RPM signal problem

[DarthBulk]

Well, I am ready to try and start my car after a LONG build. Pretty much everything is new and replaced. I have been setting up my AEM EMS, and have run into a weird problem regarding the RPM signal. I have a 3" GM MAF and version 1.3 MAF Translator installed (it is all zeroed out, because I am using the EMS for tuning). When I look at the RPM readout on the computer screen from the AEM EMS, I usually get 0 RPM reading while cranking the engine. I sometimes get a wildly fluctuating RPM reading, maxing out at over 9000 RPM, and this is with the car off...not even cranking it (I have never started it yet, because I am just setting up the EMS). When this happens, the fuel pump turns on, and the red light on the MAF-T glows dim, which indicates it is reading WOT mode. I can also hear the coil packs firing wildly. If I unplug the connector from the GM MAF to the MAF-T, this problem stops. Also, if I unplug the plug from the top of my CAS, the problem stops. If I unplug the RPM signal wire from the MAF-T to the RPM signal wire on the CAS (where I tapped it in), there is no difference, the problem persists. This wild RPM problem is not always there, it only seems to happen when the engine stops at a certain point after cranking. Keep in mind, I am not trying to start the engine yet, I am just cranking it to set up parameters in the EMS. I don't have any fuel in the car yet.
So what could cause there to be a wild RPM signal in the wiring harness? What actually generates this signal? I know there is an RPM signal at the CAS and also at the transistor pack, right? Both of these items are the original ones, so they are old with about 140000 miles on them. If one of these has failed, could that cause the problem? Any help would be very welcome!

 

[DarthBulk]

Ok, I have been working on this problem some more. I tested my transistor pack as described in the maintenance manuals, and it tests out ok. So then I focused on the CAS. I pulled the CAS from my cylinder head, and rotated it by hand while watching the AEM Pro software screen. As I slowly rotate my CAS, the RPMs jump all over the place! They go from 0 to 25000 to every point in between. And they aren't smooth jumps. They just jump around randomly. The car Tach jumps around too, and I can hear my coil packs firing whenever the RPMs jump around. If I stop rotating the CAS, the RPMS drop back to zero and the coil packs stop firing, and the fuel pump turns off. So my question is, what would cause this? Is the CAS bad? How do you test the CAS? There doesn't seem to be anything about that in the manual. Please help!

 

[DIRQUIN]

Hey how ya doin. I hope I can help. I have a print out of a CAS test. I'll start from the top and hope I don't get lost. Here we go, disconect the CAS conector and turn the ignition on. Measure voltage at cavity 3, white wire. Should have 4-5 volts. If yes, check continuity between cavity 1, black wire and ground. If yes, with the key on measure voltage at cavity 2, red wire. If voltage is within 1 volt of battery volts, replace CAS. Now back to the cavity 3 test. If no, measure voltage at cavity 22 of the ecu connetor. I don't know if you have the same connector or not but maybe you can find the same wire in your connector. Voltage should be between 4-5 volts. If yes repair the white wire for open or high resistance. If no replace ecu. Hopefully its yes. Now back to cavity 1 black wire. If continuity doesn't exist repair that wire for open. Last but not least, cavity 2 red wire, if voltage is not within batt. volts repair red wire for open or high resistance. I dont know if this will help you but its all i have. Good luck and safe driving.

 

[DarthBulk]

Thanks, I'll give it a try! This problem is really a pain. I almost hope it is a bad CAS, so I can just replace it and try starting the car.

 

[DarthBulk]

Ok, I ran these tests. First, the 4th wire is the white one on my CAS connector, while the third one is black. One is black, two is red, three is black, and four is white. At cavity 3 (black wire) I have 4.95 volts. I have continuity between the cavity 1 and ground. the red wire in cavity 2 reads 9.5 volts, while at the battery I have 11.95 volts. Is that ok? These tests will verify the wiring and everything on the car side of the CAS, but not the CAS itself, right? As soon as I plug the CAS back in, and try to rotate it by hand, my RPMs go all over the place, before I even make 1 revolution.

 

[DIRQUIN]

Ok, the red wire should have no less than 1 volt difference than the batt. My test shows there is an open or high resistance in the red wire. I would try and trace down the red wire to see if its broke some where. You may also be able to remove the red wire from the conector and run a test wire strait from the batt. I don't see a problem with this as long as you dont send power back through the ecu harness, as it may damage it, { ecu, or wire } You may also be able to hook your dvom up and wiggle test the wire harness and if the readings change, open the harness at that point and check the wire. Hope I can help. Let me know what exact year and model you have with the prodution date and i may be able to find a more elaborate wiring diagram. Let me know how it turns out. It may be later tomorow night before I get to check this thread, so be patient with me, I do want to help, I just have to take my wife to the airport . Good luck.

