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  #1  
Old 05-17-2004, 06:36 PM
Eiknujrac's Avatar
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1991 300E Diagnostic

My car has finally given one of the dreaded, incredibly uninformative "check engine light"s

I was told there was multiple ways to diagnose this light ranging from complete diagnostic systems to simple home-brewed impulse counters.

I was also told that you could use a multimeter.
Now i have a couple multimeters laying around, but do they have to be able to read something special to be able to be used for diagnostics?

Also, I'm somewhat of a car junkie, and I take every oppurtunity to get my hands dirty, but when it comes to diagnostics I'm utterly useless.

I know there is some sort of connector on the driver's side fender, a circle shaped 9-pin connecter, but is this the diagnostic port?

Assuming it is, how would I go about using the multimeter (assuming i can) to read the codes?


I know this is alot of questions, but any help would be appreciated.

Thanks in advance,
-E
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  #2  
Old 05-17-2004, 07:30 PM
LarryBible
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You don't indicate your location. It is different on a California car from a Federal (49 state) car.

Have a great day,
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  #3  
Old 05-17-2004, 07:38 PM
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It's a Federal, I'm in Texas
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  #4  
Old 05-17-2004, 08:57 PM
LarryBible
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Your best bet is to use a multimeter with a %Duty Cycle scale. If a code is set it will be a very accurate multiple of 10% with ignition on, engine off.

So, there are only ten codes. Use the duty cycle meter on pin 3 of the round connector on the ignition module and ground the other lead. The ignition module is on the left fenderwell.

Give us the %code and we will decipher it for you.

There are some guys who say that they have successfully done this measurement with a voltmeter. Since I have my $29.95 special meter with duty cycle range, I haven't tried that trick. If you don't have a duty cycle meter you might do a search to get the information.

Good luck,
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  #5  
Old 05-19-2004, 12:27 AM
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What duty-cycle multimeter would you recommend, or what multimeter with duty-cycle is the best deal?
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  #6  
Old 05-19-2004, 05:09 AM
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multimeter

Sears has one for approx $30.00. It has been mentioned numerous times on this site. I bought one myself and the duty-cycle function worked great for mixture adjustment.
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  #7  
Old 05-19-2004, 08:35 PM
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But you can use the duty-cycle to read fault codes from a check engine light right?
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  #8  
Old 05-20-2004, 09:00 AM
LarryBible
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Yes, there are 10 or less codes that are indicated by the duty cycle in multiples of 10. If you have an old fashioned dwell meter, you can also read these with some simple math. Dwell is just duty cycle except the meter scale is graduated in degrees instead of %.

Good luck,
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