ISM not working at all

[PeterBiggerstaff]

Hi Everyone,

Sorry for so many posts at the moment, If there is someone I can talk to offline or another resource I could use I'd be happy to so I'm not spamming this forum so much.

I'm still working on the issue of #10985 not revving. Last time I posted someone suggested checking the ISM and the injectors. I've had the injectors out and they are all spraying a fine mist. I backed off the mix screw until they were all just spraying and no more.

I then moved on to the idle motor and found it's currently doing nothing at all. I've heard it running before but someone said it should do a sweep when you click the ignition on, it does not. I got my multimeter out and found the following:

The middle pin of the ISM socket has 12 volts when the ignition is on, pin 5 and 3 have nothing.

On the IDLE ECU, the 4 pin socket has:
Pin 8 = Live when the idle microswitch is depressed, dead when the throttle spool moves and the switch clicks out.
Pin 12 = 0 volts when the engine is off, then pulses between 1 and 2 volts while cranking

on the 5 pin socket:
Pin 1 is live (12v) when the ignition is on

I didn't test any of the other pins as I figured some of them would be digital signal lines.

I was thinking about testing 12 volts on the ISM, but couldn't find which pins were live, signal and ground.

Even if the idle speed motor isn't working though, should this affect revving the engine? I figured it would only affect the idle?

EDIT:
I have also replaced ALL fuses and installed the DMC relay update kit, as most of my relays were rusty and none of the fuses were the correct values.

[Bitsyncmaster]

When the idle ECU is powered, you should get 12 volts on the center pin of the ISM and both outer pins should be switching on and off at 100 Hz rate on OEM ECU. That is why the ISM will buzz because you hear and feel that switching power to the ISM.

You test the ISM by measuring resistance from the center pin to each outer pin on the open connector. You should measure about 10 ohms from center to each leg on the OEM ISM.

The ECU grounds each leg of the ISM at that 100 Hz so you need to have the ISM plugged in to see the switching voltage. The motor is what pulls the outer pins up to 12 volts.

After verifying the ISM the best test of the ECU is to borrow another owners unit to plug into your car. Or plugging your ECU into another owners car. Most of the ECU failures are one of the drive transistors to the ISM is not working. But your testing indicates both drivers not working.

[Elvis]

Get yourself the manual and schematics and read read read !


basic stuff like - ISM and revving...should not come up anymore
after reading and understaning this old basic engine.

and - that's the fun behind the onwership - waste your time with
your hobby and learn many things about it :-)

[PeterBiggerstaff]

Get yourself the manual and schematics and read read read !


basic stuff like - ISM and revving...should not come up anymore
after reading and understaning this old basic engine.

and - that's the fun behind the onwership - waste your time with
your hobby and learn many things about it :-)

Hey Elvis, Is there a manual for the ECU out there? I have been using the wiring schematics for troubleshooting this far, but an actual ECU troubleshooting guide or reference manual would be great!

[PeterBiggerstaff]

When the idle ECU is powered, you should get 12 volts on the center pin of the ISM and both outer pins should be switching on and off at 100 Hz rate on OEM ECU. That is why the ISM will buzz because you hear and feel that switching power to the ISM.

You test the ISM by measuring resistance from the center pin to each outer pin on the open connector. You should measure about 10 ohms from center to each leg on the OEM ISM.

The ECU grounds each leg of the ISM at that 100 Hz so you need to have the ISM plugged in to see the switching voltage. The motor is what pulls the outer pins up to 12 volts.

After verifying the ISM the best test of the ECU is to borrow another owners unit to plug into your car. Or plugging your ECU into another owners car. Most of the ECU failures are one of the drive transistors to the ISM is not working. But your testing indicates both drivers not working.

Thanks for that, I'll test those pins tonight. Unfortunately in New Zealand, I can't just borrow parts off another DeLorean as the closest one is a 7 hour drive away. I might need to look at other options if I can't get it working as-is, because I'm getting to the stage where I don't want to keep pouring money into this engine and still not get any result.

I've asked 3 or 4 mechanics if they can help, and every one said "convert it to carb" so I might have to start looking at that a bit more seriously.

