Strange electrical thing

[rblong]

You guys are going to think I lost my cookies, but I have been working on some upgrades to my cooling system so I have been running a lot of test on water temp, fan CFM etc. In other words I have been spending a lot of time watching digital meters and the water temp gauge! I noticed that when driving at night sitting at lights that my temperature seemed to creep up (20 degrees - up from 195-210) and my oil pressure falls off (5-10lbs) . I assumed the motor was getting hotter causing lower oil pressure but it is a night and cooler than during the day - and it don't do it during the day! This weekend I was changing stats, starting to fab shroud and decided to run tests, of course it is day so I don't have the lights on. I am watching temp gauge and a digital thermometer, oil pressure gauge etc. and all great with fan cycling on and off as it should. I think what the heck is happening at night - I turn on the heaglights and guess what - the temp gauge rises 15 degrees and the oil pressure falls 5 lbs. Now I have a digital meter going and the actual temp is not changing. I am thinking that maybe I have a voltage drop with lights on so I check it. Sure enough I am running 14.5 VDC with no lights and 14VDC with. Could this cause it? I am running all VDO gauges. Now mind you this is early in the day and I hadn't had anything to drink but coffee yet!!! Too weird for me.

Bruce

[Excaliber]

Sounds like a bad ground to the fan motor, which runs OK with the lights off. But with lights on the load is 'just great enough' to cause the fan speed (rpm) to drop.

I could actually HEAR the difference in my fan speed when I got the ground AND the primary connections better. Until then, I never knew the fan could go that fast! Fans draw a LOT of amps, which is not reflected as much by the VOLTAGE drop. If the AMPS fall off the fan slows down.

I suspect the oil press drop is a 'red herring'. At any rate, focus on engine water temp.

[Excaliber]

Friend of mine had his car radio go off during the DAY, but it would work fine at NIGHT!

Turns out his alternator was OVER CHARGING and the radio would auto shut down with the higher voltage. Turn on the LIGHTS, voltage dropped, radio worked fine!

Don't ya just love electrical stuff!

[rblong]

Excaliber - I suspected the fan slowing down also, but using a digital submerged in the same water and the water temp is not actually changing - just what the gauge is reading is changing. I will use a strobe on the fan just to make sure though.

Yea, these kind of things, even though not serious, bug the heck out of me till I figure them out. I'll bet that radio thing was a hoot to figure out!

[Excaliber]

That radio thing had me tearing my hair out!

I noticed while testing several temp gauges on some Formula Ford race cars:

The gauge reading was affected by where the sensor TUBING was located. The first few inches of the tubing, coming from the sensor end, can dramatically affect the gauge reading. I heat wrapped the first few inches of the sensor tubing and, in some cases, relocated it to get a more accurate reading. I was REALLY surprised how much difference it can make. It IS subject to outside air temps, fan air flow on or off, DRIVING air speed slow or fast, header heat, etc.

I wonder as well if the gauge readings are being affected by a voltage change, lights on or off.

[sidewaysinthird]

There are always a few things that can be overlooked when something that seems so simple. With that in mind and just a guess from someone else who isn't looking under your hood like you are: The digital meter you are using to check the water temp is in the _____ and the actual temp sender for the gauge is located? With the fan running the radiator temp may stay fairly constant and the sender if located in the either the head or on top of the manifold may show the actual temp. The other guys are correct about the lights being on causing a voltage drop which could slow the fan speed, but it can also slow the idle speed. However just the 100rpm or so drop from a 60amp alt at full charge doesn't seem enough to cause a reduced water flow enough to cause such a rise in water temp. Another guess might be to check as was said before all the grounds for the fan and the dash panel. Just to ensure less of a voltage drop with the lights on you could hook a second battery via jumper cables and see if the gauges give the same reading.

Like I said these are such simple little things which of course makes them very difficult to hunt down. Just my thoughts. Best of luck.

Mike

[slithering]

Bruce,

I have a 90 amp alternator, it will not create the correct voltage until I exceed 1200 rpm. You might check your headlamps, just to make sure the positive and negative post are not touching, at the lamp itself.

Chris

[jarendall]

factory built cars have a voltage regulator for the instruments and instrument lights, this keeps a constant voltage to them at all times, you most likely do not have such a voltage regulator, neither do I, this will cause the instruments to read different under varying voltage loads, most instruments are designed to work under certain voltage, i.e. 10 to 14 volts, check with the manufacturer of your instruments and see what the voltage ratings are for the instruments, you may have to install a regulator for them, or your alternator maybe swinging to much, it has a built in regulator

[rblong]

I tend to agree with papajohn. Because I think I used the wrong term when I said the temp gauge creeped up, because it does not creep as much as it immedietly rises, and the oil pressure immedietly falls the values indicated in my original post. I will contact maufacturer since after more testing my temp and oil pressure are not actually changing just because I turn on the lights. It has to be a voltage thing whether a conection, ground, alternator or lack of regulator. I have been over the car inside and out over the last 18 months and have never seen anything like a voltage regulator and I do see a voltage swing under different loads.

