Replaced distributor and coil... Tach now acting up?
Thread Starter
Junior Member
Joined: Nov 2007
Posts: 98
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From: Pennsylvania
Car: 1987 Trans Am
Engine: 5.0 L
Replaced distributor and coil... Tach now acting up?
Not too long ago my distributor quit making spark, and I ended up replacing everything including the coil (coil outside of cap model). I finally got the car working again only to notice that when i'm at a high rate of speed (45-60) and coasting with my foot off the accelerator, the tach becomes goes out of control. If i'm going at 1500 rpm's the tach will jump as high as 2000 rpm's and dip to about 1250. Assuming that are the plugs are firing (Car does not appear to ride rough when this happens, I don't hear the engine revving or dying), is there any way to test/fix this issue?
Supreme Member
Joined: Jan 2006
Posts: 3,341
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From: Mooresville NC
Car: LOWERED ♦ CRIMSON METALFLAKE
Engine: ► 400 KUBES ◄
Transmission: 765R4
Axle/Gears: EATON POSI 4.56
60 Is A High Rate Of Speed (In A Parking Lot)
We assume you’re using stock parts because you didn’t mention otherwise.
Try putting a 10KΩ resistor in line with the tach lead.
Then you can post the results.
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Happy Racing!
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I Didn’t Jump The Start, The Flagman Was Late
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Try putting a 10KΩ resistor in line with the tach lead.
Then you can post the results.
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Happy Racing!
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I Didn’t Jump The Start, The Flagman Was Late
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Thread Starter
Junior Member
Joined: Nov 2007
Posts: 98
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From: Pennsylvania
Car: 1987 Trans Am
Engine: 5.0 L
Re: 60 Is A High Rate Of Speed (In A Parking Lot)
You're saying which parts are stock?
The Tach, obviously, but the whole distributor, cap and everything was replaced including the coil.
The Tach, obviously, but the whole distributor, cap and everything was replaced including the coil.
Supreme Member
Joined: Jan 2006
Posts: 3,341
Likes: 10
From: Mooresville NC
Car: LOWERED ♦ CRIMSON METALFLAKE
Engine: ► 400 KUBES ◄
Transmission: 765R4
Axle/Gears: EATON POSI 4.56
60 Is A High Rate Of Speed (In A Parking Lot)
Stock means the replacement is the original type of part.
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Happy Racing!
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He Must Have Been Be Cheating - Because I Was And He Almost Beat Me.
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Happy Racing!
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He Must Have Been Be Cheating - Because I Was And He Almost Beat Me.
.
Thread Starter
Junior Member
Joined: Nov 2007
Posts: 98
Likes: 0
From: Pennsylvania
Car: 1987 Trans Am
Engine: 5.0 L
Re: 60 Is A High Rate Of Speed (In A Parking Lot)
Ah, gotcha, excuse my lack of jargon comprehension. I've never this before, but I know that the tach lead goes to one of the wires on the coil. Can you tell me which it is?
EDIT: gray coil wire, black and white?
EDIT: gray coil wire, black and white?
Supreme Member
Joined: Jan 2006
Posts: 3,341
Likes: 10
From: Mooresville NC
Car: LOWERED ♦ CRIMSON METALFLAKE
Engine: ► 400 KUBES ◄
Transmission: 765R4
Axle/Gears: EATON POSI 4.56
60 Is A High Rate Of Speed (In A Parking Lot)
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Happy Racing!
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How bout those too scared to drive without headlights in the daytime
and the ones that need headlights for a little rain
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Thread Starter
Junior Member
Joined: Nov 2007
Posts: 98
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From: Pennsylvania
Car: 1987 Trans Am
Engine: 5.0 L
Re: Replaced distributor and coil... Tach now acting up?
Unfortunately putting that resistor in line did not solve the problem. What's my next step?
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Supreme Member
Joined: Jan 2006
Posts: 3,341
Likes: 10
From: Mooresville NC
Car: LOWERED ♦ CRIMSON METALFLAKE
Engine: ► 400 KUBES ◄
Transmission: 765R4
Axle/Gears: EATON POSI 4.56
60 Is A High Rate Of Speed (In A Parking Lot)
OK, take the resistor out of line.
Because it didn’t work, the problem is probably in the dash.
However you can do one quick test before you perform auto surgery.
Use a hand-held tach, and see if it exhibit’s the same symptom.
If it does NOT, and is smooth, you will know the problem is in the dash tach.
There is circuitry in the dash near the tach.
It’s contained on a small plug-in PCB or integrated in the main board.
On the PCB you’ll see a resistor chip.
Desolder pins 4 & 10.
Use a 500K potentiometer.
Solder it across traces 4 & 10.
