CPU's and Grounding
03-08-2014, 10:34 PM
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Track Day
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CPU's and Grounding
Hi everyone,
I am curious, do the CPU's when mounted in the tray need to be bolted to the sidewall of the car to receive a ground? Or do they receive the ground through the harnesses that are connected to them?
I am still chasing an intermittent running issue and after replacing the relays noticed the tray was loose.... apparently the last shop that worked on this car only put 2 of the screws for the LH on (and they were not tightened - I found the other 2 under the carpet) plus the whole tray was loose with only one screw on the bottom.
Any thoughts?
Thanks in advance,
Brett
1987 928 S4
Auto
I am curious, do the CPU's when mounted in the tray need to be bolted to the sidewall of the car to receive a ground? Or do they receive the ground through the harnesses that are connected to them?
I am still chasing an intermittent running issue and after replacing the relays noticed the tray was loose.... apparently the last shop that worked on this car only put 2 of the screws for the LH on (and they were not tightened - I found the other 2 under the carpet) plus the whole tray was loose with only one screw on the bottom.
Any thoughts?
Thanks in advance,
Brett
1987 928 S4
Auto
03-08-2014, 11:33 PM
#2
Team Owner
the computers are grounded via the grounding lugs on the harness, not the boxes,
To test this simply unplug your computer connectors,
swap in a good set of computers,
sit them on the floor and start the engine
To test this simply unplug your computer connectors,
swap in a good set of computers,
sit them on the floor and start the engine
03-08-2014, 11:56 PM
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Thank you for that information Mr Merlin, it is much appreciated. Okay so I am still in search of the problem - whenever it should decide to rear its ugly head again. Being that it is intermittent I still haven't found the trigger that sends everything down hill. I think it is about time I pull the CE and take a look behind it to see if everything is proper. I also seem to remember someone posting about cold solder joints on their rebuilt CPU creating a similar issue. I know mine was rebuilt in 2001 but this problem actually started circa 2009 so maybe thats worth a look also.
Thank you for taking the time out to share you knowledge.
Have a good weekend,
Brett
Thank you for taking the time out to share you knowledge.
Have a good weekend,
Brett
03-09-2014, 12:36 AM
#4
Rennlist Member
Brett, the "cold solder joints" was probably a cruise-control brain, I don't think it is an issue with the LH or EZK. The EZK, and the LH once rebuilt, are pretty robust. The various sensors, not so much.
I think the most likely culprit for an intermittent failure is the CPS (crank position sensor), conveniently located beneath the airbox and MAF, on top of the bellhousing. The sensor itself will sometimes get intermittent and eventually die in the most inopportune situation, but more often the connector crumbles and falls apart. It's worth pulling the airbox and MAF and having a good look, and also a careful check of the u-shaped fuel hose that is also under there.
Cheers, Jim
I think the most likely culprit for an intermittent failure is the CPS (crank position sensor), conveniently located beneath the airbox and MAF, on top of the bellhousing. The sensor itself will sometimes get intermittent and eventually die in the most inopportune situation, but more often the connector crumbles and falls apart. It's worth pulling the airbox and MAF and having a good look, and also a careful check of the u-shaped fuel hose that is also under there.
Cheers, Jim
03-09-2014, 01:15 AM
#5
Team Owner
Follow Jims advice
03-09-2014, 01:56 PM
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Hi Jim,
Thank you for the input. I just checked my records and the CPS was replaced in 09 while a shop was trying to diagnose this problem (note: the car has been in storage since then). I was however just under the airbox recently and replaced the harness side of the CPS connector because it looked shady. As it turned out the wires inside the harness area were cracked so I had to cut back and add in some new wire to make it all work properly.
The fuel hose you mentioned was also checked just recently and I replaced the FPR and rear fuel damper because they looked suspect.
The biggest problem is that the car runs fine (well that is stretching it - it runs okay) and then out of nowhere all of a sudden it starts doing the whole near stall gasping thing. Sometimes it happens when its hot - sometimes when I just first start it. Very strange!
This has become a very long episode of "In Search Of".....
Thank you for the ideas, any others??
