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Discussion Starter #1
Hi guys,

Back in October, my "EMI" fuse (15 amp, powers transmission shift solenoids & O2 sensor heaters) blew while driving on highway (car dropped from 4th to 3rd unexpectedly, SES lit up).
Got home, checked for shorts on O2 sensor wiring, none found. Replaced with new 15 amp fuse, problem solved.
Today, while at full throttle, car shifted to 2nd way early, SES lit up.
Checked fuse, blown again. I've replaced it but haven't driven yet.
Anyone else have problems with this fuse blowing, what could be root cause, assuming wiring is not chafed/shorted?

Thanks
 

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If you can scan live data you could check that the O2 sensors are working.


If you start blowing this fuse regularly try unplugging the O2 sensors. If the problem stops it may be one of the heaters inside the O2s.


If I understand you correctly it has blown once at steady throttle (transmission not changing) and once at WOT (transmission trying to shift). This would lower the probability that a shift solenoid is bad but does not rule out bad wiring.
 

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Discussion Starter #3
If you can scan live data you could check that the O2 sensors are working.


If you start blowing this fuse regularly try unplugging the O2 sensors. If the problem stops it may be one of the heaters inside the O2s.


If I understand you correctly it has blown once at steady throttle (transmission not changing) and once at WOT (transmission trying to shift). This would lower the probability that a shift solenoid is bad but does not rule out bad wiring.
Correct, fuse blew once at steady throttle, once at WOT.

Is it possible for an O2 sensor to be malfunctioning/faulty without the SES lit up?

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Is it possible for an O2 sensor to be malfunctioning/faulty without the SES lit up?
The claim on the internet is that contamination will slow down the sensor's reaction to lean/rich and the PCM will not activate the SES(MIL malfunction indicator lamp). I have been working on a Crown Vic and found out that some PCMs monitor the current to the O2 heaters and some O2 diagnostics suggest ensuring the exhaust has a good ground for the heaters.(I wounder how many "slow O2s " were replaced for just having a bad ground?)



A O2 with a damaged heater will not react the same as a good one. I was hoping that the heater might be banging around and going open(cool) as well as shorting out. If this was happening you would see it in the O2 numbers.



my "EMI" fuse (15 amp, powers transmission shift solenoids & O2 sensor heaters)
Usually the 94 and 95 FSM are the same. When I looked up the EMI fuse it does not match your discriptiion. Where did you get your information?



EMI fuse #4 10 Amp O2 sensors, EGR solenoid, Emission canister solenoid, MAF, air injector pump, and underhood light.


Trans fuse #5 10 Amp TCC and shift solenoids



Both are 10 amp not 15 amp.


If it is the EMI fuse I remember reports of problems with the air injector pump, and underhood light blowing fuses. A forum search of air pump fuse will give many delete listing but maybe some help. I would spend more time with this than a low probability O2 issue.

Have a look at the engine wiring for all parts connected to the fuse.
 

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Discussion Starter #5 (Edited)
The claim on the internet is that contamination will slow down the sensor's reaction to lean/rich and the PCM will not activate the SES(MIL malfunction indicator lamp). I have been working on a Crown Vic and found out that some PCMs monitor the current to the O2 heaters and some O2 diagnostics suggest ensuring the exhaust has a good ground for the heaters.(I wounder how many "slow O2s " were replaced for just having a bad ground?)



A O2 with a damaged heater will not react the same as a good one. I was hoping that the heater might be banging around and going open(cool) as well as shorting out. If this was happening you would see it in the O2 numbers.




Usually the 94 and 95 FSM are the same. When I looked up the EMI fuse it does not match your discriptiion. Where did you get your information?



EMI fuse #4 10 Amp O2 sensors, EGR solenoid, Emission canister solenoid, MAF, air injector pump, and underhood light.


Trans fuse #5 10 Amp TCC and shift solenoids



Both are 10 amp not 15 amp.


If it is the EMI fuse I remember reports of problems with the air injector pump, and underhood light blowing fuses. A forum search of air pump fuse will give many delete listing but maybe some help. I would spend more time with this than a low probability O2 issue.

