ECU: testing circuits & interpreting results '76 280Z

[Jennys280Z]

Hi everyone!

Well I did my little circuit tests tonight from my ECU's 35-pin harness connector and made a table on my clipboard to record the results. A few things on it look good but unfortunately a few also look very bad. I was hoping a few of the wizardly Z people around here could offer some sage wisdom about my results, from your experience, or whatever you'd like to say that's most likely stuff I don't know!

I ran all the tests that overlapped with the symptoms I've experienced with my car recently, namely loss of power, backfiring and/or afterburning. Tests were run at approximately 50 degrees F.

I don't know how to do a table in HTML off the top of my head so I'll try to type it manually in this format:

Test # (according to 1976 FSM and 1975 EFI Bible), Reading, and Expected Value

1-(1), 0.7 Ohms, Continuity

1-(2), skipped

1-(3a), 226 Ohms, 180 Ohms

1-(3b), 199 Ohms, Continuity

1-(3c), 125.9 Ohms, 100 Ohms

1-(4), 3960 Ohms, 3250-4150 Ohms

1-(6), 4290 Ohms, 3250-4150 Ohms

1-(8), 68.7 Ohms, Continuity

2-(1), 10.67V, Battery Voltage

2-(2), 10.88V, Battery Voltage

2-(3a), 10.87V, Battery Voltage

2-(3b), 10.87V, Battery Voltage

2-(3c), 10.86V, Battery Voltage

2-(3d), 10.86V, Battery Voltage

2-(3e), 10.85V, Battery Voltage

2-(3f), 10.84V, Battery Voltage

Battery Voltage between posts after tests completed: 11.23V

So it seems that my Air Flow Meter is in trouble, doesn't it? OH NOOOO :paranoid:

And I don't think that 68.7 Ohms is exactly "Continuity" in 1-(8) either. sigh

I would guess that everything else looks fine, aside from the undercharged battery, I hope all my injectors are getting a healthy % (~96.7%) of that voltage and I would presume they are.

I've never opened up my AFM and touched the little valve in it before so I won't really know how it's supposed to feel when it feels right. Or the throttle body's either for that matter.

Hope to hear from you guys soon on what you would do if you were me at this point. Or what you would do if you were you! hehe

Thank you!

[Zed Head]

Those are almost the exact numbers I had on my bad AFM. I had 227 and 126.

The car would idle well, but popped and surged when you tried to drive it at part throttle. If you hit the gas though, it ran fine. Of course, I couldn't drive around at full throttle all the time, in my neighborhood anyway, so I replaced it.

I got a rebuilt one from MSA (thezstore.com) and the car ran great after that. Until an injector died, but that took a while and is another story. MSA will either rebuild yours after you send it in, or send you one off the shelf if they have it, but with a $400 core charge. Apparently they are running out of cores to rebuild because people don't send the bad ones in, hence the huge core charge.

Someone out there might have a used one if you want to save some money.

[WingZr0]

Call Stephen's Automotive at 530.671.3820 . He specializes and Z's and used to be a Datsun Dealership mechanic when these where new. Shop is in California but he can likely tell you what your readings mean and how to fix it. FWI

[sblake01]

You might try removing the cover from the AFM and spraying the contact area of the potentiometer with a contact cleaner such as Deoxit or the like. And then re-check the readings. You might also see if running the tests with at fully charged battery changes the readings. (Obviously the voltage based readings will change but I'm referring to the resistence based readings)

[Zed Head]

Well, now you have the experienced guys on the case.

Call the guy in CA. I would like to know if these are fixable without a rebuild too. If the core charge hadn't been so big I might have kept my old one to dink around with (maybe MSA knows that). They seem simple in concept.

To be clear, the MSA rebuilts cost $169 and MSA did not give me any problems returning the core charge. As long as the aluminum body is not cracked or broken, the charge will be returned. But it is a risk, with a lot of money.

To add to sblake01's suggestion, if you try the Deoxit, make sure you let it dry out before testing it. I tried something similar and tested it while still moist after and the numbers were all over the place. That contact area is really sensitive. After it dried though, they went back to the start values (the bad ones:cry:). But you might have some luck, it's worth a shot. Since you are working at the ECU connector, you should probably clean the connector at the AFM too. And that 68.7 reading could be a corroded connector too.

Good luck.

[sblake01]

Yep, I've cleaned them and had them work and I've cleaned them and had them be just as bad as they were. It all depends on what's causing the bad resistence, corrosion or wear. I don't actually use Deoxit but a similar product called 'Ideal Electronic Switch and Contact Cleaner'. But it stiil needs to be dry before you test it.

[Jennys280Z]

Thanks everyone! I want to do everything I can to save this AFM before having to replace it and I'm really pretty upset about this. I hate how my manuals really don't have a good exploded drawing of my AFM and all the parts named for me. Even when you guys say "front cover" I feel like I can't be 100% sure.