 

[DarthBulk]

I'll check that red wire when I get home from work tonight, and see if I can find why it has only 9.5 volts. I do have my battery re-located to the hatch, but I checked and I have good voltage (same as at the battery) at the positive wire end that is run to the engine compartment. I really appreciate the help!

 

[DIRQUIN]

Glad to help. Have fun and drive safe. Talk at ya later.

 

[DarthBulk]

Well, I tried to trace that red wire but it becomes impossible after it enters the large bundle of wires going around the intake manifold. Do you know where the other end is? I assume it is at a fuse box somewhere? I am looking through my factory manuals now.

 

[DIRQUIN]

Sorry, not egnoring you, wife trashed room packing for her trip and i can't find my papers. I'm still looking for them so hang in there and I will find them shortly.

 

[DarthBulk]

I really appreciate the help. I believe that if this RPM problem will be solved, I will be able to start my car finally, after a couple of years of complete rebuild

 

[DIRQUIN]

OK, sorry about that. damn women. Can't leave anything alone. Well I looked at my diag. flow chart and all it says is that wire should have batt. volts. and so does my diagram. It doesn't show any fuses or relays that it goes to. I dont know. I'm stuck at that point. I dont have access to the sys. that I got this from, so I can't get a better diagram. All I can suggest now is check all your fuses, see if everything else is working, maybe it's tied into some other power source for something else, and maybe its not working too. .......?... You might be able to go to a dealer and con them into giving you a diagram of that circuit. I'm sorry I'm all tapped out. As I said before you may be able to run a new hot wire to your CAS and see if it works. My flow chart says it should have batt volts and so does my diagram. So it can't hurt to put a good wire to it to test it out. Well I may be able to get a better diagram if I can talk my friend into helping out. Let me see if I can pursuade him. I'll keep in contact and let you know if I find any thing. Talk at ya later.

 

[DarthBulk]

Ok, I finally had some time today to check the wiring to my CAS. I traced the red wire, wire #2, to the MPI control relay which is bolted in the center console by the ECU. I also traced the main power wire to this MPI relay from the fusible link on the end of the positive battery cable. I discovered that I have 12.2 volts (battery voltage, I just charged it) on this wire, which is black with a red stripe. So the MPI relay is definitely getting +12V. Then I checked with my digital voltmeter the red wires leaving the MPI relay, one of which goes to pin 2 on the CAS. There is only about 8.95 volts on this wire! Shouldn't there be +12V? This could be the root of my problem. On the AEM dashboard, I get a battery voltage reading identical to the red output wire on the MPI relay, the same wire that goes to the CAS. And here is the really strange part...when I plug in my MAF translator, that voltage immediately drops to 4.95 Volts. So it seems that my MPI control relay is not working. Can anyone offer some insight into this?

 

[DarthBulk]

Wow, I spent 2 days tracing wires and checking electrical connections all over my car. I tested my MPI relay, and it seems to be fine. I can ground pin 8 and it turns on, sending 12V out on the correct pins. I now believe that the RPM problem I have been experiencing is just a symptom of the real problem. After much troubleshooting, I have found that pin 105 on my AEM EMS (which is wastegate solenoid control, or PW #2) and pin 66 (supposed to be "MFI On", but with the AEM it is listed as Lambda #2 available input) are causing the problems. I wired pin 105 to one side of my Perrin EBCS, and the other side of the EBCS is connected to switched +12V power, just like the directions say. Well, this is not working. When I have this wire connected to pin 105, the AEM dashboard is reading 5V battery voltage. This is the voltage being sent to the CAS, the MAF, the injectors, etc. It causes major problems. If I disconnect the wire from pin 105, the voltage drops to ZERO. This means the MPI relay is not turning on. So, I grounded pin #8 on the MPI relay, and I hear it turn on...now the AEM dashboard is reading +12V, and I can spin the CAS and see the correct RPM, hear my injectors pulse appropriately, hear my coils fire correctly, etc. In other words, everything seems fine.
So this leaves 2 problems. Why does the voltage coming out of my MPI relay drop to 5V when I connect pin 105 to the Perrin EBCS, and why doesn't my MPI relay turn on when I disconnect pin 105? Can anyone help me with this?

 

[DIRQUIN]

I'm out on this one. I don't know anything about how the AEM or ebcs works. Sorry, good luck though. Dave.

 

[DarthBulk]

I appreciate the help that you gave, actually it was your advice that pointed me in the right direction. I would not have made it this far without your test