Electrical 91 Lx - Factory Tachometer Issues

cdurbin · Apr 10, 2014

Well, I'm having some tachometer issues I'm hoping to get some input on. Yesterday my tachometer needle started intermittently jumping around but the engine was at a steady RPMs. Last time this happened my TFI module went out and the car just flat shut off in traffic (not a fun day). So naturally I'm thinking the TFI is on it's way out again so I replaced it today. Took it for a test drive and about 2 miles down the road the damn thing started jumping again. Could the Tachometer be going bad? Not sure what else to check.

jrichker · Apr 10, 2014

Simple things first.:
You might want to look at the connections between the instrument cluster and the dash. It's sheet of plastic with copper etching on it like a printed circuit board. If the connections get corroded or loose, you can have some unusual problems.

The next step is to dump the codes and look for code 16: The same circuit that feeds the computer with information also feeds the tach. Problems with that wire, resistor and connections can do goofy things with the computer and tach.

Dump the codes: Codes may be present even if the Check Engine Light (CEL) isn't on.

Dumping the computer diagnostic codes on 86-95 Mustangs

Revised 26-July-2011. Added need to make sure the clutch is pressed when dumping codes.

Codes may be present even if the check engine light hasn’t come on, so be sure to check for them.

Here's the way to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great. You watch the flashing test lamp or Check Engine Light and count the flashes.

Post the codes you get and I will post 86-93 model 5.0 Mustang specific code definitions and fixes. I do not have a complete listing for 94-95 model 5.0 Mustangs at this time.

Be sure to turn off the A/C, and put the transmission in neutral when dumping the codes. On a manual transmission car, be sure to press the clutch to the floor.
Fail to do this and you will generate a code 67 and not be able to dump the Engine Running codes.

If your car is an 86-88 stang, you'll have to use the test lamp or voltmeter method. There is no functional check engine light on the 86-88's except possibly the Cali Mass Air cars.

The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the self test connector.

89 through 95 cars have a working Check Engine light. Watch it instead of using a test lamp.

The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the self test connector.

WARNING!!! There is a single dark brown connector with a black/orange wire. It is the 12 volt power to the under the hood light. Do not jumper it to the computer test connector. If you do, you will damage the computer.

What to expect:
You should get a code 11 (two single flashes in succession). This says that the computer's internal workings are OK, and that the wiring to put the computer into diagnostic mode is good. No code 11 and you have some wiring problems. This is crucial: the same wire that provides the ground to dump the codes provides signal ground for the TPS, EGR, ACT and Map/Baro sensors. If it fails, you will have poor performance, economy and driveablity problems

Some codes have different answers if the engine is running from the answers that it has when the engine isn't running. It helps a lot to know if you had the engine running when you ran the test.

Dumping the Engine Running codes: The procedure is the same, you start the engine with the test jumper in place. Be sure the A/C is off, and clutch (if present) is pressed to the floor, and the transmission is in neutral. You'll get an 11, then a 4 and the engine will speed up to do the EGR test. After the engine speed decreases back to idle, it will dump the engine running codes.

Trouble codes are either 2 digit or 3 digit, there are no cars that use both 2 digit codes and 3 digit codes.

Your 86-88 5.0 won't have a working Check Engine Light, so you'll need a test light.
See AutoZone Part Number: 25886 , $10

Alternate methods:
For those who are intimidated by all the wires & connections, see Actron® for what a typical hand scanner looks like. Normal retail price is about $30 or so at AutoZone or Wal-Mart.

Or for a nicer scanner see www.midwayautosupply.com/Equus-Digital-Ford-Code-Reader/dp/B000EW0KHW Equus - Digital Ford Code Reader (3145It has a 3 digit LCD display so that you don’t have to count flashes or beeps.. Cost is $22-$36.

Code 16 - Electronic ignition - IDM circuit fault - Ignition Systems. Bad wiring, possible bad TFI. The computer isn’t seeing the proper IDM signal coming from the TFI circuit. There is a 22,000 ohm resistor in the dark green/ yellow wire that goes to pin 4 on the computer. Disconnect the TFI connector and remove the passenger side kick panel. Then loosen the 10 MM bolt to disconnect the computer connector. Use an ohmmeter to measure the resistance between pin 4 and the dark green/ yellow wire on the TFI connector. You should see 21 - 23 K ohms (21,000-23,000 ohms). If not the dark green/ yellow wire is broken or has a bad connection. If this checks out OK, measure between the dark green/ yellow wire and ground. You should see 1 M Ohm or more, OR an infinite reading. Less that 100 K ohms means a short to ground somewhere in the dark green/ yellow wire.
Wiggle and jiggle the TFI wiring harness wire while testing it to try and find breaks and short circuits.

Diagram courtesy of Tmoss & Stang&2birds

Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs

cdurbin · Apr 11, 2014

jrichker said:
Simple things first.:
You might want to look at the connections between the instrument cluster and the dash. It's sheet of plastic with copper etching on it like a printed circuit board. If the connections get corroded or loose, you can have some unusual problems.