I took a few photos of the underside of my AFM by sinking my camera down under it, because I can't see under there no matter which way I try to look, and it's a long stretch from the bottom-up even if I jacked up my car and got underneath (which I don't like doing, and like to have at least 2-3 reasons to do it before I do ).

I see a big black plug under there which I guess is a main connector I should clean...I have some newbie questions and I'm scared to touch my AFM already because of what the mechanic said so please help me! Is that the big plug I need to disconnect and clean? If so how does it come off? It looks like it just pulls straight off but I'm scared to tug on it until someone tells me how. And even if it comes off easily enough, is it keyed so it's easy to plug back in? It looks like it will be impossible to plug back in without being able to see what I'm doing and I don't think my my arm is long enough to plug it back in from the bottom if that's what is best!

Also if you look at the upper left bolt (blurry) in the photo of the bottom of the AFM there is a single wire that is connected to that bolt (possibly with a ring-type end on it). Is this a ground? And if you look at the top of my AFM, right under the front cover there is a bolt there, and another wire connected to that (is that also a ground?).

sblake01 thank you and I'm going to try to clean the Potentiometer like you said, and the AFM connectors too. I got some Dexoxit delivered today (I just found it at the door yaay!)! I have a newbie question about that too though that I hope you or anyone can answer for me. The cover (the black plastic cover) has some red stuff that looks like it's sealing it closed and you can see it on the photo. Does the cover just come straight off with a little pull from my hand? I don't see anything else holding it on there and again, I know how sensitive this AFM is and I don't want to put ANY kind of force on it that isn't correct/productive. And then once I have the cover off, how do I put it back on? Should I get some of that red stuff to put around the edges to seal it up again before I put the cover back on?

The potentiometer looks complicated to me from the drawing of it I see in my EFI Bible and I wouldn't know what to look for to check if something is wrong so I'll just clean it with the Deoxit spray only and let it dry.

Then after cleaning of course I'll make sure my battery is a solid 12.6V again from charging like you said as that might have an effect on my resistance values.

Tonight I'm at least going to clean some other connectors that I've recently located/loosened: Oil pressure sensor (always read low on my gauge), Thermotime, Coolant Temp sensor, Firewall ground, EFI powers, EFI ground, bullet connectors, fuse box connectors...a few more I am forgetting. I'm not going to clean or spray anything on my ECU pins or connectors because they look so nice and clean already. Unless I can be sure I won't hurt anything by spraying Deoxit on it. Sorry I'm so wordy tonight! Someone please help!

Thank you guys sblake01, Zed Head, WingZr0 and everyone else in advance!

[Jennys280Z]

Caig Deoxit D5 has done an awesome job on all the connectors I've cleaned so far. Last night I cleaned my water temp sensor, thermotime, oil pressure sensor, fusible links, EFI harness connectors, firewall ground. The water temp sensor was very corroded, and my car's temp gauge wouldn't get higher than about 130-140degF no matter how long I drove/ran it last. My oil pressure gauge always read zero when I'm just idling so I hope cleaning it will make a difference. My fusible links were in bad shape too so I sanded/filed/cleaned them and now they look shiny and new. Tonight I'm going to clean my fuse box connections, bullet connectors, alternator connections, battery posts, and even my horns which are currently off the car!

Speaking of horns! HONK!! Anyone answer my newbie questions about disconnecting the plug and black plastic cover on my AFM? I know my question isn't good, but please? Otherwise I'm just going to have to start pulling on stuff. Can anyone who's done it before tell me if there's anything I should know before I touch it? Like...if I should pull the black cover up and out, instead of just out. Or if the connector has a clip I have to pinch. And/or should I disconnect the AFM itself first to have better access to the connector, et al. Advice like this would be so appreciated!!! Anyone? I want to clean my AFM/Potentiator worse than anything but I'll put it off another day in hopes I can get help/reassurance here.

PS I can do repairs on my car and have done quite a bit of things successfully already. In time I could be an asset to this forum as much as I am a consumer of it now but I need to learn first like everyone else. I think everyone who has a Z should try to learn as much as they can about them so we can keep these cars on the road, if for nothing else, because we love them! Besides, even knowledge itself keeps the cars alive (keeps cost of ownership down, and by extension, interest in ownership up!).

[Zed Head]

It looks like someone may have already been inside the AFM, I don't think that they came sealed with red sealer. But I could be wrong. But it would not affect your bad resistance readings anyway.

The black wire is a ground wire.

The best way to clean it would be to take it out. Loosen up all of the hose clamps and hoses. Take out the three big Phillips head screws that hold the metal frame the AFM is mounted on. That's the bottom of the mount you're looking at in the second picture. Work off one of the hoses, I think the front one is easiest, and lift that end of the AFM up clear of the hose. Work it out of the other hose, but not too far, you will need to reach under it and get the wires clear of the mount point on the car. Then you should have room to turn it and see the connector, and be able to disconnect it. I think the connector is a little difficult to get off, it may have a wire clip or something holding it. It's been a while. The black wire you saw connects the AFM to the frame so will come with the assembly.

