Idle ECU repair

[sdg3205]

I am beside myself. Does anyone have any tips for repairing the idle ECU?

I fried 2 ECUs (in both 3205 and 2510) in less than 5 minutes by plugging in a faulty ISM. I tried the ISM in 2510 where it was replacing a buzzing ISM with a stuck valve, and when it
didn't seem to work I tried it in my perfectly running 3205, where it also did not work. Puzzled, I plugged back in the original ISMs and nothing happened. The car wont stay running without the system in place. After checking fuses, pins and diodes I took the now suspect ECUs over to my buddy who owns 2991.

We verified his ISM and ECU were working as they should then simply swapped in my ECUs. Nothing. They are fried. Went back to his ECU and the car ran beautifully again.

I've heard of ways to fix them. I'm all ears. Bitsyncmaster.... I'm looking at you!

[Bitsyncmaster]

It does sound like the ECUs may be bad. A short circuit on an ISM could blow the output driver transistor (or a diode). Normally I would say to unplug and plug in the bulkhead connectors but with two cars not working that would be not the issue. You want to be sure you find that short circuit before playing with another ECU. Most likely the connector at the ISM. Pull the cover off the harness at that ISM connector and look if the wires have broken and are touching. If you want me to verify what blew out on the ECUs, I can do that if you ship them to me.

It's tricky to open the idle ECU box. There are clip tabs (six of them) that hold that connector and PCB in the case. It's the clips on the short sides that are hard to let go.

[sdg3205]

Dave,

The ism harness are okay. I backed off the rubber cover to ensure the terminals had not backed out. It was the actual used ism I was testing before installing.

The new, used ism has no resistance readings between the pins - dead short. It almost instantly fried the ECUs.

I have already opened up the units to look for any obvious damage:

http://img.tapatalk.com/d/13/09/21/7y6uqu8a.jpg
http://img.tapatalk.com/d/13/09/21/e7yhepuz.jpg
http://img.tapatalk.com/d/13/09/21/u6e4ujus.jpg
http://img.tapatalk.com/d/13/09/21/8yta4abu.jpg

I tried ringing out the burn traces and comparing resistors between the two boards but in not electronics savy enough to know where to look. The output diodes are the three legged black ones?

[DMCVegas]

Just for the heck of it, check the continuity of wires going to the ISM. Long story short, the bend in the wiring harness on the plug was too severe on my car, and one of the wires was damaged having allot more resistance than the others. I ended up splicing a new wire in and got my Idle Speed Circuit functioning again like new.

Do a quick Google Image Search on "DeLorean Engine", and you'll see that most cars seem to have way too stiff a bend in the wires just aft of the connector. It could go a long way to preventing any further issues.

[sdg3205]

Just for the heck of it, check the continuity of wires going to the ISM. Long story short, the bend in the wiring harness on the plug was too severe on my car, and one of the wires was damaged having allot more resistance than the others. I ended up splicing a new wire in and got my Idle Speed Circuit functioning again like new.

Do a quick Google Image Search on "DeLorean Engine", and you'll see that most cars seem to have way too stiff a bend in the wires just aft of the connector. It could go a long way to preventing any further issues.

Hey Rob,

Thanks so much for the suggestion. I'll check for the sake of future issue prevention.

A good ECU sends 12v to the ISM with the key at "run" but I measure just a tad over 6 v with the bad ECUs.

I verified this on Dan's car (2991) last night when i shooting out the ECU's.

I also ran the test of unplugging the inner harness on the ECU, which (should) cause the idle to speed up to 2,500 RPM. The result was as expected, with his ECU causing the desired (good) reaction of 2,500 RPM and both my fried ECUs doing nothing at all. I tested this function on BOTH my ECUs last week and verified both ECUs were good with the good ISM from 3205, which ultimately allowed me to narrow down the seized ISM on 2510.

When the replacement ISM arrived I quickly went to plug it in to 2150's harness and confirm function by simply listening and watching for the valve to move over the course of 30-40 seconds. When nothing happened, i walked over to 3205 and tried it there. Again, nothing, so of course i moved the plug back to 3205's good ISM which was then i noticed it was no longer humming. This is when my heart sank and i began running the tests that ultimately determined the new/used ISM had fried both ECUs upon plugging in.

I bought the the ISM from a forum member in good standing, a fine gentlemen who has already refunded my money for the bad unit. These things happen, and it was a risk i took buying a "working" used unit. These are old parts prone to failure, and thats just part of DeLorean ownership. Sure enough the meter shows a dead short across all motor pins.

I am going to try and get the ECUs in to my electronics whiz that does the Craig/ASI refurbs I sell.

I am officially and forever the guy that fried two ECU's in less than 5 minutes with a bad ISM.

[Bitsyncmaster]

From memory, the 12 volts to the center pin comes from a diode on the circuit board. If you have any experience with an ohm meter you can probably find the blown components. Check those large diodes (Black cylinders with silver writing and silver band) near the two driver transistors. Also do an ohmmeter check on the two driver transistors (normal NPN type). You may have to pull the componts to find an open componet.