Have a look at the engine wiring for all parts connected to the fuse.
I wouldn't think it makes any difference, but my car is a 95 RMW.
"Automatic Transmission" is what's indicated in owner's manual for fuse #4. Car won't engage 1st or 4th when this fuse is open.
It's been a 15 amp fuse since I bought the car... hmmm...
My air pump is deleted.
 

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Usually the 94 and 95 FSM are the same.
I wouldn't think it makes any difference, but my car is a 95 RMW.
I would like to help but I do not have a 95 FSM. The ABS changed from 94 to 95 and it looks like they moved some fuses around.



"Automatic Transmission" is what's indicated in owner's manual for fuse #4. Car won't engage 1st or 4th when this fuse is open.
Can't argue with results. On a 96 fuse #4 is shown as 20 Amp and you would lose all the solenoids.But this is OBD2



94 and 96 diagrams are all here:


Goldswagon diagrams


But it does not look like 95 is the same as 94. I have found errors in the 94 FSM but you have to start somewhere.

If you have no way of getting a circuit diagram for a 95 I could get one in a few days from Alldata.



I would hope someone else will post what the fuse connects to on a 95.
 

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Discussion Starter #7
The claim on the internet is that contamination will slow down the sensor's reaction to lean/rich and the PCM will not activate the SES(MIL malfunction indicator lamp). I have been working on a Crown Vic and found out that some PCMs monitor the current to the O2 heaters and some O2 diagnostics suggest ensuring the exhaust has a good ground for the heaters.(I wounder how many "slow O2s " were replaced for just having a bad ground?)



A O2 with a damaged heater will not react the same as a good one. I was hoping that the heater might be banging around and going open(cool) as well as shorting out. If this was happening you would see it in the O2 numbers.




Usually the 94 and 95 FSM are the same. When I looked up the EMI fuse it does not match your discriptiion. Where did you get your information?



EMI fuse #4 10 Amp O2 sensors, EGR solenoid, Emission canister solenoid, MAF, air injector pump, and underhood light.


Trans fuse #5 10 Amp TCC and shift solenoids



Both are 10 amp not 15 amp.


If it is the EMI fuse I remember reports of problems with the air injector pump, and underhood light blowing fuses. A forum search of air pump fuse will give many delete listing but maybe some help. I would spend more time with this than a low probability O2 issue.

Have a look at the engine wiring for all parts connected to the fuse.
Looks like EMI fuse is 15 amp...
 

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The color coding on the lid is great since the OM does not list sizes. Keep a eye out for the three book FSM it usually pays off in one use.



A inspection of the EGR solenoid, Emission canister solenoid, MAF, air injector pump, and underhood light wiring would be first on my list. Next would be a wiggle test with the car running. Careful bending and shaking of the wire harness may trigger a short if that is the problem.


There is nothing so far that rules out a bad component but that is harder to test for. You mentioned no issues with the transmission.



Have you replaced O2s, done exhaust work, engine or transmission removal? This can cause damage on old wiring.



Intermittent problems are the worst to sort out. Total parts failure is much easier.
 

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I searched the forum for "fuse 4"


Burned harness on the drivers side o2.
https://www.impalassforum.com/vBulletin/34-engine-problems-maintenance/244756-emi-fuse-what-all-goes-through.html


it eneded up being the oxygen sensor that was shorting the fuse out.
https://www.impalassforum.com/vBulletin/26-electrical/1027649-emi-fuse.html


There were several members suggesting that O2 wiring burning was a problem. It would be harder to prove a O2 fail. Unplugging the O2 will start codes but would be one way.

There was a thread suggesting a bad transmission solenoid but no verified outcome. If I had no other choice I would cut the power wire to the transmission and add a inline fuse(maybe 7.5-10 Amp) and see if it would blow before the main fuse. This would be my very last choice. I would hope you would feel transmission issues or be able to pull transmission codes instead of doing this.

Your turn to search EMI fuse.
 

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Discussion Starter #10
After learning that 96 cars have a 20 amp fuse, I replaced my 15 with a 20 amp. So far, so good.
The 96 cars have 2 more O2 sensors on this circuit, but I don't know if the wire on the load side of the 96's EMI fuse is a thicker gage than a 95's.
 
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