The black cover just comes straight off of the AFM. There's really not much holding it on except the sealer. You might need to cut the sealer with a blade and carefully pry the cover off. Inside you'll see the black contact pad and the sliding contacts and the weight and spring. Rotate the weight or move the valve inside the air path and you'll see how the potentiometer works.

No one is talking probably because if yours was working correctly, this would be "Not a good idea." But since you'll have to take it out anyway if you have it replaced, you might as well go for it.

Have fun. It is a strange device.

[Jennys280Z]

It looks like someone may have already been inside the AFM, I don't think that they came sealed with red sealer. But I could be wrong. But it would not affect your bad resistance readings anyway.

Tom Thorman of Z Service Unlimited, to be exact!

No one is talking probably because if yours was working correctly, this would be "Not a good idea." But since you'll have to take it out anyway if you have it replaced, you might as well go for it.

Have fun. It is a strange device.

Thanks so so so much for your help!! It's bizarre that Tom would have sealed it like that. That's how we got the car back after he worked on it, and that was the time he warned my b/f not to mess with it (pointing to the AFM cover). I remember that well because he said it in an almost accusatory tone that implied my b/f had tried to tamper with it (I'm nearly positive he did NOT). Since it's so sensitive (and the fact that there's red goop of some kind on the cover that will be Lord-knows-how-hard to take off after all this time ) , I'll take a minimalistic approach to tampering with it.

1. First fully charge the battery as sblake suggested.

2. Retest AFM from the ECU

3. If readings the same, pull off the cover and clean the Potentiometer contacts as sblake suggested.

4. Retest AFM from the ECU

5. If readings the same, not sure what this step will be. Not sure if it would be prudent to open the front hose and manually check the valve's operation, or unfasten the AFM unit as you described (thanks!) and clean the connectors -- if that will even make a difference in the readings I don't know.

And 6. Retest again...

Step 3 might just be to start my baby up and see how he runs. I didn't mention yet but your car's symptoms were very similar to mine now (as are the resistance results of course!). Though I did momentarily apply full throttle in my neighborhood --funny you mentioned that too -- and it was still running poorly (at idle, part throttle, full throttle, didn't matter :disappoin)

I couldn't believe how fast my fuel gauge went to Empty...could six gallons disappear so fast? I was idling in the driveway and the fuel gauge went from about 20% to touching the E line! Perhaps a failing AFM is a great reason to run way too rich (and therefore like crap) but how come I didn't see any black smoke? That still baffles me. Fast Woman I think asked me if my exhaust smelled like gas and it kinda did, but I really don't have a frame of reference to compare it to, to tell if it smelled unusual. It always kinda smells like gas don't it?

Did I not thank you enough already? Nope! THANK YOU!!!

[Pomorza]

Tom Thorman of Z Service Unlimited, to be exact!

I couldn't believe how fast my fuel gauge went to Empty...could six gallons disappear so fast? I was idling in the driveway and the fuel gauge went from about 20% to touching the E line! Perhaps a failing AFM is a great reason to run way too rich (and therefore like crap) but how come I didn't see any black smoke? That still baffles me. Fast Woman I think asked me if my exhaust smelled like gas and it kinda did, but I really don't have a frame of reference to compare it to, to tell if it smelled unusual. It always kinda smells like gas don't it?

Did I not thank you enough already? Nope! THANK YOU!!!

Gas gauges in Z's (or at least mine to particular) are never accurate. It isn't abnormal for mine to read F one second and then half a minute or two later. 6 gallons of gasoline is roughly 1/3 of a tank (I believe they have 17 gallon tanks) so it should read 1/3. Unless your car is running ridiculously rich burning six gallons of gas while idling is pretty impossible in my view unless your were running it for hours on end and even than that's a stretch. In my opinion your sender, the gauge or the wiring are the culprit here and the readings your getting are off.

hope that helps

Jan

[WingZr0]

As a side note if your oil guage is hoovering around zero it's likely the guage is broken and not the sender. Most folk here will tell that from experience including me . Find a replacement from the many Z forum classifieds or ebay. If seller don't know if the guage works don't buy it. Should take only ten easy minuts to replace. Take out the 4 screws on the center dash/ AC face plate and unplug the light bulbs to it. There are 4 metal clips that those 4 screws go into mounted on tabs behind the face plate, be sure to keep track of those clips and DON'T loose them. Look inside and under the oil guage is one last screw holding the strap up on the guage. Slide guage from hole, unplug it and replace with new guage. Be sure to tuck all the wires to everything out the way of the AC vents so the face plate will fit right. If still not working then ok, it's the sender .

Edited February 5, 2010 by WingZr0