[David T]

Thanks for that, I'll test those pins tonight. Unfortunately in New Zealand, I can't just borrow parts off another DeLorean as the closest one is a 7 hour drive away. I might need to look at other options if I can't get it working as-is, because I'm getting to the stage where I don't want to keep pouring money into this engine and still not get any result.

I've asked 3 or 4 mechanics if they can help, and every one said "convert it to carb" so I might have to start looking at that a bit more seriously.

Problems with the idle motor system are rarely caused by a faulty ECU. The most common causes of problems are:

Misadjustments of the idle switch or sticking throttle
Wiring problems
Vacuum leaks
Dirty/sticking vane in the idle motor
Defective idle motor
Defective ECU

I prioritized the order. Putting a carb on the car is like saying you don't understand how to fix the motor on a boat so put some sails on it! Check the voltages and grounds to the ECU and prove out the wiring. Refer to D:05:03 in your Workshop Manual.

[Horsebox]

I sent you a PM, but a couple of pointers on the idlespeed....

You've verified that the ECU gets +12V with the ignition on, and also that the ISM gets +12V at the same time.
You've verified that the microswitch is tripped by the throttle and the grounding signal is sent all the way back to the ECU pin 8.

(please correct me if the above is wrong)

1) You should check the grounding of the ECU i.e. does the harness side of pin 2 on the ECU make a good connection to the battery negative terminal?

2) As mentioned you should check the resistance between the centre pin of the ISM and each of the outer pins separately, and post what it is.

3) The 8 pin white bulkhead connector near the ignition coil is responsible for most (all?) of the idlespeed connections from the engine harness to the interior harness of the car, so it's worth pulling that out and cleaning the connectors (you may have to insert a tiny screwdriver into the male pins to get them to splay out and make good contact with the female connector). Particularly the bottom two, which are for the two outer pins of the ISM, which you seem to having trouble with.

4) If none of that works remove the ISM from the car and rattle it about in your hand - the valve should snap open and closed freely. You can also test the ISM by connecting +12V to the centre pin and then very briefly grounding each outer pin separately - the valve should snap open and closed.

Good luck...

[PeterBiggerstaff]

I sent you a PM, but a couple of pointers on the idlespeed....

You've verified that the ECU gets +12V with the ignition on, and also that the ISM gets +12V at the same time.
You've verified that the microswitch is tripped by the throttle and the grounding signal is sent all the way back to the ECU pin 8.

(please correct me if the above is wrong)

1) You should check the grounding of the ECU i.e. does the harness side of pin 2 on the ECU make a good connection to the battery negative terminal?

2) As mentioned you should check the resistance between the centre pin of the ISM and each of the outer pins separately, and post what it is.

3) The 8 pin white bulkhead connector near the ignition coil is responsible for most (all?) of the idlespeed connections from the engine harness to the interior harness of the car, so it's worth pulling that out and cleaning the connectors (you may have to insert a tiny screwdriver into the male pins to get them to splay out and make good contact with the female connector). Particularly the bottom two, which are for the two outer pins of the ISM, which you seem to having trouble with.

4) If none of that works remove the ISM from the car and rattle it about in your hand - the valve should snap open and closed freely. You can also test the ISM by connecting +12V to the centre pin and then very briefly grounding each outer pin separately - the valve should snap open and closed.

Good luck...

Thanks for those pointers, I'll have a look at these tonight and post the results. I also emailed you (before I saw this post) so if you don't get anything, it may be in your spam.

[Jonathan]

Hey Elvis, Is there a manual for the ECU out there? I have been using the wiring schematics for troubleshooting this far, but an actual ECU troubleshooting guide or reference manual would be great!

Not specifically for the ECU, but the workshop manual includes it:

http://dmctalk.org/showthread.php?102-Workshop-Manual

Read through the Fuel section for starters. Page D:04:15 has a brief diagnostic sequence you can do by reading off a dwell measuring instrument to confirm whether certain aspects are functioning correctly.

As others mentioned, it may very well be that the ECU is fine and the problem is another component. That 46 page section on fuel and emissions explains well what everything does and will help you figure out where your problem is.