The next step is to dump the codes and look for code 16: The same circuit that feeds the computer with information also feeds the tach. Problems with that wire, resistor and connections can do goofy things with the computer and tach.

Dump the codes: Codes may be present even if the Check Engine Light (CEL) isn't on.

Dumping the computer diagnostic codes on 86-95 Mustangs

Revised 26-July-2011. Added need to make sure the clutch is pressed when dumping codes.

Codes may be present even if the check engine light hasn’t come on, so be sure to check for them.

Here's the way to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great. You watch the flashing test lamp or Check Engine Light and count the flashes.

Post the codes you get and I will post 86-93 model 5.0 Mustang specific code definitions and fixes. I do not have a complete listing for 94-95 model 5.0 Mustangs at this time.

Be sure to turn off the A/C, and put the transmission in neutral when dumping the codes. On a manual transmission car, be sure to press the clutch to the floor.
Fail to do this and you will generate a code 67 and not be able to dump the Engine Running codes.

If your car is an 86-88 stang, you'll have to use the test lamp or voltmeter method. There is no functional check engine light on the 86-88's except possibly the Cali Mass Air cars.

The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the self test connector.

89 through 95 cars have a working Check Engine light. Watch it instead of using a test lamp.

The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the self test connector.

WARNING!!! There is a single dark brown connector with a black/orange wire. It is the 12 volt power to the under the hood light. Do not jumper it to the computer test connector. If you do, you will damage the computer.

What to expect:
You should get a code 11 (two single flashes in succession). This says that the computer's internal workings are OK, and that the wiring to put the computer into diagnostic mode is good. No code 11 and you have some wiring problems. This is crucial: the same wire that provides the ground to dump the codes provides signal ground for the TPS, EGR, ACT and Map/Baro sensors. If it fails, you will have poor performance, economy and driveablity problems

Some codes have different answers if the engine is running from the answers that it has when the engine isn't running. It helps a lot to know if you had the engine running when you ran the test.

Dumping the Engine Running codes: The procedure is the same, you start the engine with the test jumper in place. Be sure the A/C is off, and clutch (if present) is pressed to the floor, and the transmission is in neutral. You'll get an 11, then a 4 and the engine will speed up to do the EGR test. After the engine speed decreases back to idle, it will dump the engine running codes.

Trouble codes are either 2 digit or 3 digit, there are no cars that use both 2 digit codes and 3 digit codes.

Your 86-88 5.0 won't have a working Check Engine Light, so you'll need a test light.
See AutoZone Part Number: 25886 , $10

Alternate methods:
For those who are intimidated by all the wires & connections, see Actron® for what a typical hand scanner looks like. Normal retail price is about $30 or so at AutoZone or Wal-Mart.

Or for a nicer scanner see www.midwayautosupply.com/Equus-Digital-Ford-Code-Reader/dp/B000EW0KHW Equus - Digital Ford Code Reader (3145It has a 3 digit LCD display so that you don’t have to count flashes or beeps.. Cost is $22-$36.

Code 16 - Electronic ignition - IDM circuit fault - Ignition Systems. Bad wiring, possible bad TFI. The computer isn’t seeing the proper IDM signal coming from the TFI circuit. There is a 22,000 ohm resistor in the dark green/ yellow wire that goes to pin 4 on the computer. Disconnect the TFI connector and remove the passenger side kick panel. Then loosen the 10 MM bolt to disconnect the computer connector. Use an ohmmeter to measure the resistance between pin 4 and the dark green/ yellow wire on the TFI connector. You should see 21 - 23 K ohms (21,000-23,000 ohms). If not the dark green/ yellow wire is broken or has a bad connection. If this checks out OK, measure between the dark green/ yellow wire and ground. You should see 1 M Ohm or more, OR an infinite reading. Less that 100 K ohms means a short to ground somewhere in the dark green/ yellow wire.
Wiggle and jiggle the TFI wiring harness wire while testing it to try and find breaks and short circuits.

Diagram courtesy of Tmoss & Stang&2birds

Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs

See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring http://www.veryuseful.com/mustang/tech/engine/ Everyone should bookmark this site.

Complete computer, actuator & sensor wiring diagram for 91-93 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/91-93_5.0_EEC_Wiring_Diagram.gif

Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif

Ignition switch wiring
http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif

Fuel, alternator, A/C and ignition wiring
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

O2 sensor wiring harness
http://www.veryuseful.com/mustang/tech/engine/images/mustangO2Harness.gif

Vacuum diagram 89-93 Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg

HVAC vacuum diagram
http://www.veryuseful.com/mustang/tech/engine/images/Mustang_AC_heat_vacuum_controls.gif

TFI module differences & pin out
http://www.veryuseful.com/mustang/tech/engine/images/TFI_5.0_comparison.gif

Fuse box layout
http://www.veryuseful.com/mustang/tech/engine/images/MustangFuseBox.gif

87-92 power window wiring
http://www.veryuseful.com/mustang/tech/engine/images/mustang87-92 PowerWindowWiring.gif

93 power window wiring
http://www.veryuseful.com/mustang/tech/engine/images/mustang93PowerWindows.gif

