Timing and vacuum advance
01-30-2009, 08:53 PM
#91
Rennlist Member
I think 120/600 is reasonable, but dropping to 120 at "part-open" sounds wrong. That would make the injectors provide just enough fuel for idle when that happens. Can you post a pic?
01-30-2009, 10:54 PM
#92
Instructor
Thread Starter
OK, I don't know how well you can see this. And it was hard to take the picture while holding the valve open some. But here is a shot. Resistance is definitately going up and down as valve is opened. Setting up for this shot, it started at 120 ohms full closed, then went down to about 80 ohms when open slightly, then increased to 120 and then 600 ohms, then went back down to about 150 ohms, then back up to 600, then down to about 200 ohms (when the picture was taken). This was in the first 30 degrees or so of travel. And it keeps that up-down-up-down pattern all the way to the full open stop (although when I did it just now, each "up" was a little higher resistance then the previous "up").
Last edited by Tom Rathjen; 11-27-2011 at 10:08 PM.
01-30-2009, 11:00 PM
#93
Rennlist Member
How does the potentiometer look? Is it worn? I have some non-linearity in my 84 manual, and I think its because of some worn spots. And there are a couple of performance lags during accel, esp. in 1st.
Real curious of the pro feedback on your finding.
Real curious of the pro feedback on your finding.
01-30-2009, 11:07 PM
#94
Administrator - "Tyson"
Lifetime Rennlist
Member
Lifetime Rennlist
Member
Testing the AFM:
From this thread:
https://rennlist.com/forums/928-foru...ring-date.html
Do an advanced search with "AFM" and subject only - lot's of interesting reading
Hacker,
Well there's actually a hot, ground,signal return, ref return, temp.
If you have the manuals, there's a whole section in there about the AFM.
Anyway, if you try ohming it out just using a pole to pole method and a medium level meter (non-lab grade) you'll see all sortsa jumpy numbers - even on a brandnew AFM. If you really want to see what it does, my recommendation is interceptor box and datalog. 0-10v on the datalogger for L-Jet.
Even though the carbon strip is basically just a rheostat - it's a bit more complicated than that. You'll never move it smoothly enough by hand to see linear readings. It's also not really going to read linearly in straight resistance mode. It needs juice across it and then tap out the V from the wiper - then you'll see the linearity you're looking for.
The AFM top pops off and you can look at the carbon trace and the metal wipe and get a better visual idea. If you look at the electrical diagram you might get some idea of what I'm talking about.
I'd write more but I gotta run.
Greg
Well there's actually a hot, ground,signal return, ref return, temp.
If you have the manuals, there's a whole section in there about the AFM.
Anyway, if you try ohming it out just using a pole to pole method and a medium level meter (non-lab grade) you'll see all sortsa jumpy numbers - even on a brandnew AFM. If you really want to see what it does, my recommendation is interceptor box and datalog. 0-10v on the datalogger for L-Jet.
Even though the carbon strip is basically just a rheostat - it's a bit more complicated than that. You'll never move it smoothly enough by hand to see linear readings. It's also not really going to read linearly in straight resistance mode. It needs juice across it and then tap out the V from the wiper - then you'll see the linearity you're looking for.
The AFM top pops off and you can look at the carbon trace and the metal wipe and get a better visual idea. If you look at the electrical diagram you might get some idea of what I'm talking about.
I'd write more but I gotta run.
Greg
https://rennlist.com/forums/928-foru...ring-date.html
Do an advanced search with "AFM" and subject only - lot's of interesting reading
01-31-2009, 04:34 AM
#95
Rennlist Member
Great stuff, Erik -- thanks! BTW, the three or so wiring diagrams I saw show a plain variable resistor, so I guess it's easy to assume it acts like one. The link provided in that thread(Post #2) gives a great procedure. While it seems you were left wanting some real voltage values to test for, I think that for troubleshooting Tom's problem it is enough to determine if there is a sudden drop in voltage as the AFM opens.
I can't tell from Tom's pic how worn the resistor is, but my impression is that there are grooves. Worth a closer look IMHO.
