Strange electrical problem - Engine won't stop!

[Chris1]

Firstly, apologies if this has been done before, I've tried searching and not found anything useful so far!

I recently bought myself a 1990 G-Limited Mk2 MR2, its not perfect, but picked it up as a bit of a project car.
After driving for a week or so I noticed the red engine compartment warning light coming on. The issue soon became clear, the "vent" relay in the rear fuse box was missing!
Ordered a new relay and in the mean time used a small length of cable to bridge the connectors so that the engine compartment fan was on all the time that the ignition was on, just to be safe.

When the new relay arrived, I plugged it in, started the engine and all seemed well, at least until I tried to switch the engine off. Took the key out and the engine kept running. I had to kill the engine by stalling the car. I also noticed that one of the fans at the front of the car was now running (hadn't heard it run before) and kept running for some time after stopping the engine.
I guess this is why the relay had been pulled in the first place.

I've reverted back to my cable bridge for the time being, but would love to know what's going on and hopefully sort this out!

Any help would be really appreciated!

Thanks!

[shinny]

Hmmm - sounds like a short on the relay somewhere. Where did you get the relay from?

For reference: http://shinny.co.uk/toyota/MR2_SW_1993_ ... S/eccf.pdf

[Chris1]

Cheers for the reply.
Just from a breaker on eBay, relay seems fine, all the continuity checks out.
I'm thinking it's something else just because it would explain why the previous owner had pulled the original relay in the first place.

[shinny]

It's hard to figure out how simply bridging the contact is ok but putting a relay in isn't. I do have a hunch though. Looking at the diagram I posted, if pins 2 and 3 (green / white and blue / black wires) were shorted together, it could explain it. Using a multimeter you might be able to test that theory. If I'm right, I wouldn't be surprised if your fan control unit is a little fried too.

Edit: Ignore the above. Checking the power source diagram makes me thing it couldn't work: http://shinny.co.uk/toyota/MR2_SW_1993_ ... wrsour.pdf

One other thing to try is to disconnect the fan control unit (behind the boot carpet on the engine bay firewall, just on the passenger side of centre in a plastic case) and leave the relay in, and see what the behaviour is then.

[Chris1]

Thanks, I'll give that a go and get back to you.
Looking at the diagram, could be a coincidence but the two things causing me issues (ignition and front fan) are both going into terminal 1 of the relay, could be a switch sticking somewhere?

[shinny]

Thanks, I'll give that a go and get back to you.
Looking at the diagram, could be a coincidence but the two things causing me issues (ignition and front fan) are both going into terminal 1 of the relay, could be a switch sticking somewhere?

I think somehow there is a short from pin 3 or 4 to pin 1 - thus, when the ignition is switched off, there is still 12V at pin 1 from the VENT fuse (which is turn is powered because there is 12V on the black/orange wire at pin 1.

An alternative theory would be that that's an internal short between pin 9 and pin 3 on the control unit causing exactly the same effect.

Did the engine bay fan actually spin when the relay was shorted? (You can test this by turning on
IGN, shorting the relay ports and unplugging the temperature sensor on the engine lid)

[Chris1]

With the control box disconnected, the engine starts and stops fine.
With the relay connectors shorted, the fan runs all the time the ignition switch is on, regardless of whether the temperature sensor is plugged in or not.

Does this point towards a fried control box?

[shinny]

With the control box disconnected, the engine starts and stops fine.
With the relay connectors shorted, the fan runs all the time the ignition switch is on, regardless of whether the temperature sensor is plugged in or not.

Does this point towards a fried control box?

So, with the control box disconnected and the relay plugged in, the car behaves normally? IN which case, yes, I'd suggest you replace the control box. I think I have a spare somewhere - if you want to PM me your address and I'll get it in the post to you when I'm next near a post office

[Chris1]

That's right, yeah.
That'd be great, thanks very much. I'll let you know how I get on with it.

[_Al_]

Haven't got the physical wiring diagrams but isn't it possible that the fault is "downstream" of the control box? It could be shorting to another circuit. That's precisely what my old FTO did.

Would it be possible to bridge out the control box just to be sure before you start posting things?

[shinny]

Haven't got the physical wiring diagrams but isn't it possible that the fault is "downstream" of the control box? It could be shorting to another circuit. That's precisely what my old FTO did.

Yes, that's altogether possible. Tricky to find though...

Would it be possible to bridge out the control box just to be sure before you start posting things?

Yes, you could... but the part's already in the post and, TBH, swapping the part is a heck of alot easier

[_Al_]

Good point, well made!

[Chris1]

Quick update on this.
New control box worked a treat, everything now appears to be working perfectly!

Thanks a lot for the help, and for the new unit.

[shinny]

Quick update on this.
New control box worked a treat, everything now appears to be working perfectly!

Thanks a lot for the help, and for the new unit.

Great stuff, glad the fix was simple in the end