Regards,
Brett
Thank you for the input. I just checked my records and the CPS was replaced in 09 while a shop was trying to diagnose this problem (note: the car has been in storage since then). I was however just under the airbox recently and replaced the harness side of the CPS connector because it looked shady. As it turned out the wires inside the harness area were cracked so I had to cut back and add in some new wire to make it all work properly.
The fuel hose you mentioned was also checked just recently and I replaced the FPR and rear fuel damper because they looked suspect.
The biggest problem is that the car runs fine (well that is stretching it - it runs okay) and then out of nowhere all of a sudden it starts doing the whole near stall gasping thing. Sometimes it happens when its hot - sometimes when I just first start it. Very strange!
This has become a very long episode of "In Search Of".....
Thank you for the ideas, any others??
Regards,
Brett
03-09-2014, 02:15 PM
#7
Rennlist Member
I assume you cleaned the ground connections on either side of back of intake valley on engine? All the engine mgmt electronics ground there.
Can't say for certain what is going on on your 87, but battery ground strap will do that too. They can look great, measure ok, but have enough corosion that they fail intermittently to pass enough electrons to satisfy the engine electronics while bouncing down the road. This is one of those 25 yea+ failure modes that emerged. Some of our most capable troubleshooters have been surprised by this affecting their cars. On battery side, the bat gasses probably degrade the cable at the crimp joint under the plastic insulation. On the car side of the strap, if you have a car that had water in the back of the car, and/or mice back there, corrosion happens near that crimp. I've experienced both on 85 and 87 cars respectively. The 85 did the intermittent skip thing, very unpredictable, then one day wouldn't restart (thats how it was discovered). The 87 just reached a point where it wouldn't start, Charles Paynes car, and he and I troubleshooted it in a parking lot.
Other things can cause it, but the valley grounds and ground strap are two possibles.
Can't say for certain what is going on on your 87, but battery ground strap will do that too. They can look great, measure ok, but have enough corosion that they fail intermittently to pass enough electrons to satisfy the engine electronics while bouncing down the road. This is one of those 25 yea+ failure modes that emerged. Some of our most capable troubleshooters have been surprised by this affecting their cars. On battery side, the bat gasses probably degrade the cable at the crimp joint under the plastic insulation. On the car side of the strap, if you have a car that had water in the back of the car, and/or mice back there, corrosion happens near that crimp. I've experienced both on 85 and 87 cars respectively. The 85 did the intermittent skip thing, very unpredictable, then one day wouldn't restart (thats how it was discovered). The 87 just reached a point where it wouldn't start, Charles Paynes car, and he and I troubleshooted it in a parking lot.
Other things can cause it, but the valley grounds and ground strap are two possibles.
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03-09-2014, 02:37 PM
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Hi Landseer,
Thank you for the advice. Yes, I did do a complete grounding check/ clean which including the two at the back of motor. I also cleaned the connector area in the engine compartment where the hot lug is. I also replaced the under car ground strap and the battery ground strap. By the way I do know exactly what you are talking about with the battery ground..... I remember pulling mine out and upon close inspection was shocked at the poor condition even though it looked "nice" from a distance!
Thank you for the advice. Yes, I did do a complete grounding check/ clean which including the two at the back of motor. I also cleaned the connector area in the engine compartment where the hot lug is. I also replaced the under car ground strap and the battery ground strap. By the way I do know exactly what you are talking about with the battery ground..... I remember pulling mine out and upon close inspection was shocked at the poor condition even though it looked "nice" from a distance!
03-09-2014, 03:15 PM
#9
Rennlist Member
Good, cross those things off! Very thorough.
Next up, CE panel. How did you deal with that so far? My approach is to replace the 10 or so 53 relays, which include the mission critical ones for LH, Fuel pump and EZK.
Inspect back carefully for any shorts / melts. Look at every wire on every plug for evidence of issues, usually a short somewhere else in the car shows evidence at those plugs of melts.
Clean the electrodes the plugs use, especially last few plugs, I think W or X, that carry the engine signals.
New fuses.
