LH Question
10-27-2010, 03:52 PM
#16
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Should have started here, clean the battery connections all of them, Then
get a fuse relay chart here 928gt.com on page 2 under tips and links print one off this will let you see what the correct relay and position it should be in .
As Wally said the computer is probably OK ,
BUT it is probably a relay thats bad.
check all of the fuses and make sure the correct fuse is in the correct slot same goes for the relays make sure the correct relay is installed as well as the empty spots being empty.
Clean all of the fuses with an eraser, clean the ground connections above the CE panel there are 9 of them on the 2 bolts just above the top edge.
Remove all of the relays and check them for corrosion,
any relay that has corroded pins should be opened and smelled for burning and the internal contracts cleaned.
Remove the computer plug connections and inspect for corrosion( just to the right of the CE panel)
Check the hot post connections and the 14 pin connector above it for corrosion.
Check that all of the sensor plugs are installed on the engine,
Note on the aircleaner temp sensor make sure the idle valve harness isnt crossed with the temp sensor,
IIRC the temp sensor wires will have 5 volts and the ISV will have 12V ,
if they have been swapped then the temp sensor is toast,
turn on the key to run and check with a volt meter.
Also check the throttle position sensor wires have not come off,
its pretty difficult to see but its on the right side of the intake as sitting in the car.
Make sure the temp 2 sensor on the water bridge is connected ( bosch type connector).
To test it , one pin to ground,
then the other pin to ground,
Note it is a dual sensor feeding 2 different systems, so each pin to ground is the testing mode.
Make sure the MAF sensor wires and boot are in good condition.
Report back
get a fuse relay chart here 928gt.com on page 2 under tips and links print one off this will let you see what the correct relay and position it should be in .
As Wally said the computer is probably OK ,
BUT it is probably a relay thats bad.
check all of the fuses and make sure the correct fuse is in the correct slot same goes for the relays make sure the correct relay is installed as well as the empty spots being empty.
Clean all of the fuses with an eraser, clean the ground connections above the CE panel there are 9 of them on the 2 bolts just above the top edge.
Remove all of the relays and check them for corrosion,
any relay that has corroded pins should be opened and smelled for burning and the internal contracts cleaned.
Remove the computer plug connections and inspect for corrosion( just to the right of the CE panel)
Check the hot post connections and the 14 pin connector above it for corrosion.
Check that all of the sensor plugs are installed on the engine,
Note on the aircleaner temp sensor make sure the idle valve harness isnt crossed with the temp sensor,
IIRC the temp sensor wires will have 5 volts and the ISV will have 12V ,
if they have been swapped then the temp sensor is toast,
turn on the key to run and check with a volt meter.
Also check the throttle position sensor wires have not come off,
its pretty difficult to see but its on the right side of the intake as sitting in the car.
Make sure the temp 2 sensor on the water bridge is connected ( bosch type connector).
To test it , one pin to ground,
then the other pin to ground,
Note it is a dual sensor feeding 2 different systems, so each pin to ground is the testing mode.
Make sure the MAF sensor wires and boot are in good condition.
Report back
10-27-2010, 05:19 PM
#17
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I think the problem for most of us is that the LH must be happy in order to energize the fuel pump relay.
Most of us (at least me) don't understand exactly which LH sensor-interuptus is going to prevent that energization.
When these cars are pulled out of long storage, those of us who "rescue" them from multi-year naps or lack of maintenance generally find its simpler to go through all the sensors, grounds, relays and CE panel to clean. Usually, the fuel pump being dead/jammed + no power to it, is simply one piece of a multi-factor no-start condition.
I can tell you that the alarm can interrupt the EZH ignition relay (edit) VIII. The ignition relay sends power to the LH. The LH decides when it will drive the fuel pump. The LH relay has some functionality, too, that confuses me, see below link....
