measure the rpm. I had my wife hold the gas steady at different increments while I checked the rpm with the stroboscope on the crank pulley. Use rpms of 1000, 2000, 3000, and 4000 and assume the rest are right. My tach was right on when this method was used.
 

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