Parasitic Draw

[NCGermerican]

Hey Folks. I have a 951 but posting in the general 944 thread as it could apply to any of us. I had been hunting a parasitic draw ever since I owned the car (3 years). If I didn't have a trickle charger on, my battery would barely start the car after about 4-5 days.

I did the traditional test (multimeter in series with the negative terminal) and confirmed I was getting about .125 milliamps draw when it should be less than .050. I pulled every fuse and there was no change. I disconnected the power to my aftermarket amps - no change.

I then consulted with ChatGPT and asked "what the unfused circuits in a 1987 944". It responded with something I didn't even think of - the alarm module.

Basically, I unplugged the alarm module and jumpered the connector. My draw immediately went down to .020. What's also weird is that my tach no longer has any weird "floating" issues.

Bottom line - if you have a parasitic draw and can't isolate it via pulling the fuses - check your alarm module (if still connected).

I hope this helps someone and my apologies if this has been covered before.

[PSU_Crash]

Good to know! I also have a parasitic draw issue. To remedy that I've just been putting it on the tender whenever I park in the garage. One of these days I'll get around to doing some real testing.

[NCGermerican]

Good to know! I also have a parasitic draw issue. To remedy that I've just been putting it on the tender whenever I park in the garage. One of these days I'll get around to doing some real testing.

It really takes no time at all to do the testing. I was so frustrated to not find the silver bullet after pulling all the fuses. It was a big dopamine release to see the issue resolved with the alarm module.

You also a PSU fan? (as in WE ARE!)

[PSU_Crash]

Good to know! I also have a parasitic draw issue. To remedy that I've just been putting it on the tender whenever I park in the garage. One of these days I'll get around to doing some real testing.

It really takes no time at all to do the testing. I was so frustrated to not find the silver bullet after pulling all the fuses. It was a big dopamine release to see the issue resolved with the alarm module.

You also a PSU fan? (as in WE ARE!)

PENN STATE!! My alma mater back in the Paterno glory days. I'm still a fan, but after all the BS, then they pulled down Joe's statue at Beaver Stadium. That left a bad taste that I haven't forgiven. I had the pleasure of meeting the coach a few times over the years. Great guy, big smile and an iron handshake. I believe he truly cared about those kids. But, anyways ... I digress.

I know it won't take me long to find. I have all the fancy test equipment. It just hasn't left me stranded yet, so it's "on the list". Other projects have taken precenence lately. I already pulled out the aftermarket alarm. Maybe the factory unit is still in there. I'm not sure yet. Sounds like a good winter project

[ROB III]

@NCGermerican
WOW, 4-5 days and hard to start is excessive. I try to keep a battery tender on my 89 as it would go down after several weeks,but several days is shocking, no pun intended.
I always thought it was the clock which is why I got the Battery Tender in the first place.
Congrats on the sleuthing and thanks for sharing.

[cda951]

The best method to diagnose a parasitic draw in a car with blade fuses is not by pulling all the fuses, but by using a quality automotive multimeter such as a Fluke with a millivolt (mV) setting and checking the voltage drop across all the fuses in situ via the small exposed sections of the legs atop each fuse. One can get away with pulling fuses in a 944 or 968, but once cars began featuring separate CAN networks for body control functions in addition to powertrain interfaces (early 9PA Cayenne, 987/997 and anything later in the Porsche world), pulling fuses can be detrimental as it will often WAKE UP a control module. Hence the non-invasive yet tedious approach of checking voltage drop across installed blade fuses. See link to voltage drop charts below.

The other great tool in the arsenal for finding parasitic drains in cars old and new (plus checking current draw in almost any automotive circuit) is a quality low-current inductive amp clamp that can fit around most battery cables---see link below for my go-to model that I use almost every day at work. This allows one to clamp around the main battery negative/ground to establish the overall current drain (should be less than 50 milliamps/mA for almost any vehicle once all systems power down). No need to disconnect the battery negative terminal to connect an inline ammeter, which can cause issues in vehicles as old as a 944S/S2 (loses idle control adaption, radio code, etc), while newer vehicles often lose OBDII monitor status, one-touch window settings, and more. Again, non-invasive is the best approach.

If a parasitic drain is suspected, the amp clamp can be used to clamp around any wiring branches to help isolate the drain. One can pull up the 944 fuse/power distribution block (please disconnect battery negative before doing so, then use bungee cords to suspend it) and use the factory wiring diagrams to determine which wires to clamp around and isolate the drain.

If none of the factory fuses show excessive drain as posted above, the amp clamp can be used to check any aftermarket connections to the battery terminals, along with factory unfused items like the alarm detritus and the alternator, etc.

https://powerprobe.com/wp-content/uploa ... 240826.pdf

https://www.amazon.com/Precision-316-Cl ... 152&sr=8-1

[NCGermerican]

The best method to diagnose a parasitic draw in a car with blade fuses is not by pulling all the fuses, but by using a quality automotive multimeter such as a Fluke with a millivolt (mV) setting and checking the voltage drop across all the fuses in situ via the small exposed sections of the legs atop each fuse. One can get away with pulling fuses in a 944 or 968, but once cars began featuring separate CAN networks for body control functions in addition to powertrain interfaces (early 9PA Cayenne, 987/997 and anything later in the Porsche world), pulling fuses can be detrimental as it will often WAKE UP a control module. Hence the non-invasive yet tedious approach of checking voltage drop across installed blade fuses. See link to voltage drop charts below.