I think the diodes are two or three amp devices so a shorted ISM will exceed that current.

[Elvis]

Send them over and I'll repair them for you.

Schematic and testing equipment are available.


Just find a cheap way to ship them to Germany and back.

[sdg3205]

Elvis,

Thank you for the offer. Same to you Dave. I am going to exhaust all options here first.

I took them in today to a shop in Vancouver. He found 2 bad diodes and transistors in each ECU. He replaced them and I brought them home.

I plugged them in an experienced a new weird phenomenon:

The ISM doesn't make any noise, and the car doesn't idle very well, but it's... better. It doesn't die, but smells super rich. Also, when I unplug the harness from the ISM the car WILL die, which means though I can't hear anything, the ISM is doing something.

Thoughts?!

[Bitsyncmaster]

Elvis,

Thank you for the offer. Same to you Dave. I am going to exhaust all options here first.

I took them in today to a shop in Vancouver. He found 2 bad diodes and transistors in each ECU. He replaced them and I brought them home.

I plugged them in an experienced a new weird phenomenon:

The ISM doesn't make any noise, and the car doesn't idle very well, but it's... better. It doesn't die, but smells super rich. Also, when I unplug the harness from the ISM the car WILL die, which means though I can't hear anything, the ISM is doing something.

Thoughts?!

You should hear or feel the ISM vibrating when the engine is running. Have you done the "screwdriver to the ear" trick? Don't mount the ISM to the block and you could feel it. The ISM should normaly stay where it is when you unplug it quicky.

[sdg3205]

My ISM on 3205 has always been loud, but I know the ECU isn't working properly because pulling the inside plug on the ECU will NOT cause the idle to surge to 2500 rpm.

It's like the system is kinda working, but at like 50%. I noticed the circuit board was quite hot up by the socket pins, where the terminals in the ECU sockets meet the circuit board.


Edit - anyone have a schematic I could use?

[nkemp]

Has anyone ever tried one of the Volvo Idle Speed Control Modules? Seems to me that even the 4 cyl versions might work. Since they are modulating the valve as a function of RPM to maintain RPM does it really care how many cylinders? Just a thought that is a little off topic from repairing the module (If it worked, at least your car would run while you repaired the originals). Note that there are differences in the size of the connector.

[Bitsyncmaster]

Elvis may share his schematic if you ask him.

A four cyl ECU would not hold the correct RPM (would be 33% higher RPM). The ECU times RPM with two ignition pulses so the uneven fire gets nulled.

[nkemp]

A four cyl ECU would not hold the correct RPM (would be 33% higher RPM). The ECU times RPM with two ignition pulses so the uneven fire gets nulled.

Good input on the difference. It might be a temporary alternative while one gets their own repaired/replaced or to get the car home. If it ran 33% higher the RPM would be in the 1,000 - 1,100 RPM range (or a bit more depending on the car). The 4cyl are more abundant and as such can be had for less (I think I got one for about $12 delivered).

WARNING: YMMV or more appropriately YISMV (Your Idle Speed May Vary :-)

[Elvis]

Why should the speed go up to 2500rpm ? Never heard of that before.

It can go anywhere from fully closed to fully open and the speed will change accordingly.

[DMCMW Dave]

Why should the speed go up to 2500rpm ? Never heard of that before.

It can go anywhere from fully closed to fully open and the speed will change accordingly.

With the engine running, if you unplug the lead that goes to the thermistor and tach pulse lead, the valve will go wide open and the car will idle about 2200 rpm.

[Elvis]

Dave,
I'm sorry - yes you are right. I thought about the other connector with the power input and
motor output or the connector on the idle motor itself.


The offer ist still out there but might mainly be interesting for european owners:

I repair and test ISM units ! (and all other ECUs of this car)

You can bring them to Deloman if you don't trust me - he sends them to me anyways...


Shipping costs from the US shouldn't be that high, I shipped remote ECUs for little money,
it might be worth to check it out.

[dn010]

If trust is an issue, I can attest to Elvis doing great work on these; fixed my ECU probably 9 years ago & it was (before pulling it for EFI conversion) still working good. I think I sent it small priority mail box, cost wasn't bad but that was a long time ago.



Dave,
I'm sorry - yes you are right. I thought about the other connector with the power input and
motor output or the connector on the idle motor itself.


The offer ist still out there but might mainly be interesting for european owners:

I repair and test ISM units ! (and all other ECUs of this car)

You can bring them to Deloman if you don't trust me - he sends them to me anyways...


Shipping costs from the US shouldn't be that high, I shipped remote ECUs for little money,
it might be worth to check it out.

[sdg3205]

I don't doubt elvis does fine work. I'd prefer to exhaust all my options more locally before I start sending things overseas.

I appreciate your input!

[sdg3205]

Once again, with the help of this fantastic community, I have two working ECUs.

[nkemp]

so what did you do to get them working?

[Bitsyncmaster]

so what did you do to get them working?

He got some used boards (without the connector) from someone.:giggle: