No Spark but all ECU inputs look right

Post here for mechanical and engine topics such as fuel issues, transmission problems, rough idle, exhaust, electrical issues, etc

Moderators: GRNSHRK, ron, bfons

Post Reply
ReluctantRed
Posts: 14
Joined: Fri Oct 19, 2018 9:11 pm
Location: St. Louis USA

No Spark but all ECU inputs look right

Post by ReluctantRed »

Guy I'm new to this forum. My Brother parked a non-running red 1987 653CSi on my lot then left town...so I have to fix it. It cranks, got fuel pressure, but no spark. I sent the ECU to Solo and they say its good.

Back in 2012 a guy called "Area 52" on this forum said these cars need a lot of voltage for the ECU to kick in. He said his had 11.4v which was not enough for the ECU to start working. He changed his positive starter cable and that fixed it, but did not said how much voltage that change gave him.

Can anyone confirm the ECU voltage needs to be over 11.4v for these cars to run? I can't believe it because during cranking most batteries won't even have 11.4v.

I do seem to have a 0.5v drop from the battery to the ignition switch on the red wire that goes through C200 (despite a new battery terminal and clean connections at the battery). May have to restring that wire.

This car seems to have a Mototronic Basic system with a 35 pin ECU.
The coil's green wire gets battery voltage in run and crank. The coil's black wire stays the same as the green wire.

My Pin 4 voltage is only 9.98v when cranking
Pin 18 is hot all the time 12v usually, but 10v when cranking
Pin 35 has 12v in run, 10v when cranking
Pins 5,10,16,17,19, 27, 25 are all grounded to body, engine, and battery negative.
Pin 26 to 25 Cylinder ID sensor is 896 ohms, 0.8Vac when cranking
Pin 8 to pin 27 Engine speed sensor is 900 ohms, 18Vac when cranking
Pin 13 Coolant temp 2.5K ohms to ground
Pin 2 grounded with throttle closed
Pin 3 grounded when throttle wide open
Pin 22 Air temp is 1.9Kohm to Pin 6
Pin 9 to Pin 6 air door resistor is 562 ohm
Pin 7 to Pin 6 varies 800-50ohms as air door is moved
Battery negative to Car chassis good connection
Engine cranks at 253 rpm

I greatly appreciate any suggestions.
imo6en
Posts: 13
Joined: Tue Sep 04, 2018 10:57 pm
Location: TN

Re: No Spark but all ECU inputs look right

Post by imo6en »

Have you checked the resistance at the crank position/reference sensors? Those are a common culprit and will give fuel but no spark. I'd start there.
User avatar
Brucey
6 Series Guru
6 Series Guru
Posts: 10077
Joined: Fri Apr 09, 2004 7:17 am
Location: Cambridge, UK

Re: No Spark but all ECU inputs look right

Post by Brucey »

10V should be enough. Batteries usually droop to around this level during cranking. Some systems fail at 9v but most tolerate a lower voltage than this.

Some things to check;

1) that the main relay is working correctly. I suggest replacing it with jumpers as a test. The reason you need to do this test is that the coil/injectors /ECU are fed by two separate switch poles in the relay and they can go bad such that you measure 12V but when they are under load (i.e. when the injectors are firing and/or the coil is drawing current etc) the voltage can drop to a low level because of internal resistance in the relay.

2) It is very unusual but a bad contact in the wiring harness can do the same thing as a bad main relay

3) do you have fuel pressure? Fuel pumps run on cranking only (until the engine runs). Again the relay can be replaced with jumpers as a test. Included in the list of things that can inhibit the fuel pump relay is the immobiliser relay that is built into the OBC computer. if this goes bad you can disable it by bridging the relevant pins on the OBC computer.

4) do you have CSI function? This is an entirely separate fuel delivery system from the main injectors (i.e. it uses a seventh injector) and is not fitted on cars built 6/87 or later. If the inlet manifold stinks of fuel after cranking then this is normally a sign (but not proof positive) that the CSI is working and you have some kind of fuel pressure.

