Low Voltage on Fuse #4

[whodean]

I have been trying to troubleshoot a problem with my instrument cluster with digital speedo on my 1989 240SX s13. No illumination and the digital speedo (and HUD) do not work. Before sending it off for repair, I want to make sure it is a problem with the cluster and not something else. During troubleshooting I checked the fuses and while testing them I also tested the voltage at the fuse panel while the car was running. Fuse #4 (the 7th fuse down on the left of the driver side fuse panel) only reads 8 volts. This fuse connects power to the HUD, TCU, Radio, Digital Gauges and Rear Window Defogger Relay. I don't believe this is the correct voltage (but want to confirm) since I would expect it to be 14v when the car is running.

For example, the 9th fuse down (#2) and the 3rd down on the right (#18) both give readings of 14v.

So, should fuse #4 only give a reading of 8v and if not, what could be the cause.

[VStar650CL]

Pretty much no fuse on any system should ever be reading 8V. Anything dragging the voltage down that far would probably blow the fuse, so chances are you have some resistance in the power wires. To find out, pull the fuse and see what the input voltage is with no load. If it's 14V, put a bulb-type test lamp on it to provide a load and see if the bulb is dim. If so, there's resistance in the wiring between the battery and fusebox. If it's bright, then something downstream is pulling the voltage down. Take the devices supplied by the fuse loose one at a time to find the culprit.

[VStar650CL]

PS - If the fuse is supplied through a relay, crappy contacts would be a prime suspect.

[whodean]

Thanks so much for the quick response and for the confirmation!

So, you inspired me to do a bit more testing. And as is often the case with electrical gremlins, it gets stranger.

When I pull the fuse and place a voltmeter across the fuse contacts I get 8V. Then I placed one of my multimeter probes on one side of the fuse connector (the righthand side) and the other on the chassis ground and 14V! Placing the probe on the lefthand side of the fuse connector and the other on chassis ground--6V! Which I believe explains the 8V reading across the fuse connector.

So somehow, I must be getting 6V back fed from somewhere (on what I believe to be the equipment/accessory side of the fuse). Does this seem to make sense and any idea how to find it?

[VStar650CL]

That 6V could be backfeeding from inputs (or blown hardware) in the devices the fuse is feeding, so I'm not sure it means anything. The negative probe when you place the meter across the fuse is measuring the difference between 14 and 6, so I'm not sure that means anything either. Put a test lamp on it and see if the side which reads 14V lights up normally or dim with the gator clip on a chassis ground. I suspect it will be dim and mean you have some upstream resistance, but you won't know until you put a power load on the circuit.

[VStar650CL]

PS - If you were measuring across the fuse in your first test, that isn't how you check a fuse. You check both sides to a chassis ground, both should read battery voltage. If only one side reads battery voltage, the fuse is blown. If neither reads battery voltage, there's a problem with the supply.

[whodean]

Okay, more troubleshooting but none the wiser (maybe a bit). Apologies if I wasn't clear on how I was testing the fuses. I did both by leaving them in as you described and testing both sides and pulling them and checking voltage on both sides of the empty fuse connector as well.

Anyway, I spent some time investigating the TCU which is downstream from the #4 fuse. I ran through the diagnostics from the Electrical System Manual (EL-55) and everything seems to check okay. I then went upstream to the instrument cluster (EL-40). I removed it and checked all the traces on the back that form the panel illumination circuit which connect the 12V bulbs. Applying 12V to pin 1 (more like trace 1) and ground on the other side of the circuit illuminates the panel confirming the circuit is good. I tested pin 1 on the plug that supplies power to the instrument cluster lights (from the TCU) and sure enough it supplies 12V when the headlights are turned on. However, they still do not illuminate which indicates they are not being grounded. And it is not obvious how ground is being supplied to them.

My question is where is the ground supplied from on the cluster? I know this sounds odd but I could not figure-out how the instrument cluster is grounded. The diagrams also are not clear on this point.

[VStar650CL]

The illumination circuit is separate from cluster power, the illumination is both powered and grounded through the TCU. It looks like Red/Blue is the high side and Red/Yellow is the low side. One of them should be modulated in the TCU for dimming, but it isn't clear from the diagram which one. See EL-52. The cluster main grounds are the Black wires on connectors I4 and I8 at the cluster, plus I3 on the Heads Up. See EL-58.

[whodean]

Thanks again for your post. Studying the wiring diagram (EL-52) I don't believe ground is coming directly to the Instrument Cluster illumination circuit. The R/L (Red/Blue) wire comes out of the TCU interconnects through the Dimmer Switch (pin 4) and then terminates in the Instrument Cluster plugs on pins 1 and 14. This wire carries power to these pins (and is marked on the flexible circuit board on the back of the Instrument Cluster as +ILL). The R/Y (Red/Yellow) wire does not connect to the Instrument Cluster (at least not the WD version--With head-up display) rather it connects to the Head-Up Display Control Unit. Similarly, the R/L wire also interconnects to the HUD Control Unit but continues on to the illumination circuit via pins 1 and 14.

There is an L (Blue) wire that connects directly from the fuse panel to pin 10 on the Instrument Cluster which is for the Liquid Crystal Clock. These are the only wires shown on the EL-52 wiring diagram that connect to the Instrument Cluster.

So, my guess is that ground is being supplied by the HUD Control Unit to the Instrument Cluster. What is not shown on the diagram is the fact that a Red wire comes out of the HUD Control Unit and connects to the flexible circuit board via a screw on the back of the Instrument Cluster. The HUD CU must be able to bring this to ground which means if the HUD CU is bad then the Instrument Cluster Lights won't work--ie., it is not being properly grounded.

I had a chance to test this by hooking the Instrument Cluster back up (w/o the HUD CU) and grounding the Instrument Cluster circuit via the screw mentioned above to chassis ground. It worked fine illuminating when the head lights are switched on and back off when they are switched off.

So my conclusion is that I have a bad HUD CU.

[VStar650CL]

Illumination circuits are almost all dimmed from the low side, but the LCD cluster is a little different because the backlights are LED's. LED's can't be controlled with voltage like an incandescent bulb, they have to be PWM'd for dimming. So it would make sense for the HUD to be in charge of that. Sounds like you're on the right track.

[VStar650CL]

PS - You might want to open up the HUD unit and see if there's an obviously-blown transmission. The PWM drive is almost certainly an N-channel transmission or MOSFET, so you might be able to replace it and salvage the unit.

[whodean]

So, it is the HUD CU. I did a bit more research and discovered that you can "test" if one of the key electrolytic capacitors, which is prone to fail, has in fact failed by heating it up with a soldering iron. I plugged just the HUD CU back in and heated the suspect capacitor and voila! the speedo came to life. (I had to use the wayback machine to find the post with the pics: https://web.archive.org/web/20100423001 ... h-hud.html).

I now need to replace the faulty capacitor but of course it won't be easy...it's a small SMD capacitor that is in an awkward place on the board.

[VStar650CL]

From the look of it, you could replace that 'lytic cap with a 1210 or 1812 ceramic MLCC, or maybe an SMC tantalum. That would make replacing it a whole lot less awkward.

[VStar650CL]

Here's a nice size chart. Measure the pad distance and see what will fit:
https://resources.pcb.cadence.com/blog/ ... ize-charts

[whodean]

Cheers!

[VStar650CL]