In this link, Rich talks about a difference between what you are working on and the 87 S4, contrasting the function of th LH relay between years (as ref / just trying to help here). He's saying that for the pre 87 LH cars, the LH relay drives nothing more than the hot wire burnoff of the MAF ---- https://rennlist.com/forums/928-foru...lh-wiring.html
Here's another partial explain / clue set from one of the applicable test plans. Note here the reference to the ignition switch, and guess what, we see them routinely fail on the old neglected 928 also:
Most of us (at least me) don't understand exactly which LH sensor-interuptus is going to prevent that energization.
When these cars are pulled out of long storage, those of us who "rescue" them from multi-year naps or lack of maintenance generally find its simpler to go through all the sensors, grounds, relays and CE panel to clean. Usually, the fuel pump being dead/jammed + no power to it, is simply one piece of a multi-factor no-start condition.
I can tell you that the alarm can interrupt the EZH ignition relay (edit) VIII. The ignition relay sends power to the LH. The LH decides when it will drive the fuel pump. The LH relay has some functionality, too, that confuses me, see below link....
In this link, Rich talks about a difference between what you are working on and the 87 S4, contrasting the function of th LH relay between years (as ref / just trying to help here). He's saying that for the pre 87 LH cars, the LH relay drives nothing more than the hot wire burnoff of the MAF ---- https://rennlist.com/forums/928-foru...lh-wiring.html
Here's another partial explain / clue set from one of the applicable test plans. Note here the reference to the ignition switch, and guess what, we see them routinely fail on the old neglected 928 also:
Last edited by Landseer; 10-28-2010 at 12:26 AM.
10-27-2010, 05:38 PM
#18
Rennlist Member
If you read my no start" file on my website you will see how the flow of RPM info triggers the EZ-F which in turn wakes up the LH ECU. The LH controls fuel punp, O2 sesnor, ISV, injectors..... all the things you have mentioned.
Now, please check you don't have a kickdown relay fitted if the car is a 5 speed......
Now, please check you don't have a kickdown relay fitted if the car is a 5 speed......
10-27-2010, 05:54 PM
#19
Team Owner
Hi Doc the purpose of my post is to provide you a checklist follow besides the WSM.
If you go through all of the steps, there is a strong possibility that you will find the problem .
]It sucks when someone with a 928 does an intake refresh to cure a running problem only to find out the battery terminal and or the LH relay were bad, or the fuel pump fuse was corroded, since you not familiar with the 928,
doing the things I posted will get you through the common failure points
Note there are 4 important relays that should all be replaced LH, EZK, Fuel pump and ignition, they are all 53 relays , if you like replace one at a time to isolate the issue but replace these 4.
Cleaning battery terminals, hot post and fuse/ relay connections is the next big non running issue.
How many times has it been posted i just washed my car and it wont run
Well, things to look at,
rear ground strap to hatch floor it may have gotten wet and its corroded
No hot post cover fitted and the 14 pin connector is wet and shorting
Water has dripped onto the CE panel from a bad blower box seal or clogged well drain and shorted an important relay or fuse or both, be specific on your repair sequence the more detailed info you provide the easier it will be to point you in the right direction,
BUT first follow the outline, then you know you will have ruled out common issues
Oh and please do as Wally said ,make sure the relay is not in for the kickdown position on the CE panel if you have a 5 speed.
this relay in that position will prevent the engine from running its happened more than a few times
If you go through all of the steps, there is a strong possibility that you will find the problem .
]It sucks when someone with a 928 does an intake refresh to cure a running problem only to find out the battery terminal and or the LH relay were bad, or the fuel pump fuse was corroded, since you not familiar with the 928,
doing the things I posted will get you through the common failure points
Note there are 4 important relays that should all be replaced LH, EZK, Fuel pump and ignition, they are all 53 relays , if you like replace one at a time to isolate the issue but replace these 4.
Cleaning battery terminals, hot post and fuse/ relay connections is the next big non running issue.
