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really WEIRD engine electrical hiccup. .. by gtxbullet
Started on: 06-08-2014 01:08 PM
Replies: 7 (106 views)
Last post by: phonedawgz on 06-08-2014 10:07 PM
gtxbullet
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Report this Post06-08-2014 01:08 PM Click Here to See the Profile for gtxbulletSend a Private Message to gtxbulletEdit/Delete MessageReply w/QuoteDirect Link to This Post
yesterday and today my 1988 Pontiac Fiero base coupe 5 speed had an electrical issue regarding the engine.
The tachometer would jump a few thousand rpm for a split second and almost bog-out when accelerating....

any thoughts
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Custom2M4
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Report this Post06-08-2014 02:43 PM Click Here to See the Profile for Custom2M4Click Here to Email Custom2M4Send a Private Message to Custom2M4Edit/Delete MessageReply w/QuoteDirect Link to This Post
Sounds like an ignition control module.

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phonedawgz
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Report this Post06-08-2014 05:56 PM Click Here to See the Profile for phonedawgzClick Here to visit phonedawgz's HomePageClick Here to Email phonedawgzSend a Private Message to phonedawgzEdit/Delete MessageReply w/QuoteDirect Link to This Post
Better filling of the cylinders with air (lower RPM and open throttle) causes more air to be in the cylinder. More air means more voltage is needed to jump the spark plug gap. Higher voltage in the secondary means weak insulation can be jumped. That is why this trouble occurs at low RPM high throttle times. Secondary voltage is jumping through the coil from the secondary to the primary. Check the resistance of your plug wires. They should be less than 10K per foot. Look at your distributor posts and rotor for corrosion. Make sure your plugs are gaped correctly. Then replace your ignition coil.

Wait long enough and your ICM will fail also.

[This message has been edited by phonedawgz (edited 06-08-2014).]

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Custom2M4
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Report this Post06-08-2014 09:07 PM Click Here to See the Profile for Custom2M4Click Here to Email Custom2M4Send a Private Message to Custom2M4Edit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by phonedawgz:

Better filling of the cylinders with air (lower RPM and open throttle) causes more air to be in the cylinder. More air means more voltage is needed to jump the spark plug gap. Higher voltage in the secondary means weak insulation can be jumped. That is why this trouble occurs at low RPM high throttle times. Secondary voltage is jumping through the coil from the secondary to the primary. Check the resistance of your plug wires. They should be less than 10K per foot. Look at your distributor posts and rotor for corrosion. Make sure your plugs are gaped correctly. Then replace your ignition coil.

Wait long enough and your ICM will fail also.





How would this effect the tach? He didn't say the engine jumped, just the tach..
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gtxbullet
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Report this Post06-08-2014 09:22 PM Click Here to See the Profile for gtxbulletSend a Private Message to gtxbulletEdit/Delete MessageReply w/QuoteDirect Link to This Post
Correct. Just the tach jumped and the engine kinda cut out.
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FieroJimmy
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Report this Post06-08-2014 09:36 PM Click Here to See the Profile for FieroJimmySend a Private Message to FieroJimmyEdit/Delete MessageReply w/QuoteDirect Link to This Post
The tach jumped because the ignition module was firing the coil too fast, causing the stumble. This is definitely an ignition fault. Likely the module, possibly caused by plugs/wires/cap and rotor.
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gtxbullet
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Report this Post06-08-2014 09:40 PM Click Here to See the Profile for gtxbulletSend a Private Message to gtxbulletEdit/Delete MessageReply w/QuoteDirect Link to This Post
I have DIS ignition as it is an 88 2.5 coupe.

The coils are new as off today. The wires are fine and Ill check the plugs again but they will probably be fine as they are less than a year old.
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phonedawgz
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Report this Post06-08-2014 10:07 PM Click Here to See the Profile for phonedawgzClick Here to visit phonedawgz's HomePageClick Here to Email phonedawgzSend a Private Message to phonedawgzEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by Custom2M4:

How would this effect the tach? He didn't say the engine jumped, just the tach..


The ICM generates an ignition pulse about 10 deg btdc. The ICM passes that pulse onto the ECM on the Purple/White wire. It is and also out sent out of the ICM on the tach port. Once then engine is rotating over 400 RPM the ECM puts a voltage on the Tan/Black that the ICM should be firing now on the ECM's generated signal. That signal will have the spark timing occurring perhaps 40 deg BTDC. The ECM sends that signal on the White wire and that signal is what is used to actually fire the coil. (Go figure that how does the ECM generate a signal that occurs BEFORE the reference signal from the ICM. It does)

Anyways that signal is ran into the coil and the coil produces that high voltage output. If that high voltage output instead of jumping the spark plug gap instead shorts back to the ICM, there will be a pulse at about 40 BTDC and a second pulse at 10 BTDC.
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