Auxiliary Fuse Box Location - C4

[TimP]

It probably sounds really stupid but I hadn't realised that my C4 has power locks. The penny dropped when I took the door card off to inspect the passenger lock and I saw the actuator in the base of the door. They haven't been working since I had the car so I hadn't really missed them but now I know that they are there I feel they should work. Ignorance really is bliss.

Everything mechanically looks ok but neither side work so the first thing to do was to check the appropriate fuse which lurks under the passenger side dashboard beneath the 'hush pane'l/light surround in the auxiliary fuse box. Here....


The hush panel is the black plastic box that is screwed to the upper part of the dashboard with two 7mm screws. A third screw is hidden behind the fuse panel at the right hand side of the dashboard.

You have to remove the three Torx head screws first which releases the main fuse box surround....


... and the third 7mm screw holding on the hush box is found at the lower right...


After releasing the velcro that holds the red dash material round the hush box, the box can be manoeuvred down to reveal the auxiliary fuse panel, centrally placed but far in the distance....



You need to be a bit of a contortionist to get to it and see what's going on, but the door lock fuse is the second fuse from the left (blue 15 Amp) - fortunately at the front.

The fuse was blown but disappointingly fitting a new one hasn't fixed anything so further investigation is required. Probably new actuators ...Let's see what Rockauto can come up with....

 

[teamzr1]

There are two fuses related to power door locks
Do the door locks and hatch latch work when using the key FOB ?

Check out the KPE module, CCM and the grounds for it and the door locks

 

[TimP]

Thanks for sending this diagram. I should have said which year my car is - sorry for that. My 1992 C4 doesn't have the PKE key fob. I think this was introduced on the 1993 model which possibly explains the fuse rating difference (15A on the '92 and 20A on the '93) as well. Presumably the diagram for the '92 model will be different?

 

[teamzr1]

OK here is for 1992
As I marked, starting at top a 30 amp circuit breaker

Then that goes to the power lock fuse, test at that fuse to see if getting the 12 volts from that breaker
Check starting at bottom right the main ground for door locks
Check pin A of each switch to see if getting the 12 volts
As noted each DL motor has a circuit breaker

Else issue is the CCM or wiring from it to the DL motors

 

[TimP]

OK here is for 1992
As I marked, starting at top a 30 amp circuit breaker

Then that goes to the power lock fuse, test at that fuse to see if getting the 12 volts from that breaker
Check starting at bottom right the main ground for door locks
Check pin A of each switch to see if getting the 12 volts
As noted each DL motor has a circuit breaker

Else issue is the CCM or wiring from it to the DL motors


View attachment 33710

Many thanks for sharing this diagram and your recommendations for trouble shooting - I'll be back on this over the next couple of days so I'll go through these steps and see what I find.

 

[teamzr1]

You're Welcome
Also check if the DL switches work for lock and/or unlock as you might find they work one way but
not the other would help you to debug the problem

 

[TimP]

You're Welcome
Also check if the DL switches work for lock and/or unlock as you might find they work one way but
not the other would help you to debug the problem

Once again, many thanks for posting your suggestions. Fortunately I didn't need to go this far. Having convinced myself that replacing the fuse hadn't fixed anything, I surprised myself today by pressing the door lock switch and the actuators kicked in perfectly. In my haste to see if I fixed everything with the new fuse, I think I might have 'tested' the door locks with the passenger door lock switch still detached thereby giving a false reading. Everything seems to work just fine now.

Now onto the radio ... having removed the driver's side front amp I find another set of destroyed power transistors .... I have a new set of capacitors and transistors and I might just be lucky that the circuit board hasn't been damaged by the heat. Either way, I'll do a different post for this though.

 

[teamzr1]

Question would be is what caused that fuse to blow and what else is on that same fused circuit ?

 

[TimP]

Good question. As far as I am aware the only circuit that is protected by this fuse is the power door lock itself so if I am correct there shouldn't be a knock on to any other circuit if this fuse blows (although presumably the car anti-theft system will not work if the power locks circuit is not working). However, now that I have bought a complete set of new fuses I am going to go through the fuse box and check if everything else is working Ok. I haven't been aware of anything else not working so there shouldn't be any nasty surprises.

As the doors haven't been working for a while - probably before my ownership - it's not possible to match an event to a failure so I could spend a lot of time looking for something that may not show itself. One thought I did have is that the passenger side door lock cylinder was originally very stiff. If the actuator was drawing a lot of current to turn the latch then it's possible that this might have caused the fuse to blow. I have lubricated the lock and the cylinder now turns much more easily even if the key is still reluctant to go in all the way.

I'm going to keep testing the power locks periodically to see if they keep working. If the fuse fails then I will look deeper and replace the pax door lock cylinder (which I might do anyway).

Many thanks for your suggestions - much appreciated!

 

[teamzr1]

You may be correct that it was a current load that caused the fuse to blow

Notice in the schematic I posted above where it says "important"
Each door's has a small motor to move the level from lock/unlock and the motors have a breaker
That when too much current load trips and will reset when load is gone

Maybe as you mention a switch would jam causing a motor to stay on, overload on current and cause fuse to blow
Else some 12 volt wire for door locks has a cut of insulation and sometimes goes to ground ?
Try a contact cleaner on those slider switches to loosen up how they slide and their contacts cleaned