5) do you have spark?

6) do you have a really good ground connection from the ECU and from the engine harness to the chassis, and from the chassis to the battery negative terminal? These all need checking carefully; all it needs is for there to be a little too much resistance to ground from the ECU and the injectors won't fire. BTW the voltages etc can look fine except during that short period of time the injectors are meant to fire.

hth

cheers
~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
ReluctantRed
Posts: 14
Joined: Fri Oct 19, 2018 9:11 pm
Location: St. Louis USA

Re: No Spark but all ECU inputs look right

Post by ReluctantRed »

I do have fuel pressure, the previous guy replaced the main relay with the wrong type but I have the correct one now with two pin 87's.

Since I have fuel pressure, the OBC relay must be working correct?

I have no spark , no primarily voltage at the coil.

I have no pulses at the main injectors.

I do smell gas so maybe I have the 7th injector. Where is that located?

Both engine speed and cylinder ID sensors read 900 ohms which I believe is correct. I'm getting strong pulses and the correct ac voltage from each sensor. I have oscillograms of the sensor output if you like to see them. Cylinder ID pulses once per revolution, is this correct?

I still haven't tracked down this voltage drop, standby for that result. Thanks for your suggestions guys.
User avatar
Brucey
6 Series Guru
6 Series Guru
Posts: 10077
Joined: Fri Apr 09, 2004 7:17 am
Location: Cambridge, UK

Re: No Spark but all ECU inputs look right

Post by Brucey »

7th injector is (on most E24 82-87) is below the main plenum. The hose leading to it connects to the fuel rail near the #4 injector.

Do be absolutely certain that you do have the right kind of main relay; there are several different sorts that have two outputs and they are not all the same.

When the ignition is on, you should have a good voltage feed (i.e. one that doesn't droop when you draw a couple of amperes ) on both the injectors and the coil. Do you have that?

cheers
~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
ReluctantRed
Posts: 14
Joined: Fri Oct 19, 2018 9:11 pm
Location: St. Louis USA

Re: No Spark but all ECU inputs look right

Post by ReluctantRed »

I know exactly what you are taking about Brucey. The previous guy put in a main relay that had a diode between the 87's.. in the wrong direction at that. And I saw another relay that fit but had NO/NC instead of NO/NO. I'll look for the 7th injector tomorrow.
User avatar
Brucey
6 Series Guru
6 Series Guru
Posts: 10077
Joined: Fri Apr 09, 2004 7:17 am
Location: Cambridge, UK

Re: No Spark but all ECU inputs look right

Post by Brucey »

no voltage at the coil means the feed from the main relay is bad. You need to sort that out, else the car will never go.

cheers
~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
ReluctantRed
Posts: 14
Joined: Fri Oct 19, 2018 9:11 pm
Location: St. Louis USA

Re: No Spark but all ECU inputs look right

Post by ReluctantRed »

I do have 12v at the coil's green primary lead. My ECU is not grounding the black primary lead. Sorry I was not clear. The ECU actually does ground at one time, just when it first cranks, but just one pulse of .25ms or so. I do see the normally ringing after that one pulse, but I never get a train of pulses.

I just when through all the checks again at the ECU connector and it all looks right, just as before.

I cranked it with a 200A car starting charger attached to the battery, which gave it an extra half volt, but no joy with that idea.

I'm starting a new thread to ask about the Check Engine light which is a mystery right now.
User avatar
Brucey
6 Series Guru
6 Series Guru
Posts: 10077
Joined: Fri Apr 09, 2004 7:17 am
Location: Cambridge, UK

Re: No Spark but all ECU inputs look right

Post by Brucey »

have you tested the various 12V feeds to be sure that they hold up under load? A bad contact can show 12V on a DMM but be nigh-on useless when you actually draw current. It is basically the same deal with ground connections too.

cheers
~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Post Reply