Rereading, two troubleshooters far more experienced than me pointed out CPS. They fail intermittently also. And, some dont have the metal probe end as tightly bonded to the plastic body, so it can be twisted or stressed on installation, I'd replace that again.
Next up, CE panel. How did you deal with that so far? My approach is to replace the 10 or so 53 relays, which include the mission critical ones for LH, Fuel pump and EZK.
Inspect back carefully for any shorts / melts. Look at every wire on every plug for evidence of issues, usually a short somewhere else in the car shows evidence at those plugs of melts.
Clean the electrodes the plugs use, especially last few plugs, I think W or X, that carry the engine signals.
New fuses.
Rereading, two troubleshooters far more experienced than me pointed out CPS. They fail intermittently also. And, some dont have the metal probe end as tightly bonded to the plastic body, so it can be twisted or stressed on installation, I'd replace that again.
03-09-2014, 03:28 PM
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Read through my recent saga that started out much like yours did, Brett:
https://rennlist.com/forums/928-foru...ermittent.html
https://rennlist.com/forums/928-foru...ermittent.html
03-09-2014, 05:24 PM
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Hello again - okay I am fueled up and ready to go (another words I had breakfast).
Landseer - I haven't dove into the CE panel yet. I did just replace all the 53's a few days ago plus I replaced the window winder relay because it was HOT! As it turns out I think I know why that happened.... the passenger window button was stuck in the close window position (duh!). The fuses are also all new (and I tested them before installing them to be sure). You mentioned cleaning the electodes the plugs use..... what do you mean? Is that behind the panel?
As for the CPS I am going to have to jump back in there to take another look to see if anything looks wrong. I seem to remember everything looking proper & new as the past mechanic had replaced it but as an interesting note, when he replaced the CPS the problem did not go away (this was an ongoing problem which started in 09 - I took the car to the shop 3 separate times only to have it die as I pulled out of there parking lot each time). After that I had moved to Florida so the car was in storage until about a year ago.
The whole "plastic bonded twisted stressed" thing does sound interesting so I will take another look for peace of mind.
Randy V - Thank you for the link. I did read though it but what was the final outcome with your problem? I think that may have been the thread that gave me the idea of the cold solder points....
Thanks,
Brett
Landseer - I haven't dove into the CE panel yet. I did just replace all the 53's a few days ago plus I replaced the window winder relay because it was HOT! As it turns out I think I know why that happened.... the passenger window button was stuck in the close window position (duh!). The fuses are also all new (and I tested them before installing them to be sure). You mentioned cleaning the electodes the plugs use..... what do you mean? Is that behind the panel?
As for the CPS I am going to have to jump back in there to take another look to see if anything looks wrong. I seem to remember everything looking proper & new as the past mechanic had replaced it but as an interesting note, when he replaced the CPS the problem did not go away (this was an ongoing problem which started in 09 - I took the car to the shop 3 separate times only to have it die as I pulled out of there parking lot each time). After that I had moved to Florida so the car was in storage until about a year ago.
The whole "plastic bonded twisted stressed" thing does sound interesting so I will take another look for peace of mind.
Randy V - Thank you for the link. I did read though it but what was the final outcome with your problem? I think that may have been the thread that gave me the idea of the cold solder points....
Thanks,
Brett
03-09-2014, 10:27 PM
#13
Nordschleife Master
The "plugs" he is talking about is the row of plugs on the bottom (front) of the CE panel. All the wires coming out of that area are actually in colored plugs.
Letter designated A to W (I is missing). There's a little red tab in the middle that is the locking mechanism.
If you look at the diagrams Alan is doing, you can see what I mean.
Edit to add link to Alan's thread:
https://rennlist.com/forums/928-foru...all-years.html
Letter designated A to W (I is missing). There's a little red tab in the middle that is the locking mechanism.
If you look at the diagrams Alan is doing, you can see what I mean.
Edit to add link to Alan's thread:
https://rennlist.com/forums/928-foru...all-years.html
03-09-2014, 10:40 PM
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Hi Joe,
Thank you for elaborating on that. That will work into tomorrows to do list. I have not pulled the CE panel yet - kind of got preoccupied with the coils. I decided to give them a good cleaning and indeed there is some corrosion on the inside of the connectors... plus the large black wires that connect from the brackets look shady. First up I will finish doing a cleaning on them plus take a look at the rotors & caps. At the very least that could contribute to the "not so great" running condition.