I appreciate the input. I'll check the codes this weekend and see what I get.
I remember when I pulled the cluster to put in my LED bulbs that the sheet of plastic was peeling around the edges.
That was over a year ago and I haven't had any problems until now. I may just buy a new cluster on Ebay or something.
I'm wondering if it's heat related now. This morning on the way to work the tachometer worked normal and the outside temp was a cool 55 deg. When it was acting up it was later in the day when the temps were around 75-80 degrees and I had been driving the car for a bit. Might not mean anything. I guess I'll see if it starts acting up after work today. It's supposed to be 80 degrees. Any thoughts on that theory? Also, the TFI module is not a Motorcraft brand. I've heard that can make a difference.
Thanks again

cdurbin · Apr 11, 2014

I went out for a ride at lunch today after and it started doing it's dance again. I'll pull codes and test the new TFI module after work. Could it possibly be the PIP going bad?

cdurbin · Apr 11, 2014

Well I swung by Oriellys on the way home to have them run for codes but the "kid" wasn't real bright so I'll end up doing it myself over the weekend.
The only code he managed to pull was a P0300 misfire code. I did notice that it appears to be running a little rich and my gas mileage is horrible the last couple of days.

jrichker · Apr 11, 2014

cdurbin said:
Well I swung by Oriellys on the way home to have them run for codes but the "kid" wasn't real bright so I'll end up doing it myself over the weekend.
The only code he managed to pull was a P0300 misfire code. I did notice that it appears to be running a little rich and my gas mileage is horrible the last couple of days.

NO such code on 86-93 5.0 Mustangs. All the codes are 2 digit.
Post your results and I will give you 5.0 Mustang specific code definitions and fixes.

cdurbin · Apr 11, 2014

jrichker said:
NO such code on 86-93 5.0 Mustangs. All the codes are 2 digit.
Post your results and I will give you 5.0 Mustang specific code definitions and fixes.

That's exactly what I thought..lol
I'll post up any codes I get over the weekend.
Thanks again

cdurbin · Apr 12, 2014

jrichker said:
NO such code on 86-93 5.0 Mustangs. All the codes are 2 digit.
Post your results and I will give you 5.0 Mustang specific code definitions and fixes.

I cheated and bought a reader for $30

KOEO - 34
KOER - 94, 44, 34

Grabbin' Asphalt · Apr 12, 2014

My tach jumped and bounced but rpms were steady when my msd distributor went out. Swap that dizzy with a bud.

cdurbin · Apr 13, 2014

Grabbin' Asphalt said:
My tach jumped and bounced but rpms were steady when my msd distributor went out. Swap that dizzy with a bud.

Unfortunately, none of my friends are cool enough to drive a fox mustang. lol
I'm leaning towards replacing the dizzy. Reason being is that I just replaced the TFI module and it did not resolve the problem. I'll probably have it tested just to be sure. Is there a certain brand of dizzy I should get or stay away from? Most of the local parts houses sell basically the same type stuff.
Thanks

Grabbin' Asphalt · Apr 13, 2014

Go to napa, I bought a remanufactured Ford Motor Company one from them. You can get it without the module, just add your module to it, it was around $90-$110. I have it as a spare in my trunk right now, my mechanic swapped a used one so I just left it, the gear was perfect shape so I just left it.

cdurbin · Apr 13, 2014

Grabbin' Asphalt said:
Go to napa, I bought a remanufactured Ford Motor Company one from them. You can get it without the module, just add your module to it, it was around $90-$110. I have it as a spare in my trunk right now, my mechanic swapped a used one so I just left it, the gear was perfect shape so I just left it.

I was thinking NAPA as well. They tend to have a better quality of products.
It'll drive me crazy until I get this fixed.

Thanks for the input. Appreciate it

Grabbin' Asphalt · Apr 13, 2014

It was under another name when they looked it up but when they brought it to me I was pleasantly surprised, just get them to bring a few to you and see what kind they are.

Grabbin' Asphalt · Apr 13, 2014

Wait I will go take a pic of the part number and box, hold on.

Grabbin' Asphalt · Apr 13, 2014

Believe its under the Napa brand...

cdurbin · Apr 13, 2014

Grabbin' Asphalt said:
Believe its under the Napa brand...

Awesome! Thanks

Grabbin' Asphalt · Apr 13, 2014

D@mn I don't even get a like for that, you're a tough one

cdurbin · Apr 14, 2014

My apologies.
My mind is on too many things at once lately. My job, working on remodeling the house, my car....etc
Thanks again

cdurbin · Apr 14, 2014

Grabbin' Asphalt said:
Believe its under the Napa brand...

I've never replaced a dizzy before so forgive me if this is a stupid question.
I'm assuming that I will need to transfer over the top portion of the old dizzy (cap..etc)

cdurbin · Apr 14, 2014

Grabbin' Asphalt said:
Believe its under the Napa brand...

Next question...do I need cast iron or steel gear? As far as I know it's the stock cam

Electrical 91 Lx - Factory Tachometer Issues

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