Hello Everyone,

I've been trying to diagnose the infamous "Add Coolant" buzzer on a friend's FD, and before anyone suggests searching the forum - I already have! Below is a detailed timeline of the issue, the steps taken so far, and where we're currently stuck.

Vehicle: Stock 94 FD

Background:
This all started after we upgraded the vacuum lines, injector o-rings, and the fuel pulsation damper. Once everything was reassembled and we fired up the car, the coolant buzzer immediately began sounding. We checked the coolant level -- it wasn't low, but it was dirty, so we performed a coolant flush. We then inspected the coolant level sensor wiring and found a couple frayed/split sections. We repaired the wiring, but the buzzer persisted.

Next, we replaced the coolant level sensor, pulled the UIM and ignition coils off to check the rear rotor house ground is where it should be, and also the ground that connects to the back of the UIM. These grounds were verified to be correctly connected. We also verified that the coolant level sensor wiring was correct -- the black/red wire leading into the harness behind the alternator connected properly to the brown/white wire that's behind the power steering pump into the harness. Everything checked out there as well.

We then tried replacing the CPU2, but unfortunately that didn't resolve the issue either. I've read that grounding the level sensor wire can silence the buzzer, but that feels like masking the issue rather than fixing it. I've also seen suggestions to desolder the speaker on the CPU2 which we would like to avoid if possible.

Important notes:
1. add coolant light is NOT illuminated when the car is running, only the buzzer sounds.
2. This vehicle had this buzzer issue before; the dash was sent out for repair and returned and was working with no issue before we began the maintenance.

At this point we are stuck and not sure what else to check. Any ideas or suggestions would be greatly appreciated!!

Mike

 

CPU 2 gets a signal from the cluster to sound the buzzer for two conditions, overrev and low coolant. Unfortunately, the wiring diagrams and body electrical manual do not specify what the trigger from the cluster to CPU 2 is so we have to deduce what the input trigger could be based on the information that is available. I believe both conditions from the cluster to the CPU 2 buzzer uses the same output.






When reviewing the instrument cluster wiring I only see one output to the CPU with no direct correlation. We can see that the low coolant ground signal goes into the instrument cluster "brain" at pin 3D and a signal comes out on a (BR)own wire at pin 3B into harness "I" and crosses over to Harness "D" at connector X-17.



Continuing our look, the (BR)own wire enters CPU 2 through pin 1L at connector "J4-01".




Try depinning 1L at the CPU2 connector J4-10, it's the brown wire, below the retainer tab.
If that works you have a faulty cluster, or a short in the wire between the cluster and CPU.

If that does not work, you have have a short internally in CPU 2 that is causing the buzzer to sound continuously.

 

Interesting.... Perhaps not related to the OP's question, but the table above got me thinking about my FD's buzzer operation, and might be a clue towards the OP's issue. In my case, I tested the operation of the low coolant alarm buzzer & light during my last coolant change and it works perfectly - all you need to do to test is key on (but don't start!) the car before you add coolant, and you'll see the light stay on and the buzzer will be screaming continuously as it says it should in the table above. Once you add enough coolant to reach up to the sensor, the light goes out and the buzzer stops screaming.

Now on my FD, that same buzzer NEVER goes off when I rev past 7500 RPMs as the table says it should. I always just assumed that's because I'm running an aftermarket ECU (Link G4+ Fury), so there would be no input from the OEM ECU to tell the buzzer to go off during an over rev situation. But my Link ECU does send a PWM signal on an AUX output back to the cluster to drive the tach, which works as it should. Specifically, that would be via the L/Y wire in the (F) harness that I tapped off of the ECU connector B1-01, pin 1N that ends up terminating at the cluster in pin 4C of the C1-01 cluster connector. So that has me thinking if you're correct that the cluster & CPU2 are what really drives the buzzer, I should be getting my buzzer to go off during an over rev beyond 7500 RPMs, since the tach works and is getting its engine speed input from a (substitute) ECU.

 

Thanks for this info! We will look into this next and I will report back my findings!

Mike

 

Right now it's a hypothesis and I'm unable to test any of it because I switched over to a digital dash nearly 2 years ago! I'm curious if anyone who repairs clusters could enlighten us, do you have an open dialogue with Geoff since you bought the car from him? I know he repaired the clusters for a period of time.

 

Haven't been in touch with Geoff in a while, but yeah he might know since he used to do cluster repairs.

 

Is there ab re-occuring air bubble in your system from the system not fully burped after a coolant change?

 

That was a thought as well, but the coolant system was properly burped after the coolant change was completed.

 

Hey I'm facing a similar issue on my 94, did you figure anything out? Also you should ground the coolant level sensor as a quick sanity check

 

So, we found the problem, and it turns out it's the coolant buzzer.... it's the airbag module underneath the dash that is screaming. Not sure what could have caused this to trip out ... the diagnosis continues!

Mike

 

Screaming air bag module?

 

That is correct. Noticed that it beeps simultaneously with the airbag light flashes (about 8-9 times). Then the buzzer remains constant while airbag light is off.

 

The matrix below is from the attached body electrical troubleshooting manual. Download the manual and navigate to the air bag section to follow the troubleshooting procedures for the amount of flashes and beeps you have. Did you recently remove an air bag crash sensor or swap the steering wheel for a non air bag unit?

 

I came across this diagram as well. Nothing was touched or removed pertaining to the air bag system at all. Very confused on what triggered it.

 

Check inner dash for mouse or rat activity.

 

This is what I am afraid of