How many times has it been posted i just washed my car and it wont run
Well, things to look at,
rear ground strap to hatch floor it may have gotten wet and its corroded
No hot post cover fitted and the 14 pin connector is wet and shorting
Water has dripped onto the CE panel from a bad blower box seal or clogged well drain and shorted an important relay or fuse or both, be specific on your repair sequence the more detailed info you provide the easier it will be to point you in the right direction,
BUT first follow the outline, then you know you will have ruled out common issues
Oh and please do as Wally said ,make sure the relay is not in for the kickdown position on the CE panel if you have a 5 speed.
this relay in that position will prevent the engine from running its happened more than a few times
10-28-2010, 12:22 AM
#20
Burning Brakes
Relays seem to be the theme here, and it makes sense, but I find that for diagnostic purposes its best to jumper the three critical relays to get that out of the equation. If it doesn't start with them jumpered then you can look elsewhere. My '85 will not start if the Temp II sensor is disconnected, don't ask how I know, but a bad sensor can behave just like a disconnected one.
10-28-2010, 12:52 AM
#22
Team Owner
installing fresh relays into the 4 positions one at a time will assist in isolating the running problem if it is a relay issue,
jumpering the elex system may work for one relay,
but doing multiple relay positions isnt the smart thing to do IMHO
jumpering the elex system may work for one relay,
but doing multiple relay positions isnt the smart thing to do IMHO
10-28-2010, 05:26 AM
#23
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The best way to quickly find out if relays are the problem is to unplug the LH, EZF (or EZK if '87 or newer) and fuel pump relays. Check that there is battery voltage at all 30 positions at the relay socket.
Next, run a jumper wire from 30 to 87 on all 3. Crank the car and if it starts, there is a problem with relays OR the control circuit to the relays (what turns them on). You'll have to unplug one of the jumpers to turn the engine off. Now, unplug the jumpers one by one and plug the relay back in. When the engine doesn't start, that is the socket with the bad relay or problem with the control signal to the relay.
If the car doesn't start with all 3 relay positions jumpered, the problem isn't with relays or the control circuit and you look elsewhere. The factory workshop manuals have very detailed diagnostic trees that will lead you to the problem.
Always look for the low cost stuff first, grounds to chassis, ground strap from the engine to the chassis, battery connections, fuse connection etc.
Other than different pin-outs at the connectors, the 25 pin and 35 pin LH systems are quite similar. The main difference is that in the 1984 (Euro) and 1985 - 1986 928s the fuel pump relay powers the fuel injectors. In the 1987 and new 928s the LH relay powers the fuel injectors. In all cases the LH (and L-Jet) modules just provide the switch between the fuel injectors and ground to turn them on and off. So don't over look checking to ensure that there is battery voltage at the fuel injectors when the associated relay socket is jumpered between 30 and 87.
Next, run a jumper wire from 30 to 87 on all 3. Crank the car and if it starts, there is a problem with relays OR the control circuit to the relays (what turns them on). You'll have to unplug one of the jumpers to turn the engine off. Now, unplug the jumpers one by one and plug the relay back in. When the engine doesn't start, that is the socket with the bad relay or problem with the control signal to the relay.
If the car doesn't start with all 3 relay positions jumpered, the problem isn't with relays or the control circuit and you look elsewhere. The factory workshop manuals have very detailed diagnostic trees that will lead you to the problem.
Always look for the low cost stuff first, grounds to chassis, ground strap from the engine to the chassis, battery connections, fuse connection etc.
Other than different pin-outs at the connectors, the 25 pin and 35 pin LH systems are quite similar. The main difference is that in the 1984 (Euro) and 1985 - 1986 928s the fuel pump relay powers the fuel injectors. In the 1987 and new 928s the LH relay powers the fuel injectors. In all cases the LH (and L-Jet) modules just provide the switch between the fuel injectors and ground to turn them on and off. So don't over look checking to ensure that there is battery voltage at the fuel injectors when the associated relay socket is jumpered between 30 and 87.
