I'm having electrical issues with my car. I thought about posting this in 240 general chat, but I'd like a better understanding of the issues I'm currently dealing with. Car is a 93 s13, small battery in the trunk, problem is that my aftermarket (Prosport) water and oil temp gauges read 40 degrees hotter when the engine is running than when the key is in the acc position. I have all of my gauges (boost, wideband, oil pressure, oil temp, water temp) wired through a relay pulling constant power from the alternator and switched from my radio harness. I know the temp senders are just thermistors, but could ~14v from the alternator vs the ~12v from the battery have that much of an impact on the readings? The gauge should be regulated so I can't think of what would be causing such a crazy drop between engine on and engine off.

Something else to note is that the readings change when I'm accelerating as well. If I accelerate through a gear, the faster I go the lower the needles go, almost to the readings I'm getting with the engine off. Obviously my alternator has something to do with what is going on. Sounds like bad grounds, and both oil and water temp senders are grounded to the same spot, so I'll need to check that.

Battery, starter, and alternator show 12.21 volts with the car off. Tomorrow's to-do list is check it with the engine running, as well as cross-reference coolant temp with my consult cable to see what the temp really is, then re-check the grounds from the senders.

Anyone ever had a similar issue or know enough about electricity to help my diagnose the issue? Thanks!

A bit drunk still but bare with me, thermistors work based on resistance right? So the higher the temperature, the lower the resistance in the sensor/sender, the more amperage gets through, the higher the reading, correct? So for you to be seeing an increased reading under system voltage with the engine on, vs battery voltage with the engine off, means there must either be either more resistance in the sensor with the engine off or more voltage with the engine on. Since the answer to that question is obvious, what does that leave you with? It sounds like a simple problem, the gauges only read correctly when using system voltage, but you're right, they should be built/programmed to display correctly whether the engine is on or off. One possibility is perhaps they are only meant to display correctly under system voltage? In leu of that, is it possible your gauges' regulation system is malfunctioning? If this happened on both gauges at the same time this seems unlikely. The acceleration thing puzzles me too, as I don't believe an alternator loses any efficiency the faster is spins, voltage is constantly regulated to ~14.4 volts. Here's what I would do, check the system voltage when the engine is off, on, and revved to say 3K, get hard figures for those three conditions and then get back to us. It may spark an idea, FWIW, that's all I'm trying to do right now. Whatever you do, don't let out any magic smoke

Still haven't checked the voltage with the car running, haven't had a chance to. In the mean time, here's something else to think about.

If it's not something that has to do with the voltage going through the gauge, maybe it's something on the sender side. The colder the engine, the more resistance the thermistor has. This means that when I turn the engine off, there is suddenly more resistance in the circuit. I'm getting power from the alternator charging post, and the sender is grounded to the alternator casing. I'll measure the resistance I'm getting between the alternator casing and a chassis ground, but technically a ground is a ground and wouldn't matter. Could there be something else that suddenly changes the resistance when the engine is turned off?

Is the post on the alternator? I'd check to see if it gets consistent power.

It does indeed get consistent power, it's the same as the battery.

Engine off has the system at 12.21 volts. Engine on is 13.41 volts, regardless of the rpm. The thermistor's range is 32 to 925 ohms, cooler temps mean more resistance. I measured the resistance when the engine was cold and it was over 1000 ohms, and since the gauge doesn't start reading until 100 degrees that makes sense. With the engine off the resistance went up, which shouldn't be happening.

Resistance is additive, so somewhere in my sender circuit I have extra resistance, but only when the car is off. I re-checked where everything is grounded, and found where that extra resistance is probably coming from. I have all of that stuff grounded to the stud on the engine mount, which means the ground has to go through the painted engine bracket before reaching the block, which I know for a fact is grounded. Technically the alternator casing is also a ground since it bolts to the block, so that contradicts my bad ground theory.

So now the question is why does resistance change when the engine is off? I'll re-do all of those grounds when I get home from work, but until then can anyone think of a reason for resistance to change?

OK how about this for a theory, when the engine is on, electricity is coming from the alternator, and therefore must ground back to the alternator to complete the circuit. When the car is off, the electricity is coming from the battery, and so must ground back to the battery to complete the circuit. So if this theory is correct, then your problem is that your engine block to battery ground has too much resistance. Trying cleaning the connections or relocating the negative battery cable the grounds to the intake manifold, see if that's where your problem is.

Fixed! It was the sender grounds, something was happening with the engine off that created more resistance, thus the higher readings. I went ahead and grounded everything to the chassis then drove around monitoring temps with my consult cable, both gauge and ecu read within 5 degrees of eachother. Thanks for helping me troubleshoot and toss ideas around!

Chris28 wrote:Fixed! It was the sender grounds, something was happening with the engine off that created more resistance, thus the higher readings. I went ahead and grounded everything to the chassis then drove around monitoring temps with my consult cable, both gauge and ecu read within 5 degrees of eachother. Thanks for helping me troubleshoot and toss ideas around!

A fully charged battery should read 12.6 volts. Test the battery with a hydrometer. If all cells do not show or accept a full charge its time to replace the battery.

About $10



Telcoman

Don't you love it when you reply to someone's problem thread only to find out they solved the problem a month ago... LOL

(I kid) happened to me just the other day...

SX APPEAL wrote:Don't you love it when you reply to someone's problem thread only to find out they solved the problem a month ago... LOL

(I kid) happened to me just the other day...

It does happen!

But what is funnier about Telco's post is that the quote in it is the specific post from the OP that says the problem is fixed!

Z

Chris28 wrote:Battery, starter, and alternator show 12.21 volts with the car off. Tomorrow's to-do list is check it with the engine running, as well as cross-reference coolant temp with my consult cable to see what the temp really is, then re-check the grounds from the senders.
Anyone ever had a similar issue or know enough about electricity to help my diagnose the issue? Thanks!

My response to Chris was over his measurement of 12.21volts with the car off. A 12v battery will show that reading with a dead cell. And yes a proper ground is important and not having a good ground will produce weird problems