Thank you so much for all the help and suggestions - it is greatly appreciated!
Brett
Thank you for elaborating on that. That will work into tomorrows to do list. I have not pulled the CE panel yet - kind of got preoccupied with the coils. I decided to give them a good cleaning and indeed there is some corrosion on the inside of the connectors... plus the large black wires that connect from the brackets look shady. First up I will finish doing a cleaning on them plus take a look at the rotors & caps. At the very least that could contribute to the "not so great" running condition.
Thank you so much for all the help and suggestions - it is greatly appreciated!
Brett
03-10-2014, 03:02 AM
#15
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Brett,
Engines are all about air, fuel and spark-- in the right amounts, at the right time.
I've gone through your previous posts-- fuel pump, CPS, wiring repair etc. It might be helpful do recap the current status, and in particular what the symptoms are. Does the car stall completely? Or just run crappy? All the time, or just intermittently? [It sounds like crappy and not completely stalled, and just intermittent, and both warm or cold-- is that correct?]
When it runs crappy, is it lean or rich? Overly-rich is easy to detect at the exhaust, but beyond that the biggest problem is lack of reliable data.
CPS issues (sensor or wiring) usually results in a dead engine, there is generally no "running poorly". Temp-II, MAF and LH issues usually (not always) throw the fueling off (rich or lean) and result in poor running. Grounding and relay issues usually (not always) result in dead engine. Hall and knock-sensors just result in a lack of power (6-deg retard by the EZK). Ignition problems (wires, coils, grounds) generally (not always) exhibit as missing under load, and a bad coil wire can mimic poor fueling.
One often-overlooked test is the fuel-volume test called out in the WSM: Disconnect the fuel-return hose at the top of the tank (at the fuel level-sensor), connect an extension and route that to a gallon jug. Then jumper the fuel pump for 30 seconds, it should pump 1.25L into the jug. The beauty here is that the fuel is pumped from the tank through the filter, regulator and return line, back to the tank-- the complete loop.
Are you confident in the CPS wiring repair? Soldered connections, properly insulated?
Is there anyone nearby that you can swap MAF's and LH's with? I understand they are both fairly recent rebuilt's, but when the likely suspects are all ruled out then the unlikely becomes probable...
Engines are all about air, fuel and spark-- in the right amounts, at the right time.
I've gone through your previous posts-- fuel pump, CPS, wiring repair etc. It might be helpful do recap the current status, and in particular what the symptoms are. Does the car stall completely? Or just run crappy? All the time, or just intermittently? [It sounds like crappy and not completely stalled, and just intermittent, and both warm or cold-- is that correct?]
When it runs crappy, is it lean or rich? Overly-rich is easy to detect at the exhaust, but beyond that the biggest problem is lack of reliable data.
CPS issues (sensor or wiring) usually results in a dead engine, there is generally no "running poorly". Temp-II, MAF and LH issues usually (not always) throw the fueling off (rich or lean) and result in poor running. Grounding and relay issues usually (not always) result in dead engine. Hall and knock-sensors just result in a lack of power (6-deg retard by the EZK). Ignition problems (wires, coils, grounds) generally (not always) exhibit as missing under load, and a bad coil wire can mimic poor fueling.
One often-overlooked test is the fuel-volume test called out in the WSM: Disconnect the fuel-return hose at the top of the tank (at the fuel level-sensor), connect an extension and route that to a gallon jug. Then jumper the fuel pump for 30 seconds, it should pump 1.25L into the jug. The beauty here is that the fuel is pumped from the tank through the filter, regulator and return line, back to the tank-- the complete loop.
Are you confident in the CPS wiring repair? Soldered connections, properly insulated?
Is there anyone nearby that you can swap MAF's and LH's with? I understand they are both fairly recent rebuilt's, but when the likely suspects are all ruled out then the unlikely becomes probable...