# Problems with Gauges



## piotrsko (Dec 9, 2007)

If you are running brushed DC then you are electrical polluting the sensor data.

Fuel gauges on fords go from zero ohms (full) to Eighty ohms at empty on a 5 volt bus. You would never see 0 ohms installed in a tank.

Try running a bit on just the marine battery with no dc-dc see if you get symptoms. 11 volts wouldn't be too bad with lights on, just cant do that more than say 20 miles.


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## sailfish11 (Sep 15, 2007)

Thanks piotrsko. Mine is an AC system (Azure). Also, it's not a Ford fuel gauge, it's a VDO (link above).

Interesting idea on running the car with just the bat to see what happens.....


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## arklan (Dec 10, 2012)

all the symptoms u described sounds to me like the high voltage is meeting the low voltage somewhere


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## sailfish11 (Sep 15, 2007)

That was my thought too. The only place I can think of that happens (or should happen) is the DC-DC, right?

Maybe I'll try piotrsko's suggestion and run the car on just the 12V for a few miles and see what happens.


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## gunnarhs (Apr 24, 2012)

sailfish11 said:


> That was my thought too. The only place I can think of that happens (or should happen) is the DC-DC, right?
> 
> Maybe I'll try piotrsko's suggestion and run the car on just the 12V for a few miles and see what happens.


Yes, do that, seperate the high and the low-voltage (cut out the DC/DC temporarely). If the problem remains, check out that your High-Voltage-Zero ( High voltage Battery-Zero) is isolated from the cars body ( 12 V battery Zero/ Ground)


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## rwaudio (May 22, 2008)

sailfish11 said:


> Hi All,
> 
> I've got an old Mustang I've converted. I have an eVision as my primary data gauge, but I also replaced the original gauges with 60's style, modern VDO gauges that fit my original dash. Trouble is, they're not all working.
> 
> ...


Not exactly related to your instrumentation issues, you can program the DMOC for a lower low voltage cut off, this isn't going to help your dc/dc converter at all, but might make the car drive better, especially in the cold. You aren't getting much power out of the system. I have 275v system that I can draw 350 battery amps from it easily. Once I get up in the mid 300v range I hope to have full power.

Make sure signal cables are twisted (ideally shielded and grounded at the source) and not run anywhere near the high voltage lines especially battery. I had my instrumentation cables run near my battery cables for a portion of the run and it would go very weird at high power, completely un usable.


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## sailfish11 (Sep 15, 2007)

So I went ahead and disconnected the DC-DC at the positive side - no problems. I didn't have a lot of time to work on it, but here's what I found out.


With the DC-DC connected, I had 13.8V at the battery and 13.57V at switched power.
With the DC-DC disconnected, I had 13.5V at the battery, which started dropping fairly rapidly. I had 12.72V on switch power, which was also dropping.


I took the car for a short drive with the DC-DC disconnected. I had the multi-meter connected to switched power, but not directly to the battery.



When I started driving, I had 12.3V switched. It dropped to 12.17V and stayed around there for a few minutes, then started dropping again. By the time I finished (about a 3-4 min drive), switched power was at 11.8V. I never drove faster than about 25 mph.


Things I noticed:
- The speedo read incorrectly from the beginning (showing ~10mph low), even at low speeds. After the first minute or so it stopped working altogether.
- The 12V gauge in the dash consistently read about .5V low (it is also hooked up to switched power).


The problem with the speedo may just be that it's very sensitive to incoming power - anything less than about 12.5V and it starts getting wonky. I guess that wouldn't be a problem if everything else was working as it should.



So now I'm pretty sure the DC-DC is fine. The battery is new. So I'm thinking maybe I have something drawing too much power, probably something connected to switched power. Things I've added: Daytime running lights, new radio, push-to-start system. The radio and PTS were professionally installed. I installed the daytime running lights, so my money would be on that as the culprit. I'm out of time today and have military duty next weekend, so it may be a while before I'm able to monkey around with it again. I could disconnect the lights (probably just pull out the bulbs) and see if that makes a difference. If not, I'm not sure what to try next. The radio would probably be next easiest to disconnect. I wouldn't have any idea how to check to the PST.



What do you guys think?


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## sailfish11 (Sep 15, 2007)

I got up early and did a little more diagnostics this morning.

Disconnected the 12V battery and read voltage at 13.6V, but dropping. Voltage from the pos terminal to car ground was 13.8V.
Waited 20 min and measured battery again, at 13.16V and fairly stable. Tried to charge it using my 12V charger, but it immediately said the battery was already fully charged and shut off.
I guess this means the battery is fine? I suppose I could take it over to Pep Boys and let them run it through their full load test (takes 15-20 min and it's free), but since it's new and seems to hold a charge I'm not sure that's worth the effort.


