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2020 Giulia TI,1988 Milano Gold Auto
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Discussion Starter #1
Does anyone know the exact function of this 7.5A Fuse located behind the 15A fuel pump fuse? During the ongoing sorting of a 70,000 mile 94LS I found that fuse was completely missing. Could this have been the cause of a (very) intermittent check engine light that did not set a code? Put a new f use in and took 15 minute drive. No check engine light yet.I'm not claiming success yet,will need some more drive time. Any ideas?
 

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1991 164L
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27,582 Posts
Does anyone know the exact function of this 7.5A Fuse located behind the 15A fuel pump fuse? During the ongoing sorting of a 70,000 mile 94LS I found that fuse was completely missing. Could this have been the cause of a (very) intermittent check engine light that did not set a code? Put a new f use in and took 15 minute drive. No check engine light yet.I'm not claiming success yet,will need some more drive time. Any ideas?
That 7.5A fuse is for O2 sensor heater only on 24v 94-95 models. Heater not fused on 91-93 12v models. Probably also used on later Euro models and all engine choices, too.
 

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Cross your fingers because the answer is maybe and maybe not as far as the CEL goes. Keep us updated please.
Charles
 

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Premium Member
2020 Giulia TI,1988 Milano Gold Auto
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751 Posts
Discussion Starter #4
Today another 30 mile drive, still no CEL. Hoping this cures the problem I've had since buying the car last year. The light has been so intermittent, but seemed to come on more during acceleration rather than steady speed, although sometimes would come on at highway speeds. The fact that it would not set code was particularly perplexing. Seemed to get better when 02 sensor was replaced.
John
 

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2020 Giulia TI,1988 Milano Gold Auto
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Discussion Starter #5
It's been a week now, and the CEL has not even flashed once. I'm hoping this is another small problem solved. Steve, is it possible that lack of 12V to the 02 sensor could cause the intermittent light? When I first got the car last year, it would set code for 02 sensor, so sensor was replaced. However, after sensor replaced, CEL came on less often, but no code was set. Thanks for any input.
John
 

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Premium Member
1991 164L
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It's been a week now, and the CEL has not even flashed once. I'm hoping this is another small problem solved. Steve, is it possible that lack of 12V to the 02 sensor could cause the intermittent light? When I first got the car last year, it would set code for 02 sensor, so sensor was replaced. However, after sensor replaced, CEL came on less often, but no code was set. Thanks for any input.
John
Let's say yes. Both the 12v and 24v models have O2 sensor heaters but only 24v has heater wires fused and a second black wire going into ECU beside green one. I have to guess the heater helps the sensor worked better.

I don't really know how it all works mostly electronic magic to me.

Enjoy the ride.
 

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But Mad North-Northwest
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It's been a week now, and the CEL has not even flashed once. I'm hoping this is another small problem solved. Steve, is it possible that lack of 12V to the 02 sensor could cause the intermittent light? When I first got the car last year, it would set code for 02 sensor, so sensor was replaced. However, after sensor replaced, CEL came on less often, but no code was set. Thanks for any input.
John
Sure, makes sense. If the heater isn't working the sensor can get too cool to operate properly after a period of idling, which can affect the output. The computer would then throw an "lambda out of range" error until the sensor heats up enough to give proper voltage readings.

My guess is the heater element in the old sensor was shorted, which was blowing the fuse. The PO just pulled the fuse rather than replace the sensor. Saw this happen on a buddy's Spider, except in that case the heater is fused along with the fuel pumps, so when the heater popped the fuse the car wouldn't run!
 

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1991 164L
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Sure, makes sense. If the heater isn't working the sensor can get too cool to operate properly after a period of idling, which can affect the output. The computer would then throw an "lambda out of range" error until the sensor heats up enough to give proper voltage readings.

My guess is the heater element in the old sensor was shorted, which was blowing the fuse. The PO just pulled the fuse rather than replace the sensor. Saw this happen on a buddy's Spider, except in that case the heater is fused along with the fuel pumps, so when the heater popped the fuse the car wouldn't run!
That is how the 91-93 164 O2 sensor is operated, too by using fuel pump relay but with no seperate fuse for O2 sensor just whole system 15A power fuse. Guess I have been lucky never had O2 sensor or fuel pump or computer blow that system fuse.

Now 94-95 164 has seperate 15A fuel pump fuse and 7.5A O2 sensor heater fuse but both still get their power off fuel pump relay.

Somebody on here had that 15S system fuse keep blowing because fuel pump relay also powers the evap cannister solenoid in left front wheel wheel and it was corroded and shorted.
 

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Question;

With ignition on (engine not powered up), what's the readings I should get at the o2 connectors (wiring harness end, NOT the o2 sensor wires itself.)?

As you may be aware, I have the rough idle / intermittent stalling issue which now seems to have got a little worse, engine dies almost all the time unless I hit the accelerator. No air leaks (double checked), clean connector at evap solenoid, seemingly OK IAC valve, new plugs, new wires etc. Suspicion now lies on either O2 Sensor itself, wiring / relay to o2 heat element, AFM or probably a totally bad evap canister.
The problem is only at idle. Starts up perfectly (no drag / lag), accelerates perfectly (no hesitation like it starves for fuel), drives fine etc. But, when I approach traffic by pressing clutch and releasing accelerator (engine goes to idle), the engine stalls out almost always now. Previously, it was intermittent.

After erasing codes, and running around, I now get 1223 error.
Previously, I've got errors 1244, 1221 also.
But upon resetting, it goes away and never returns. 1223 is the only code that returns.

Anything else that I need to suspect?

I checked 7.5A fuse, it's visibly OK and not burnt out.
How do I check if the relay (on which the 7.5A fuse is connected to) for the o2 heat element is bad?

My TS has two 2-pin connectors for the o2 sensor.

I used a multimeter to check voltage on o2 connector wires (wiring harness end, not the o2 wires)
With ignition on and engine not cranked, I get 0v, 0v, 0.14v, 0.13v in each of the 4 wires I tested.
Should I not be getting 12v in at least one wire??

If so, what should I do next? Probably a bad wiring or relay?

Thank you.
Zaid
 
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