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Old 11-16-2014, 01:28 PM
Glen's Avatar
...auto enthusiast
 
Join Date: Sep 2000
Location: Carlsbad, CA USA
Posts: 1,187
'95 E320 smog pump relay?

Hi guys,
Need a little help troubleshooting DM code 5 issue. In the 14 years I've had this car, this code has come back a few times over the years...so far with different issues each time.

The check engine light has been coming on for a couple weeks now, generally it only takes 3 to 4 restarts after clearing to come back.

Secondary air tube is clear and EGR valve works...tested at idle with vacuum source to EGR valve, this caused RPM drop and slight miss.

The air pump itself also works when 12V is applied directly.

I think the issue is that the computer is not able to engage the air pump. I used to hear it run occasionally, now I don't.

Is there a fuse or relay for the pump? If so, where is it?

Thanks,
Glen
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Old 11-16-2014, 03:07 PM
oldtrucker's Avatar
BMW Mech (70's) Germany
 
Join Date: Jan 2014
Location: San Diego County
Posts: 131
Hi Glen,
the air pump is only supposed to come on at low temperature until the engine reaches a preset temp.
As far as I can tell, it is to fake that the engine is running lean by applying additional air and has the computer, via O2 sensor, adjust for a richer run, again until the temp is rising. (Cold Start / Run)
When and how long it will run depends on ambient and coolant temperature.

Have you checked the entire pipe from the EGR valve to the intake manifold? The pipe is hard to reach at the manifold side. In my experience, it will clog right within an inch from the manifold flange. It makes sense because everything has cooled down there.
I had been chasing this problem for a long time, not believing that it was clogged.
Finally I unscrewed both ends, since the speedometer cable rotor rooter didn't work for me. I cleaned out lots of carbon deposits, but carefully, don't want to scratch the inside of the tube.
Put it back together started the engine and viola, the code and CEL was gone.
Other than that, most of the indirectly triggered codes are merely guesses by the firmware.
One really has to troubleshoot by eliminating trouble sources and the pipe is one.

Additional:
I just had a look at one of them many diagrams.
It doesn't seem that the airpump components are fused individually.
Y32 (air pump switch) and Y33 (electro mag clutch) are controlled directly (Simultaneous) from its control module N3/4 (Engine control module (Pin 15)).
It's hard to access the ECM at pin 15 if not impossible, with an DVM. If, you can get to it, you could check if the signal actually leaves the module e.g. toggles at pin 15 with respect to chassis ground.
Be careful, ESD (Electro Static Discharge) or spikes can occur if powerd up, e.g. battery connected. if you take the connector off.
Another, probably easier way is to measure at the pump or the switch over valve. One lead should be attached to chassis and the other one the control.
If it doesn't toggle, it may be too warm. If it is freezing cold, then there is something fishy... but your engine should run crappy until warmed up.
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Mostly, I don't know notin, I just know where to look.
I am looking back, to over 30 years in Electronics Design.
Electrons don't care if they move in a car, computer or relay!

95 W124 E320 M104.992 - Because, I love to repair, naaaah!
Over 221,000 Miles
Cheers,
Norbert

Last edited by oldtrucker; 11-16-2014 at 04:31 PM. Reason: Additional Info
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  #3  
Old 11-17-2014, 04:19 PM
Glen's Avatar
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Join Date: Sep 2000
Location: Carlsbad, CA USA
Posts: 1,187
Thanks for the response.
It's possible it hasn't been cool enough for the pump to come on, I will pay more attention now that it's getting cooler here in SoCal.

As far the tube being clear. I've cleared it out twice over the last 14 years, engine now has 320K miles but I had the head rebuilt 30K miles ago and the tube was cleaned then too. And as I said, if I apply vacuum to the EGR valve at idle, RPM drops indicating that the tube is clear.

I'll do some more diagnostics this coming weekend and see what I find.

Thanks again!
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