The other great tool in the arsenal for finding parasitic drains in cars old and new (plus checking current draw in almost any automotive circuit) is a quality low-current inductive amp clamp that can fit around most battery cables---see link below for my go-to model that I use almost every day at work. This allows one to clamp around the main battery negative/ground to establish the overall current drain (should be less than 50 milliamps/mA for almost any vehicle once all systems power down). No need to disconnect the battery negative terminal to connect an inline ammeter, which can cause issues in vehicles as old as a 944S/S2 (loses idle control adaption, radio code, etc), while newer vehicles often lose OBDII monitor status, one-touch window settings, and more. Again, non-invasive is the best approach.

If a parasitic drain is suspected, the amp clamp can be used to clamp around any wiring branches to help isolate the drain. One can pull up the 944 fuse/power distribution block (please disconnect battery negative before doing so, then use bungee cords to suspend it) and use the factory wiring diagrams to determine which wires to clamp around and isolate the drain.

If none of the factory fuses show excessive drain as posted above, the amp clamp can be used to check any aftermarket connections to the battery terminals, along with factory unfused items like the alarm detritus and the alternator, etc.

https://powerprobe.com/wp-content/uploa ... 240826.pdf

https://www.amazon.com/Precision-316-Cl ... 152&sr=8-1

I think this is the method that was described on the "Humbemechanic" YouTube channel a few years ago. That was the only video I saw that detailed the method you described. All of the other's were the "pull each fuse" method.

Yours would definitely be more suited for a new car with Can-bus systems. Although, at $200 that amp clamp is not cheap!

[dr bob]

My diagnostic method mirrors the method @cda951 shares above.

It doesn't take an expensive set of meters to do the testing. Until you get to the clamp-on DC ammeter that is, when reading those milliamp drains with the inductive meters gats to be more and more fun. The basic part with reading the voltage drop across each of the little blade fuses can happen with a $5 Harbor Freight DMM. You'll want to fashion some needle probes to get into the back of each fuse, perhaps as easy as a little work with a file on those HF test leads/probes.

Someplace I saw a table that purported to help determine the current flow through different size fuses, from the voltage-drop values. I'm not sure I'd put a lot of stock in that table, but it's at least a start.

I've been using a handy clamp-on multimeter from Amazon. Search for "UNI-T UT210e Digital Clamp Meter AC DC Amp Meter Clamp Multimeter True RMS 2000 Counts Voltmeter Continuity Tester Capacitor HVAC Tool Multi Tester", and know that it's a common piece available with a few different brand names on it. It's sensitive enough that I need to zero it at the attitude and nearby the wire I'm going to clamp around. It's sensitive enough to measure the earth's magnetic field plus every large electrical load in the vicinity. The extra care is worth the effort, because it reads the same milliamps I see with a regular milliamp meter in the circuit. $30 last I looked, and more than sufficient for the once-in-a-while DIY duty I force on it.

[Tom]

My diagnostic method mirrors the method @cda951 shares above.

It doesn't take an expensive set of meters to do the testing. Until you get to the clamp-on DC ammeter that is, when reading those milliamp drains with the inductive meters gats to be more and more fun. The basic part with reading the voltage drop across each of the little blade fuses can happen with a $5 Harbor Freight DMM. You'll want to fashion some needle probes to get into the back of each fuse, perhaps as easy as a little work with a file on those HF test leads/probes.

Someplace I saw a table that purported to help determine the current flow through different size fuses, from the voltage-drop values. I'm not sure I'd put a lot of stock in that table, but it's at least a start.

I've been using a handy clamp-on multimeter from Amazon. Search for "UNI-T UT210e Digital Clamp Meter AC DC Amp Meter Clamp Multimeter True RMS 2000 Counts Voltmeter Continuity Tester Capacitor HVAC Tool Multi Tester", and know that it's a common piece available with a few different brand names on it. It's sensitive enough that I need to zero it at the attitude and nearby the wire I'm going to clamp around. It's sensitive enough to measure the earth's magnetic field plus every large electrical load in the vicinity. The extra care is worth the effort, because it reads the same milliamps I see with a regular milliamp meter in the circuit. $30 last I looked, and more than sufficient for the once-in-a-while DIY duty I force on it.

This is new to me. Thanks to cda951 for posting it! If there is a voltage drop across the fuse when current is flowing, then the fuse must have more resistance than I ever realized. I guess when current flows, the element heats up and the resistance increases enough to create a tiny voltage drop??

[PSU_Crash]

I have 2 fluke amp clamps. One for high power up to 60A and another milliamp clamp. Takes all the guess work out of it. That is assuming you have room to seperate the wires. Usually that's the hard part.