Thanks to all and I will keep investigating.

Bruce

[rblong]

Well finally had a chance to talk with Siemens and they sent me a few trouble shooting guides. A part of one says :

"If your gauge's pointer moves noticeably when the lights, fans, or air conditioner turns on, you are experiencing a low voltage condition. This may be due to the following factors:

You have installed the gauges to an overloaded circuit either on the positive or negative wire. Re-wire to eliminate problem.
Voltage output of alternator during idle conditions may be too small for electrical system. Hook up voltmeter and monitor voltage. Voltage should not drop below 12.75 volts at any time. "

I have gone to some higher output lights come to think of it so I will check to see if the VDO gauges happen to be on the same circuit. I do not see the problem when the fan is on and don't see any voltage drop at battery/fan etc. Doesn't just do it at idle either. They also talk about minimum wires sizes and I will check that also. I suspect whatever circuit feeds lights/light switch powers dash gauges also.

Maybe this will help someone else also.

Bruce

[slithering]

Bruce,

Are your oil and temp gauges electrical sending units or mechanical? If you are dropping voltage at the alt. or regulator, it may show itself in your gauges, when there is no other problem than a voltage drop. Your lights should be on a circuit by themselves. This does sound like a voltage drop problem but it may be positive or negative. Have you always had this problem?

Chris

[undy]

Bruce,

I know you said you'd checked your alternator output before but your car's the same vintage as mine. LSC's electrical systems, although faily well thought and installed, were (are?) somewhat anemic. For example, the wire from the alternator output heading back towards the battery is only #10, not sufficient to feed the battery that feeds our "everything's electric" Cobras. This is especially true with what everyone uses, March's under drive pulleys. My electrical problems, although more pronounced than yours, had a major impact on my gauges as yours did. Since my modification the gauges remain stable under all electrical load conditions. With the lights on, fan running and radio blasting I'm supplying the fan (and the entire system) with 14.2 volts.

Electric pressure/temperature gauge readings, simply stated, are the product of a variable resistance to ground sending unit at a fixed voltage (hopefully) at a given temperature/pressure. If you change the voltage as well as the temp/press you'll change the gauge reading. Some of the older factory installed systems utilized dedicated voltage regulators/limiters for their pressure and temperature gauges. The Brits were famous for that... courtesy of Lucas, God of Darkness...

[rblong]

Yes, I agree with everyone's analysis. As mentioned, after talking with Siemen's the light went off about voltage drop associated with gauge movement and about the same time I noticed the drop I had changed to new high output lighting. Of course at the same time is when I did a complete drive line change so I was not thinking of just the lights. I agree the lighting should be on a dedicated circuit, but even though Lone Star did not build this car, I have not liked the way a lot of the circuitry has been run and actually (have rewired some) planned on rewiring/cleaning up the whole electrical system at some point.

I will check the main power and trace everything out this weekend. I will make whatever particular changes I need to right now to get me through the summer so I can keep driving! My car has a custom wood dash and has developed a small crack, I was thinking of pulling it and building a new dash this winter and that would be the time to rewire everything like I want it.

I will post what I find that solves the immediate issues just as an FYI. Overall I really like working through this stuff (as long as it isn't keeping me off the road)- keeps me out of the bars - not the booze, just the bars!!!!

Bruce

[REDSC400]

Bruce:
The fans and lights and horns on the ERA are all grounded to the front suspension X member via a common ground wire bundle. I had a headlight issue and parking lite issue that with Bob's help and guidance was found to be a grounding issue. We deduced and defined the overall cause as old and less than optimal connection of the ground (13 years and a wee bit of too much powder coat and surface corrosion).

In my situation, the frame was powder coated and the common ground connection on the X member needed some attention. What I did was to scrape some of the coating on the X member and buff up the wire bundle contacts. I sprayed the tapped hole with some CRC contact cleaner and then put on a dab of dielectric grease. This helped out greatly.

I then followed from my trunk mounted battery to each and every grounding strap. Where the straps or cables were bolted to the frame or engine/trannie, I removed the bolt, cleaned and buffed the frame (removing coating and again CRCing and Dielectric greasing) and also buffed the cable/strap connection.

Although this sounds like a tedious and complex deal it was easily done in just one episode of Car Talk (NPR Radio) on a Saturday morning. That said all my lights work swell. My advice to you would be to first and foremost check and insure all ground points from the battery to frame are 100% perfect.

Second, I would see if the Lone Star employs remote relays on hi or somewhat hi amperage circuts like the Headlights and fans. If not you may want to consider using them. The relay will eliminate long and resistive cable runs which sap voltage.

I would also use an ohm meter to trace typical wire resistance in the gauge you car uses. This can help point out a poor solder joint or crimp and perhaps a bad wire.

Hope this helps Bruce.