Move the potentiometer to half way.
Start the engine and compare the tach reading with a test equipment quality tach.
Dial it in, unsolder the potentiometer, measure it and install a matching resistor.
◙◙◙◙◙◙◙◙◙◙◙◙◙◙◙
Happy Racing!
◙◙◙◙◙◙◙◙◙◙◙◙◙◙◙
Does The White Flag Mean He’s Surrendering?
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Because it didn’t work, the problem is probably in the dash.
However you can do one quick test before you perform auto surgery.
Use a hand-held tach, and see if it exhibit’s the same symptom.
If it does NOT, and is smooth, you will know the problem is in the dash tach.
There is circuitry in the dash near the tach.
It’s contained on a small plug-in PCB or integrated in the main board.
On the PCB you’ll see a resistor chip.
Desolder pins 4 & 10.
Use a 500K potentiometer.
Solder it across traces 4 & 10.
Move the potentiometer to half way.
Start the engine and compare the tach reading with a test equipment quality tach.
Dial it in, unsolder the potentiometer, measure it and install a matching resistor.
◙◙◙◙◙◙◙◙◙◙◙◙◙◙◙
Happy Racing!
◙◙◙◙◙◙◙◙◙◙◙◙◙◙◙
Does The White Flag Mean He’s Surrendering?
.
Thread Starter
Junior Member
Joined: Nov 2007
Posts: 98
Likes: 0
From: Pennsylvania
Car: 1987 Trans Am
Engine: 5.0 L
Re: 60 Is A High Rate Of Speed (In A Parking Lot)
Well, I finally got my hands on a separate tach... But the results aren't good. It appears to be exhibiting the same symptom of the dash tach. I'm guessing this is either in the distributor, coil, or the engine now? Where do I go from here?
Supreme Member
Joined: Jan 2006
Posts: 3,341
Likes: 10
From: Mooresville NC
Car: LOWERED ♦ CRIMSON METALFLAKE
Engine: ► 400 KUBES ◄
Transmission: 765R4
Axle/Gears: EATON POSI 4.56
60 Is A High Rate Of Speed (In A Parking Lot)
OK if you tested with the separate tach when the dash tach was disconnected it’s a valid test.
To proceed further it’s useful to have an oscilloscope, and use it to monitor the key delivered voltage at the coil.
Maybe you can beg, borrow or steal (but not mine) one for the test.
If the voltage has no spikes, monitor the ignition module output for a clean wave.
The ignition module can put out a broken-up wave but the engine will behave properly due to still getting the spark at the correct time.
The tach is a frequency counter therefore can show the effects of a broken-up wave.
◙◙◙◙◙◙◙◙◙◙◙◙◙◙◙
Happy Racing!
◙◙◙◙◙◙◙◙◙◙◙◙◙◙◙
Slamming The Wall Is Not The Neatest Way To Remove An Engine
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To proceed further it’s useful to have an oscilloscope, and use it to monitor the key delivered voltage at the coil.
Maybe you can beg, borrow or steal (but not mine) one for the test.
If the voltage has no spikes, monitor the ignition module output for a clean wave.
The ignition module can put out a broken-up wave but the engine will behave properly due to still getting the spark at the correct time.
The tach is a frequency counter therefore can show the effects of a broken-up wave.
◙◙◙◙◙◙◙◙◙◙◙◙◙◙◙
Happy Racing!
◙◙◙◙◙◙◙◙◙◙◙◙◙◙◙
Slamming The Wall Is Not The Neatest Way To Remove An Engine
.
Thread Starter
Junior Member
Joined: Nov 2007
Posts: 98
Likes: 0
From: Pennsylvania
Car: 1987 Trans Am
Engine: 5.0 L
Re: Replaced distributor and coil... Tach now acting up?
I'll have to re preform the test, I left the dash tach on so I could compare both tachs to make sure they jumped at the same time and such. will do so shortly
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Joined: Oct 2004
Posts: 965
Likes: 2
From: SE, Ohio
Car: '86 Z28, '91 RS
Engine: 305ci, 305ci
Transmission: TH200c (no kidding), TH700r4
Axle/Gears: 2.73, 2.73
Re: Replaced distributor and coil... Tach now acting up?
If the motor isn't reacting to the dip/pulse and only the tach and doing it with aftermarket tachs I would save headaches and swap ignition modules. If you just bought it take it back and get a new one.
Thread Starter
Junior Member
Joined: Nov 2007
Posts: 98
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From: Pennsylvania
Car: 1987 Trans Am
Engine: 5.0 L
Re: Replaced distributor and coil... Tach now acting up?
That's what I was wondering. I did replace the ICM, that could possibly be the problem?
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