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Bad ground *NM*

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Greetings!

New to the forum, but not new to Porsche!

I have a '98 Boxster with under 40K miles. The other day while driving it, out of the blue, all of the idiot lights began flashing on and off, and the gauges went dead. It almost acts like it's on a turn signal relay. The fuel gauge will "twitch" on occasion when the idiot lights flash.

Here's a video of it in action: [youtu.be]

I've gone through a bunch of troubleshooting, but I've hit a wall. Battery is fine, alternator/regulator are fine, B1, B10 and E1 fuses are fine, everything else electrically in the car works perfectly. I pulled the cluster to make sure the connections were sound, and they are. I figured it had to be the ignition switch, so I replaced it with a new one but it didn't correct the issue.

The one thing I found that has stopped me in my tracks is the following:

1. The voltage at the B1 fuse with the ignition off is 11.2V

2. The voltage at the B1 fuse with the ignition on is 9.7V

3. The voltage at the B1 fuse with the engine on is 10.7V

If I pull the B10 fuse, the voltage goes up at the B1 fuse location by 1V. Voltage at other fuses on the panel is 12.7V, including the B10 fuse location, so somehow, there's a drop in voltage at B1 which of course feeds the cluster. I assume the low voltage is causing the issue, but I'm not certain.

Any insight would be most appreciated!!!!!



Edited 1 time(s). Last edit at 08/16/2018 03:41PM by Seadweller. (view changes)
The usual culprit
Boxsterra - 3 months ago
is the last thing that was touched. Who did the ignition switch? I've seen people replace a bad ignition switch incorrectly, causing this sort of problem.

If not that, it could be a bad ground, which voltage testing would not tell you. What has been serviced recently?
Greetings, and thanks for chiming in!

The car has not been recently serviced, and prior services have been routine. I replaced the ignition switch myself, but I replaced the switch to see if it would correct the issue, which it didn't, so the switch can be ruled out I believe. The entire ignition assembly was replaced in the car in 2011, so it already had the new, improved switch. When I removed the old switch and disassembled it, I knew by its condition that the switch was not the issue.

For some reason, voltage at the B1 fuse location is low. I'm getting 10.7V at that fuse location, and 12.7 at all other fuse locations, with the ignition on. The voltage rises 1V with the engine running. I assume that the low voltage is causing the issue with the gauge cluster. I just don't know where to look at this point, to determine why I have low voltage at a single fuse location.
FWIW, I've seen many people replace their ignition switch incorrectly. Just because it's seated and the electrical plug is pushed all the way in (it is, right?) doesn't mean it has been done correctly.
Quote
Boxsterra
FWIW, I've seen many people replace their ignition switch incorrectly. Just because it's seated and the electrical plug is pushed all the way in (it is, right?) doesn't mean it has been done correctly.

Understood...I have the same symptoms with the new switch, as I did with the old switch, which leads me to believe it's not the switch. I can't imagine the ignition switch could cause a voltage drop at a single fuse location.

The battery at rest is 12.7V. With the engine running, I'm getting 13.7V at the battery.

I just completely disconnected the gauge cluster, and found the following:

1. At the B1 fuse location with the ignition off, the voltage is 11.8V
2. With the key in the accessory position, the voltage is 11.8V
3. With the key in the ignition position, the voltage is 7.5V

Thoughts? Thanks for the help, this one really has me stumped!!!
Greetings!

OK, I have an update...I found that fuses B1 - B4 are fed 12V via the current distributor. This circuit is evidently not routed through the ignition switch. I have the gauge cluster removed, so it's not the cluster either, so the ignition switch and cluster are no longer suspects.

What I did find is the following, which has me perplexed:

Voltage at B1 fuse location was 11.56
Voltage at B2 fuse location was 11.49
Voltage at the B3 and B4 location could not be read. My meter was jumping all over the place and I couldn't get a voltage reading

Any idea what could cause this?
Bad ground *NM*
Boxsterra - 3 months ago
Re: Bad ground
Seadweller - 3 months ago
Could the bad ground be at the fuse box? How about the current distributor?
No
Boxsterra - 3 months ago
A simplified version of a typical circuit is:

Battery → Fuse box → Electrical part being powered → Maybe relay and/or Switch → Ground point

There cannot be a ground connection at the fuse box.

The erratic readings at your voltage meter are a fortunate find since they are probably exactly what the cluster is experiencing. Download a circuit diagram and look at where the circuit is in the car.
Clarification
Boxsterra - 3 months ago
If there was a ground at the fuse box it would be called a short circuit, not a bad ground. The former is a connection to ground where there shouldn't be one. The latter is a connection to ground that is broken or intermittent.
Figured I'd provide an update in case anyone faces a similar issue. Tested the electrical system from the battery to the pins on the cluster connectors, 12.4V the entire route. Ground continuity tested perfectly. Speedo shop called and he was able to replicate the issue, so the failure is in the cluster itself. He's going to see if he can identify the issue, but he did mention they're not easy to repair as there's evidently no schematics for the cluster board.
Chances are
Boxsterra - 2 months ago
it's a bad ground in the instrument cluster. It should be possible to trace the voltage through and see where the break is.
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