10-28-2010, 07:18 AM
#24
Under the Lift
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Based on the observation that the car responds to starting fluid but does not start when the fuel pump is running via a jumper indicates to me that the injectors are not firing. I'm making an assumption that the fuel pump is providing fuel.
The EZF relay is working since you have spark. This also means the alarm is not causing the problem since the EZF relay is signaled through the alarm via F25 on the CE panel.
The fuel pump relay receives power from the ignition switched power buss, but is swiched by a signal from the LH through W15 on the CE panel. Also, the LH fires the injectors by grounding one side. So, you are right to suspect the LH as the fuel pump is not running unless you jump the relay. Other things can take out the injectors, such as a short in any one injector lead, but the LH is a good bet even though the early LHs are not nearly as unreliable as the later ones.
The EZF relay is working since you have spark. This also means the alarm is not causing the problem since the EZF relay is signaled through the alarm via F25 on the CE panel.
The fuel pump relay receives power from the ignition switched power buss, but is swiched by a signal from the LH through W15 on the CE panel. Also, the LH fires the injectors by grounding one side. So, you are right to suspect the LH as the fuel pump is not running unless you jump the relay. Other things can take out the injectors, such as a short in any one injector lead, but the LH is a good bet even though the early LHs are not nearly as unreliable as the later ones.
10-28-2010, 07:48 AM
#25
Craic Head
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<snip>
Check that all of the sensor plugs are installed on the engine,
Note on the air cleaner temp sensor make sure the idle valve harness isnt crossed with the temp sensor,
IIRC the temp sensor wires will have 5 volts and the ISV will have 12V ,
if they have been swapped then the temp sensor is toast,
turn on the key to run and check with a volt meter<snip again>
Check that all of the sensor plugs are installed on the engine,
Note on the air cleaner temp sensor make sure the idle valve harness isnt crossed with the temp sensor,
IIRC the temp sensor wires will have 5 volts and the ISV will have 12V ,
if they have been swapped then the temp sensor is toast,
turn on the key to run and check with a volt meter<snip again>
To the OP: Where are you located? There might be someone in the area that has a spare LH to rule that out (like me).
10-28-2010, 07:57 AM
#26
When I had an LH failure on my 1990 S4, I went back to basics by and followed Porsche's LH & EZK test matrix fault finding and test procedure.
As Mrmerlin stated, his procedure covers the basics and should unearth the problem in a logical flow method.
Fortunately in Jim's technical CD the,two test procedures booklets for the LH and EZk units were included and as a final resort to get over the hit or miss random checking, I started at the beginning by test of every sensor, cleanng earths and battery battery terminals, noid lights for the injectors, fuel pressure by installing a pressure gausge and did a leak down test etc., etc., as already mentioned.
Unfortunately I did not have some of the electronic testing equipment that I now have, but with the help of members of this board finally got a LH computer to swap and we had the problem verified and repaired by John Seake.
Wally P's year electrical maintenance procedure is also a good procedure to ensure that all the electrical connections, fuses, earths, relay and battery connections etc are clean and operational, so I would recommend that you follow this logical method as well.
Intereresting to hear of your success in isolating the problem.
Tails 1990 928S4 Auto
As Mrmerlin stated, his procedure covers the basics and should unearth the problem in a logical flow method.
Fortunately in Jim's technical CD the,two test procedures booklets for the LH and EZk units were included and as a final resort to get over the hit or miss random checking, I started at the beginning by test of every sensor, cleanng earths and battery battery terminals, noid lights for the injectors, fuel pressure by installing a pressure gausge and did a leak down test etc., etc., as already mentioned.
Unfortunately I did not have some of the electronic testing equipment that I now have, but with the help of members of this board finally got a LH computer to swap and we had the problem verified and repaired by John Seake.
Wally P's year electrical maintenance procedure is also a good procedure to ensure that all the electrical connections, fuses, earths, relay and battery connections etc are clean and operational, so I would recommend that you follow this logical method as well.