I can't tell from Tom's pic how worn the resistor is, but my impression is that there are grooves. Worth a closer look IMHO.
01-31-2009, 12:18 PM
#96
Instructor
Thread Starter
OK, did the above test. Makes a lot of sense. And looks like my AFM is actually OK. My results are shown below.
My 928's AFM had 5 pins instead of the 4 shown in the 944 AMF test procedure. And, as one of the posts suggested, that 5th pin (pin 8) is a "signal return", which I assume means that is the reference voltage that the DME sees and compares with the variable voltage that comes off of the variable resistor. Anyway, I did the test twice, once by measuring the absolute voltage at pin 7 compared to ground (like the procedure in the 944 link). And then I did it with the delta voltage between pins 7 and 8. The latter gave the best results. It was very smooth all through the range of travel of the valve. When comparing voltage at pin 7 and ground, there was some slight voltage wobble when the wiper arm contacted each of the 11 "spots" (or bumps) where the circuit board conductors intersect the resistor arc. (Sorry if my description is not very good.) But there was no wobble at these points when looking at the pin 7 to 8 voltage.
The only notable difference in my results compared to the 944 results is that the actual voltage output at pin 7 was much lower than on the 944 AFM (max about 6 Volts for mine, whereas the max on the 944 AFM test was almost 9....test uses a 9 volt battery). But, I doubt that means mine is bad, just different design than the 944. I didn't find any dirty contacts.
I also tested the temp sensor resistance per the procedure in the link, and it was 2.5K ohms (within the expected range).
Regarding the condition of the resistor strip, it does have visible marks and light grooves. But since the test results are so smooth I don't think it is a problem. I did clean and reposition the wiper arm a little per the procedure.
Well, bummer. I was excited last night that I found a real problem. I need to get my head around the other suggestions in the earlier posts now and try some other things.
My 928's AFM had 5 pins instead of the 4 shown in the 944 AMF test procedure. And, as one of the posts suggested, that 5th pin (pin 8) is a "signal return", which I assume means that is the reference voltage that the DME sees and compares with the variable voltage that comes off of the variable resistor. Anyway, I did the test twice, once by measuring the absolute voltage at pin 7 compared to ground (like the procedure in the 944 link). And then I did it with the delta voltage between pins 7 and 8. The latter gave the best results. It was very smooth all through the range of travel of the valve. When comparing voltage at pin 7 and ground, there was some slight voltage wobble when the wiper arm contacted each of the 11 "spots" (or bumps) where the circuit board conductors intersect the resistor arc. (Sorry if my description is not very good.) But there was no wobble at these points when looking at the pin 7 to 8 voltage.
The only notable difference in my results compared to the 944 results is that the actual voltage output at pin 7 was much lower than on the 944 AFM (max about 6 Volts for mine, whereas the max on the 944 AFM test was almost 9....test uses a 9 volt battery). But, I doubt that means mine is bad, just different design than the 944. I didn't find any dirty contacts.
I also tested the temp sensor resistance per the procedure in the link, and it was 2.5K ohms (within the expected range).
Regarding the condition of the resistor strip, it does have visible marks and light grooves. But since the test results are so smooth I don't think it is a problem. I did clean and reposition the wiper arm a little per the procedure.
Well, bummer. I was excited last night that I found a real problem. I need to get my head around the other suggestions in the earlier posts now and try some other things.
Last edited by Tom Rathjen; 11-27-2011 at 10:08 PM.
02-02-2009, 02:14 PM
#97
Instructor
Thread Starter
Well, I didn't have time the rest of the weekend to do any more than put the AFM back on and drive around with the timing light to see if sparks cut off when "stumbling". Next on my list still be to find the O2 sensor connector and measure the voltage. I am also ordering a fuel pressure guage.
When driving with the timing light, I really couldn't notice any problems. The flashing at all engine speeds seemed steady, even when stumbling. I tried it with the timing light connected to both the coil wire and to a plug wire.
One notable observation from this drive test, though: it was a significantly warmer day than it has been for months. (Up around 60 degrees, instead of the 20s-40s it's been for all of Dec and Jan.) And the car ran better. It did still "stumble" between 3 and 4K when under load (like driving up a hill), still seemed to lack power, and the engine speed never would get above 4K. But the stumbling didn't seem as pronounced, and the car seemed to have a little more power. Not sure if this means anything.
