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I bought a car to work on with my father as a project and it sadly seems to maybe have some electrical issues after we bought it. The issue we have is that the car will not start, but it does crank. The previous owner said he inheritied from his father so it may of been sitting for a year or two. But, he had it running and idled it for about an hour to make sure it was good. Then as he was driving it shut off and wouldnt start again. Here are some things we have done to test.
1. It seems there is already a VATS bypass and security stops flashing as soon as I turn the ignition on.
2. We have confirmed it has spark.
3. We have confirmed there is fuel pressure. The weird thing with this it seems they are also bypassing the fuel pump relay because they spliced into the +12V on the distributor.
4. It cranks just fine.
5. There is a new battery inside.
6. All fuel injectors measured to 15-16 ohms.
7. According to the carfax the ECM was replaced twice within last 5 years. (Maybe mechanics were misdiagnosing another issue?)
8. With ignition on their is ~12 volts on BOTH inputs to the injectors. From my understanding of the circuits is the ECM should ground one side to complete the circuit.
9. All fuses were checked with continuity.
10. We did replace the ICM since it originally did not have any screws holding it down so we thought it got burned from last owner running it idle for an hour.
11. There is a tach signal coming out of the distributor but the RPM needle does not move at all. (Maybe issue with the filter or connector itself since the clip is broken)
12. The distributors hi and low signals to the ECM have been checked with continuity but havent confirmed that it is pulsing. I have an Oscope coming in tomorrow to probe the hi signal.
13. We also tested the coil pickup per the haynes manual we have. (The two individual lines were open and had 750 ohms across eachother)
Some things we have noticed.
1. We cannot get diagnostic mode to work with the ALDL port. It seems it has been repinned since there are extra connections then on any schematics I have seen (There are only two blanks in the current port).
2. They have the fuel pump bypass to have it on when ignition is on.
3. They did some more funky splices for the AC control lines.
4. The dash does boot up and shows a mileage and gas tank reader so I think its safe to say the CCM is still operating.
5. The schematics we have dont fully match up with the ECM harness. ( I am hoping that the harness was not completely altered from previous owners)
Is there a good spot to get a better FSM, the pdf we have the schematics are very hard to see and may not be correct. Any tips or anything we should look at is greatly appreciated.
Thanks!
For the moment, forget about what the PO said about having it running, maybe it didn't. Someone clearly botched this thing. On mine it turned out that they forgot to put the ground strap in place inside the distributor cap which caused the coil to short out through the tachometer wire. They ran it like this which caused the coil to burn so badly that the pulsations from the short were barely noticeable anymore and I only noticed when I replaced the coil. Point of the story is you probably have multiple points of failure - the original problem and then the botched "repairs"
They have been messing with the distributor and i cannot imagine why they bypassed the fuel pump relay through it. There is a reason it is there and I would start by correctly re-wiring your fuel pump relay, during the process you may find a mistake they made.
I think this is what we were gearing to look at next. The fuseable links may of fused and thats why they bypassed the fuel pump relay. He did send me a dated video of it running 2 months ago so I do want to believe the PO. Tomorrow morning we were gonna make sure all the power inputs and outputs to the ecm were also correct then move onto fixing the bypass. Im hoping my oscope comes before sun goes down so I can actually prob the lines coming out the distributor and fuel injectors. I am worried the ECM is screwed up since shorting A and B does not go into diagnostic mode on the dash. Atleast for getting the car to just start the basics we have are fuel pressure and spark so that to me points to ECM and something with harness potentially related to their bypasses.
Update: We have undone what they did to the fuel pump relay and with ignition on the relay does not click or give power to the fuel pump. Fuses were checked and relay was swapped with one that was confirmed to be clicking. Maybe this is related to ECM issue?
Does the Service Engine Soon light come on with key in RUN? If = no, the ECM is not powering on.
The SECURITY light must be OFF with the key in RUN. If it is solid-ON, the CCM is not sending the Fuel Enable signal to the ECM. (It shouldn't crank either, but it is likely that POs have bypassed the starter relay at some point in the past.)