Intereresting to hear of your success in isolating the problem.
Tails 1990 928S4 Auto
10-28-2010, 03:31 PM
#27
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Thanks for the help guys, it has all been great.
Here is an update:
I found a little more info, that the car wasn't just parked and then wouldn't start, it was parked and he tried cleaning the engine bay up with some wd-40 because he didnt' want to use water. After that it wouldn't start. Ok, new plan of attack. I went after every sensor in the engine bay I could get to, cleaned the contacts, cleaned the battery terminals front and rear, pulled the air box and cleaned all the connectors under there (note, pins 1 and 2 on the maf did have some corrosion). Put it all back together.....no start. Cleaned the grounds above the CE and tried again. No start. Just for giggles and grins, I jumped the fuel pump relay and bam it fired right up. Put the relay back in (brand new relay BTW), no start. Grabbed one of the old relays and shoved it in, bam, fired right up. Started and stopped it along with a long test drive and it started every time, so I have a feeling there was more than one issue here. Maybe he caused the 2nd issue by pulling relays that he didn't have to, when the issue was elsewhere, I don't know. I can't believe it was a relay issue alone, because I had tried numerous different relays in the various positions and still had a no start. I will say that it isn't the first time I have tried a new relay that has been bad.
Not sure what I cleaned, or what it was, but we will monitor it for a few weeks to see what happens.
Also, looks like during his cleaning, the TPS harness connector was broken (no surprise there), so that short harness will need to be replaced (looks like roger @ 928srus has them?). I can't tell if you can wiggle it in and out without pulling at least the front coolant port for access or not. Any advice there?
Here is an update:
I found a little more info, that the car wasn't just parked and then wouldn't start, it was parked and he tried cleaning the engine bay up with some wd-40 because he didnt' want to use water. After that it wouldn't start. Ok, new plan of attack. I went after every sensor in the engine bay I could get to, cleaned the contacts, cleaned the battery terminals front and rear, pulled the air box and cleaned all the connectors under there (note, pins 1 and 2 on the maf did have some corrosion). Put it all back together.....no start. Cleaned the grounds above the CE and tried again. No start. Just for giggles and grins, I jumped the fuel pump relay and bam it fired right up. Put the relay back in (brand new relay BTW), no start. Grabbed one of the old relays and shoved it in, bam, fired right up. Started and stopped it along with a long test drive and it started every time, so I have a feeling there was more than one issue here. Maybe he caused the 2nd issue by pulling relays that he didn't have to, when the issue was elsewhere, I don't know. I can't believe it was a relay issue alone, because I had tried numerous different relays in the various positions and still had a no start. I will say that it isn't the first time I have tried a new relay that has been bad.
Not sure what I cleaned, or what it was, but we will monitor it for a few weeks to see what happens.
Also, looks like during his cleaning, the TPS harness connector was broken (no surprise there), so that short harness will need to be replaced (looks like roger @ 928srus has them?). I can't tell if you can wiggle it in and out without pulling at least the front coolant port for access or not. Any advice there?
10-28-2010, 04:01 PM
#28
Team Owner
Glad you found the issue, sometimes just moving the relays will make the connection better same goes for cleaning all of them..
Anyway.
to install the harness to the TPS i use a long set of medical forcepts they can grab the connector and position it then use a long screwdriver to gently push the connector on
Anyway.
to install the harness to the TPS i use a long set of medical forcepts they can grab the connector and position it then use a long screwdriver to gently push the connector on
10-28-2010, 04:05 PM
#29
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Glad you found the issue, sometimes just moving the relays will make the connection better same goes for cleaning all of them..
Anyway.
to install the harness to the TPS i use a long set of medical forcepts they can grab the connector and position it then use a long screwdriver to gently push the connector on
Anyway.
to install the harness to the TPS i use a long set of medical forcepts they can grab the connector and position it then use a long screwdriver to gently push the connector on