5A ECU Fuse Constantly Blowing

[Anarkistic]

I am officially at the end of my automotive electrical knowledge and don't appear to figured out the answer, so I thought it best to consult the hive mind!

Quick history. 172 Ph1 (2000 year). Car started and ran fine. Left it for a month or so over winter (connected to a trickle maintainer). Went to start it....cranks fine but no fuel. Can't hear the relay clicking or the pump priming. Relays are all tested and working 100%.

Lots of electrical testing later has revealed that the 5A fuse in the engine bay fuse board blows as soon as the negative battery terminal is connected (ignition fully off). This fuse appears to sit in-line between the Battery Positive and Pin 30 on the ECU harness. To eliminate any other parts of the circuit shorting to ground, while testing this the fuel pump is disconnected. Inertia cutoff is disconnected. Relays are all pulled out.

Multimeter on the 5A fuse holder shows a nice clean 12.38V to the battery + on one side, and a direct to ground continuity on the other side (red wire going to ECU). If I disconnect the ECU harness, that continuity to ground goes away. So question 1, is that expected for it to reach ground through the ECU when the harness is connected?

A second observation is that there is continuity through the ECU between pin 30 (our 5A fuse) and pin 28 (ground wire). Which explains why the fuse goes to ground when the harness is connected, the ECU is bridging pin 30 to ground. So question 2....is this also expected?

If the answers to both questions above are "That's expected behaviour" then....why on earth is the ECU causing that 5A fuse to blow as soon as the battery negative is connected?

Attached a couple of pictures to sort of show you what I'm on about.

 

[Struggler]

Note sure how useful this is but Kitson's GEN90 thread documented the pins on the ph1 ECU (different to the phase 2 because obviously!). It confirms your pin labelling. But surely if the ECU ever bridges pin 28 to 30 the fuse will blow? Which would point me towards an ECU issue?

I think I have a ph1 ECU somewhere at home, thought its not been used in over 10 years so might be dead, i can check the pin 28 - 30 continuity if that helps. Also looks like a similar check on a ph2 ECU should yield the same result.

 

[Anarkistic]

Note sure how useful this is but Kitson's GEN90 thread documented the pins on the ph1 ECU (different to the phase 2 because obviously!). It confirms your pin labelling. But surely if the ECU ever bridges pin 28 to 30 the fuse will blow? Which would point me towards an ECU issue?

I think I have a ph1 ECU somewhere at home, thought its not been used in over 10 years so might be dead, i can check the pin 28 - 30 continuity if that helps. Also looks like a similar check on a ph2 ECU should yield the same result.

View attachment 1779100

Well I'm definitely holding on to that image! Thanks!

If you do have any ECUs you could verify if there's continuity between pins 28 and 30 that would be a huge help! I don't want to have to yank my ECU out if I don't need to.

 

[Struggler]

I've packed up for today, but can check tomorrow when I get home from work!

There's a thread from kitson about gen90 and fuel pump if you want the other half of that list as that only goes half way.

 

[Brigsy]

At a wild guess check the lambda wiring is not shorted out, Seen this happen on ph2 which pops the fuse. Might not apply to ph1 but i dont believe the wiring is massively different

 

[Anarkistic]

At a wild guess check the lambda wiring is not shorted out, Seen this happen on ph2 which pops the fuse. Might not apply to ph1 but i dont believe the wiring is massively different

I'll definitely have a check, can't hurt! But from what I've read it's usually the 30A ECU fuse that blows when the lambda is shorting, rather than the 5A one I'm struggling with.

 

[Struggler]

Had a quick check here on an old ph1 ECU. Couldn't detect any meaningful continuity between the pins, maybe a little in the order of Mehaohms.

 

[Anarkistic]

Had a quick check here on an old ph1 ECU. Couldn't detect any meaningful continuity between the pins, maybe a little in the order of Mehaohms.

Damn so 30-28 you get no continuity? That's vastly different to mine, I get 100% perfect continuity between those two. And since 28 ends up being a ground once the harness is connected, I guess that means my 5A fused circuit is shorting to ground as soon as I reconnect the negative terminal.

That doesn't seem like a good thing 🤔

 

[Struggler]

Yup basically zero.

Could moisture have got into the ECU and shorted the pins on the PCB?

 

[Anarkistic]

Yup basically zero.

Could moisture have got into the ECU and shorted the pins on the PCB?

I don't know about your multimeter, but full continuity on mine reads as zeros and no continuity reads as 1. For example if I touch my two probes together it reads as all zeros (full continuity). Just checking that when you say "basically zero" you're not actually getting the same readings as me 😅

It's not impossible that moisture might have crept in, or something has become damaged on the board. I guess I'm pulling the ECU to find out 😬

 

[hopgop1]

I don't know about your multimeter, but full continuity on mine reads as zeros and no continuity reads as 1. For example if I touch my two probes together it reads as all zeros (full continuity). Just checking that when you say "basically zero" you're not actually getting the same readings as me 😅

It's not impossible that moisture might have crept in, or something has become damaged on the board. I guess I'm pulling the ECU to find out 😬

Put your multimeter in Ohms rather than just 0/1 continuity, you'll get a reading showing what the resistance between the two pins is.

 

[Struggler]

Put your multimeter in Ohms rather than just 0/1 continuity, you'll get a reading showing what the resistance between the two pins is.

This is what I do. Touching the two probes gets about 0.6ohms resistance. I think the I/O beeps up to ~50ohm.

Between in 28 and 30 I could measure in the order of 20Mohms, note that if I put my thumbs on the two pins I get something similar so very high.