When driving with the timing light, I really couldn't notice any problems. The flashing at all engine speeds seemed steady, even when stumbling. I tried it with the timing light connected to both the coil wire and to a plug wire.
One notable observation from this drive test, though: it was a significantly warmer day than it has been for months. (Up around 60 degrees, instead of the 20s-40s it's been for all of Dec and Jan.) And the car ran better. It did still "stumble" between 3 and 4K when under load (like driving up a hill), still seemed to lack power, and the engine speed never would get above 4K. But the stumbling didn't seem as pronounced, and the car seemed to have a little more power. Not sure if this means anything.
02-02-2009, 06:45 PM
#99
Rennlist Member
I doubt this will be valuable but it just happened to me so thought I would mention it.
Almost immediately after a TB/WP job my '88 began to miss and stumble very similar to what you have described. I was getting a strong signal from the coils and assumed my problem wasn't ignition parts, but rather a timing issue.
At some point I decided to pull the distributor caps, and lo and behold, the passenger side coil wire was shorting out to the ground wire.
I slipped on a different coil wire and problem solved.
Almost immediately after a TB/WP job my '88 began to miss and stumble very similar to what you have described. I was getting a strong signal from the coils and assumed my problem wasn't ignition parts, but rather a timing issue.
At some point I decided to pull the distributor caps, and lo and behold, the passenger side coil wire was shorting out to the ground wire.
I slipped on a different coil wire and problem solved.
02-02-2009, 08:42 PM
#100
Rennlist Member
can you do the test with the car running? those voltages are much higher when the car is running vs static with the ignition on.
mk
mk
OK, did the above test. Makes a lot of sense. And looks like my AFM is actually OK. My results are shown below.
My 928's AFM had 5 pins instead of the 4 shown in the 944 AMF test procedure. And, as one of the posts suggested, that 5th pin (pin 8) is a "signal return", which I assume means that is the reference voltage that the DME sees and compares with the variable voltage that comes off of the variable resistor. Anyway, I did the test twice, once by measuring the absolute voltage at pin 7 compared to ground (like the procedure in the 944 link). And then I did it with the delta voltage between pins 7 and 8. The latter gave the best results. It was very smooth all through the range of travel of the valve. When comparing voltage at pin 7 and ground, there was some slight voltage wobble when the wiper arm contacted each of the 11 "spots" (or bumps) where the circuit board conductors intersect the resistor arc. (Sorry if my description is not very good.) But there was no wobble at these points when looking at the pin 7 to 8 voltage.
The only notable difference in my results compared to the 944 results is that the actual voltage output at pin 7 was much lower than on the 944 AFM (max about 6 Volts for mine, whereas the max on the 944 AFM test was almost 9....test uses a 9 volt battery). But, I doubt that means mine is bad, just different design than the 944. I didn't find any dirty contacts.
I also tested the temp sensor resistance per the procedure in the link, and it was 2.5K ohms (within the expected range).
Regarding the condition of the resistor strip, it does have visible marks and light grooves. But since the test results are so smooth I don't think it is a problem. I did clean and reposition the wiper arm a little per the procedure.
Well, bummer. I was excited last night that I found a real problem. I need to get my head around the other suggestions in the earlier posts now and try some other things.
My 928's AFM had 5 pins instead of the 4 shown in the 944 AMF test procedure. And, as one of the posts suggested, that 5th pin (pin 8) is a "signal return", which I assume means that is the reference voltage that the DME sees and compares with the variable voltage that comes off of the variable resistor. Anyway, I did the test twice, once by measuring the absolute voltage at pin 7 compared to ground (like the procedure in the 944 link). And then I did it with the delta voltage between pins 7 and 8. The latter gave the best results. It was very smooth all through the range of travel of the valve. When comparing voltage at pin 7 and ground, there was some slight voltage wobble when the wiper arm contacted each of the 11 "spots" (or bumps) where the circuit board conductors intersect the resistor arc. (Sorry if my description is not very good.) But there was no wobble at these points when looking at the pin 7 to 8 voltage.
The only notable difference in my results compared to the 944 results is that the actual voltage output at pin 7 was much lower than on the 944 AFM (max about 6 Volts for mine, whereas the max on the 944 AFM test was almost 9....test uses a 9 volt battery). But, I doubt that means mine is bad, just different design than the 944. I didn't find any dirty contacts.
I also tested the temp sensor resistance per the procedure in the link, and it was 2.5K ohms (within the expected range).
Regarding the condition of the resistor strip, it does have visible marks and light grooves. But since the test results are so smooth I don't think it is a problem. I did clean and reposition the wiper arm a little per the procedure.
Well, bummer. I was excited last night that I found a real problem. I need to get my head around the other suggestions in the earlier posts now and try some other things.
02-02-2009, 09:35 PM
#101
Three Wheelin'
Join Date: Sep 2006
Location: Banished to the SBC Wastelands
Posts: 1,578
Likes: 0
Received 4 Likes
on
3 Posts
now what i know from reading racing prep's on chevy's is that they want all advance in at about 3000 revs... staying there till max revs. Which is about 36° advance. In the Porsche manual i also see that 36° number, as a max advance.
Now the OEM distributor gives between 13 and 15° mechanical advance form idle to 3000 revs... and hold this further steady.
So only using this ( no vacuum lines ) i should point for 21° at idle, giving me max 36° at 3000 and further.
The distributor has 2 vacuum lines : on top it's timing advance , on the back it's timing retard.
First : timing advance : this is all about emissions , mpg etc... i think. What this does is advancing the timing when the engine is at high vacuum, so with near closed throttle... giving a other fuel mixture which can be ignited sooner ( =more advance ) to reach max explosion force just after TDC... But with the pedal down , you don't have vacuum high enough to operate the vacuum advance , so this is doing nothing at that driving condition.
So for my "racing" application i can forget that vacuum thing
... again, i think.I'm interested in this as I have no vacuum advance on my MSD Distro. If I set it to 32* @3500rpm, I want to ensure advance drops back to the correct level at idle.
What is the normal advance at idle for a Euro 16V?
02-03-2009, 06:36 PM
#102
Instructor
Thread Starter
Well, I'm not sure. When I replaced the fuel pump a couple months ago (in the process of making the car drivable), I looked at the filter closely. It was very "new "looking, especially relative to the pump I was replacing. I bench tested my replacement pump with the filter connected, and seemed to get good pressure. This wasn't a very thorough test. And now that I have been driving it around, maybe crud that had settled in the tank after sitting so long had now clogged it. I'm hoping testing the fuel pressure with the gauge I just ordered will tell me something. (I probably should have just replaced it when I did the pump. But was trying to only replace what I knew I needed....not like I haven't failed at that lofty goal already.)
Regarding a possible short of the coil while...I've looked pretty closely in the cap and in the distributor, and didn't see any problem. (I only have one on the 82.)
Regarding testing the AFM voltages with the car running....that would be tricky. Not sure how I would do it without slicing in to the wires somewhere. Or building 5 jumpers to link the connector sockets with the AFM's pins such that I can access test points. I'll have to give that some thought.
I'm headed out of town again tomorrow. Hopefully my fuel gauge will arrive while I am gone, so I can look at that as well as the O2 sensor this weekend.
Question: where are the ballast resistors located?
Regarding a possible short of the coil while...I've looked pretty closely in the cap and in the distributor, and didn't see any problem. (I only have one on the 82.)
Regarding testing the AFM voltages with the car running....that would be tricky. Not sure how I would do it without slicing in to the wires somewhere. Or building 5 jumpers to link the connector sockets with the AFM's pins such that I can access test points. I'll have to give that some thought.
I'm headed out of town again tomorrow. Hopefully my fuel gauge will arrive while I am gone, so I can look at that as well as the O2 sensor this weekend.
Question: where are the ballast resistors located?
02-07-2009, 11:12 AM
#103
Instructor
Thread Starter
Fuel pressure gauge arrived while I was out of town. Just installed it.
At idle, the pressure is about 29 PSIG. As RPM increases, the pressure steadily increases to about 36 PSIG at full throttle.
I could only test this on the passenger side fuel rail, since the driver's side rail does not have the test port cap. (I assume this is as designed.)
According to a critical engine data spec, pressure at idle should be approx. 2 bar, which is 29 psig. So...that looks right. But, is the pressure at higher engine speeds low? Looking through other posts suggests I should be seeing something in the high 40s?
At idle, the pressure is about 29 PSIG. As RPM increases, the pressure steadily increases to about 36 PSIG at full throttle.
I could only test this on the passenger side fuel rail, since the driver's side rail does not have the test port cap. (I assume this is as designed.)
According to a critical engine data spec, pressure at idle should be approx. 2 bar, which is 29 psig. So...that looks right. But, is the pressure at higher engine speeds low? Looking through other posts suggests I should be seeing something in the high 40s?
02-07-2009, 01:46 PM
#104
Rennlist Member
That matches spec. I think the 32v engines run at a higher pressure, maybe that's where you were seeing 40+.
Skimming back through the thread, I don't see any mention of cap & rotor condition. You say the plug wires seem fine, at this point I think you should check them with an ohmmeter and look for any signs of arcing/tracking in the distributor cap. IIRC the rotor has a 1K resistor between the center and outer contacts. Make sure that you don't have a layer of white crud on the distributor contacts. Clean all of that up. If there is any residue inside the cap, clean it with denatured alcohol or a solvent that will be nice to the plastic and leave no residue.
Skimming back through the thread, I don't see any mention of cap & rotor condition. You say the plug wires seem fine, at this point I think you should check them with an ohmmeter and look for any signs of arcing/tracking in the distributor cap. IIRC the rotor has a 1K resistor between the center and outer contacts. Make sure that you don't have a layer of white crud on the distributor contacts. Clean all of that up. If there is any residue inside the cap, clean it with denatured alcohol or a solvent that will be nice to the plastic and leave no residue.
02-07-2009, 02:29 PM
#105
Instructor
Thread Starter
Will check the wires, rotor, and contacts in the cap with an ohm meter. I know I don't have white crud in it and it looks clean.
I did find something else just now. The fuel return damper (the thing that looks like a fuel pressure regulator in the front of the engine that both fuel rails attach to, and the line coming out of it heads back to the tank) is not holding a vacuum (at the port where the vacuum line attaches). I don't know if this could have anything to do with my problem, but I'm sure that must be bad and should be replaced. When I ran the engine with its vacuum line disconnected and plugged, the fuel pressure readings became a few PSI lower at idle and only about 30 PSIG at full throttle.
Also, I have been trying to test my O2 sensor. It visually looks pretty clean/new with an intact wire. And I found where the connector is near the fuses and relays. But I have been unable to get any kind of a voltage signal when testing at that connection point (relative to ground). It just reads zero all the time. But, I am wondering if my volt meter is simply not sensitive enough. It does have a low voltage setting (2.5 Volts), but I read in some other post that typically analog meters are not sensitive enough to detect anything. On the other hand, disconnecting it had no affect on how the engine was running.
I did find something else just now. The fuel return damper (the thing that looks like a fuel pressure regulator in the front of the engine that both fuel rails attach to, and the line coming out of it heads back to the tank) is not holding a vacuum (at the port where the vacuum line attaches). I don't know if this could have anything to do with my problem, but I'm sure that must be bad and should be replaced. When I ran the engine with its vacuum line disconnected and plugged, the fuel pressure readings became a few PSI lower at idle and only about 30 PSIG at full throttle.
Also, I have been trying to test my O2 sensor. It visually looks pretty clean/new with an intact wire. And I found where the connector is near the fuses and relays. But I have been unable to get any kind of a voltage signal when testing at that connection point (relative to ground). It just reads zero all the time. But, I am wondering if my volt meter is simply not sensitive enough. It does have a low voltage setting (2.5 Volts), but I read in some other post that typically analog meters are not sensitive enough to detect anything. On the other hand, disconnecting it had no affect on how the engine was running.