The 90 has onboard diagnostics that display in the Speedo LCD.
Jumper A (top right) to G (bottom left) and turn the key to RUN.
Watch the speedo. There are (2) Modules. The little number is the Module. 1 = CCM, 4 = ECM. The " - - - " indicates "end of string." An "ERR" in Module 4 indicates a communication problem, possibly the ECM itself.
Codes are displayed as History and Current. A C-code is always accompanied by the corresponding H code. Except Module 1 where C12 means "No CCM Codes Present. In diagnostic mode."
See if the onboard diagnostics help.
Thanks for the help! The ses light does not come on. I will take out the ecm and inspect it then. I have verified co tinnitus from aldl port to the ecm harness though. We also were not getting 5v reference out of the ecm
Update, it seems its not the ecm. We got the engine to almost catch. But the ECM fuse keeps popping for whatever reason. They do have an aftermarket radio installed
The only item on the ECM fuse 3 Amp is the ECM itself. Pnk/blk wire at Cavity 10 on the red connector. Unless the wiring hacks have caused a short, it sounds like you have a bad ECM. Unusual.
The ECM fuse gets its power from the AIR BAG fuse. The intent is to discourage disabling the SIR by removing its fuse. If it is removed or blown, the ECM isn't powered and the engine won't run. Something to check.
according to the carfax the ecm has been replaced twice so I think its something with the connection between ecm and the air bags. The weird thing is it pops almost instantly when ignition is on run. Then when doing a continuity check on the fuse that was blown weird enough it beeps with ignition off but stops beeping when ignition is on. This is testing continuity with it across the fuse. Could something be wrong with the derm or whatever the airbag circuit is?
Probably not. The DERM 10-cavity connector must be plugged in to complete the circuit from the AIR BAG fuse to the input of the ECM fuse. A short in the DERM would pop the AIR BAG fuse, which would also interrupt the power to the ECM fuse. But the DERM can't pop the ECM fuse.
Simple check for wiring or the ECM causing ECM fuse to blow: Disconnect the red ECM plug. Install a new ECM fuse. Turn key to RUN. Does the ECM fuse now hold? If = yes, it is the ECM.
EDIT/ADD: If the ECM fuse still blows with the red ECM connector unplugged, I would look for any wiring hacks involving the pink/blk wire at cavity 10 at the red ECM connector, or under the RH dash in the harness that goes through the firewall on the RH side. If someone has tapped that wire/circuit for the "fuel pump always on" hack, the fuel pump draws considerably more than 3 amps. I suggest looking under the RH dash area as well because that is where the fuel pump relay is located.
Does the ECM fuse still blow with the red ECM connector unplugged? << This is a valuable clue as to where to look for the short.
the fuse was not blowing when red ecm was out. We also disconnected the SIR system. It looks like the ignition switch is grounded. Doing a continuity test on the air bag fuse BOTH sides have continuity to ground with key in ignition and not. Going to investigate the switch to see when they did the vats bypass with resistors in a line if they fucked up something
After further inspection it seems the actual fuse box is just really weird. Per the picture below what is pictured in red all have the same continuity to ground (Without ECM even connected). Even the ECM fuse connections that connects to B10 is shorted to ground. It is important to note that this is with the SIR disconnected and most relays we could see as being disconnected. It does look like there is a bus bar going along that column but it would make no sense to me why you would ever want to short out the whole column of fuses to ground, that would defeat the purpose of an inline fuse if you want to give +12V to the ignition feed of the ECM. Any advice? Tomorrow we are going to pull out the fuse box and do some more inspection of it. Do any of you guys have pictures of the back of your fuse box?
In automotive circuits, I don't think you understand that "continuity to ground" often exists without there being a short to ground that can blow fuses.
On circuits like those fuses in the red circle, there may well be continuity to ground through other devices like light bulbs, relay coils, and injectors that are connected to a fuse. Also, because the fuses are are all hot in ignition, they are common to each other on the left side, so all it takes is one device to ground on one of the fuses, and all of them see the same device. Your meter or test will see these devices as "continuity to ground" but this is completely normal.
Do you have any wiring diagrams for your car? If you did, are you knowledgeable enough to know what you're looking at? I can take pics of pertinent pages in my FSM and post them. Not ideal, and a PITA for me, but I will if you know how to read electrical schematics.
Yes we have schematics, I am not sure if they are from the FSM. I design PCBs and schematics for embedded systems so I can read schematics. It was an observation that the whole column on the right has continuity to ground regardless of position of ignition switch because there is also a bar on the opposite side, i havent had the time to look at the circuits that the other fuses in that column go to yet. Some fuses on the back side do not have any wires going to them and are completely empty, so tomorrow i want to make sure no wire was pulled out and connecting to that bar. The right of the ecm fuse goes directly to B10 on the ECM. So with the ecm harness and fuse pulled out we should not be getting any continuity with ground according to the schematics it should be isolated when fuse is taken out. Power should be flowing from the air bag fuse to the ECM fuse through the derm connector. We also finally got C41 to show up on the dash when my dad moved around the wires around on the back side of the fuse box so something is definetly up. The ecm fuse was out so it makes sense to show up. We have never got the dash to display any C or H codes, so it's progress! We will try more tomorrow it feels like we are closer, I have been enjoying the Easter hug hunt. Thanks for your tips!
If the added words in brackets makes the above a true statement:
WHAT does this tell you?
Hint: The answer is in post #11. It's quite clear, actually.
EDIT: Sorry i did not see this reply. It actually did seem it was still blowing when ECM was unplugged, we were mistaken. So I agree with you thats why we spent today looking behind the fuse box and the ignition switch today.We just ran out of sun light sadly before we could fully dive into the wiring of the fuse box/RH side wiring. The probing we were doing of the fusebox was the very last thing we did. All the relays have been pulled out during our testing. At the very least we are definitely learning about the car lol.
Last edited by Jordanstw; Dec 13, 2025 at 10:34 PM.
Update! We got the car started without the ecm fuse going off. Now we dont really know what was the fix or maybe the problem is still there. We took out the fuse box and we have it kind of floating right now. We also replaced the fuses that were rated to high for the position. The PO also had a wildly wrong timing order on the distributor so now it idles a lot better. Only current issues we need to keep looking at are: codes stopped showing up again when we short a and g (a and b seems to work since both fans turn on), oil and engine temperature gauge does not move, rpm gauge does not move its almost like its stuck way below the 0 line, AC and trip buttons also do not do anything. They did other hot wire hacks for the AC so a lot of that may just be destroyed since it was always on. Some cables were also just unplugged so these may be the only fixes needed but at the very least all the weird wiring hacks they did have been undone. Hopefully I can test drive it tomorrow but ir sounds amazing!
Just as another update incase anyone ever runs into this issue, we did have ECM fuse keep popping and verified all the wires in the interior after the last update. The issue (we think) actually was the engine harness. Since the loom was so old it was cracking away on the harness bracket so everytime the engine/transmission moved when shifting into reverse/park it kept cutting into ONLY the pink/black wire that connects to the ECM fuse.(I forgot to take a picture of the wire and harness bracket but it was definetly cut open from it and confirmed it was the wire that goes to the ECM) So now we took out the whole harness added new loom and more electrical tape everywhere we could. We have mostly everytihng back in and should be able to test drive it for longer! We just have to install the distributor back on and connect back up the starter to see if this officially fixes it. But, if it doesnt I may as well just get a matchbook lol.
The issues we encountered that were not consistent:
1. When turning ignition on the ECM fuse pops instantly.
2. If it does not pop in situation 1 it would pop when going into reverse or park SOMETIMES. (Did not happen frequently and noticed this too late since it was not consistent.)
3. With iginition on and fuse not popping try to yank on the engine harness by the firewall where the wire harness bracket is. (We tested this very early on and it did not cause a pop but when we moved it a lot harder it caused the pop)
Atleast the positive of the story is that the new harness should be protected for the next 40 years and I have some spare ECM's now
1990 C4 Debugging Help