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## sailfish11 (Sep 15, 2007)

One more thing.
Since my pack is very sensitive to cold, I've been running the car in "Econ" mode, which raises the min pack voltage, limiting my amp draw to about 80 amps. The car still works fine except going up hills, when I slow to about 25 mph. Doesn't make the cars behind me too happy, but the hills are fairly small so it doesn't happen for long.
To minimize this issue, I tend to "floor it" as I approach a hill, to build up some speed to get me going. Since I'm limited to ~80 amps, flooring it doesn't do a whole lot....
But here's what I'm noticing, the speedo works fine as I'm flooring it and building up speed. It works fine as I start up the hill (with it still floored and pulling ~80 amps). But as the car starts to slow - still pulling ~80 amps - that's when the speedo starts to jump around.
Since I'm driving, I can't just stare at the gauges so I can't say for sure, but perhaps that's the point at which the 12V voltage starts to drop below whatever voltage the speedo needs?
I have no idea why this is happening, but it seems there's more going on than just amp draw, since that remains fairly constant when flooring it in Econ mode.


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## dougingraham (Jul 26, 2011)

Speedometers generally connect to the transmission with a cable. The cable is driven from the transmission output shaft geared in some way. The rotary motion is used to spin a magnet. The magnet is usually inside an aluminum cup and the cup is directly connected to the needle that displays the speed. The needle/cup is spring loaded to return to zero mph or kph. The eddy currents induced in the cup by the rotating magnetic field cause it to rotate against the spring in a linear fashion thus displaying the vehicles speed. This has been done this way pretty much forever (At least as far as cars are concerned) because it is so simple that it is very reliable. It does not need any electricity to operate.

If the speedo uses this arrangement and it doesn't work then there is something wrong with the speedo mechanism, with the transmission or the most likely there is something wrong with the cable. I would check the cable.


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## dougingraham (Jul 26, 2011)

I looked up your VDO meter. It is electronic so the cable probably spins a disc which is used to generate pulses and then the time for those pulses is measured to get an RPM reading. The computer in the meter is told how to map RPM to speed displayed. I would still suspect the mechanical cable is somehow at fault. You could drive the individual instruments from their own 12 volt source such that they do not connect at all with the rest of the vehicle but these kinds of devices draw so little power and tend to have a lot of filtering because ICE vehicles are really noisy systems that must be tolerated. For example when you are starting an ICE the battery is providing a few hundred amps and this causes the battery voltage to sag. For an aftermarket device you expect it to be able to operate during this severe voltage sag (6 or even lower volts). When the key is released and the solenoid opens there is a huge voltage spike that occurs and for brief periods the voltage across the battery can spike to something less than 60 volts. 60 volts is typically the design voltage used to protect the electronics. If your 12 volts is so bad a meter designed to operate in this kind of hostile situation is having problems there is a lot of stuff that would not work. You could drive just that one meter with a separate 12 volt battery directly connected to the meter to verify it is not just a loose connection.

Make certain the cable is properly attached at both ends and that it is not pinched or kinked. I have seen speedo cables that were broken and the ends of the break would touch and it would read correctly when they did.

I am curious to know what you end up finding.


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## sailfish11 (Sep 15, 2007)

I haven't had time to really look at the system, but I have made some more observations while driving. The speedo malfunction is clearly related to amp draw (or voltage droop), but not absolutely.
For one thing, the system always works fine for the first few minutes, no matter how many amps I'm drawing - even with the headlights on.
Also, once it starts to go bonkers, it tends to stay that way, even once the amp draw falls. This is especially true with the headlights on (which, because of the time of year, I'm doing more and more).
But it is still at least loosely related to amp draw/v droop, which is why I tend to think it is not a physical problem with a loose wire or sensor. With the headlights off, if I drive in econ mode (limiting my amps) I can drive indefinitely with no speedo problems.
I suppose it could be multiple problems in which case I'm going to have a LOT of fun figuring it out .
I've also added a few 12v accessories over the years (radio, push-to-start, daytime running lights) and have more I want to add (power steering, power windows, seat heaters). So it's possible I've wired something up incorrectly - I don't think so, but at this point I have to consider anything and everything.


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## jwiger (Oct 18, 2014)

The voltage range for that speedo is pretty wide: 10.8-32Vdc

http://www.vdo-gauges.com/media/instructions/TU00-0751-5504620 Cockpit Int'l Speedometers.pdf

I'm curious how the sensor is hooked to the transmission. If it has a cable or plugged directly into the transmission in place of the cable.

The delayed on-set/current draw relationship is interesting to me. My instinct tells me it's related to something on the high volt side warming up and starting to short out on the chassis, perhaps finding a ground path through the gauges. The speedo would be interesting because flex cable (usually metal core/jacket) or not, there is possible electrical path from the motor/transmission through the speedo sensor into the chassis of the speedo. 

You mentioned the speedo starts acting up at low RPMs, is it possible you can remove the sensor and spin it with a drill while having someone watch the speedo? That could confirm the sensor operating properly.

I'm also curious on how you're driving the ampmeter. Is it from an actual shunt or an output from your controller? If it's an actual shunt what is it, where is it mounted (physically and in your HV path) and how is it isolated from the chassis?


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## dougingraham (Jul 26, 2011)

sailfish11 said:


> I haven't had time to really look at the system, but I have made some more observations while driving. The speedo malfunction is clearly related to amp draw (or voltage droop), but not absolutely.


This is easy to test. Disconnect the Speedo from the 12 volts and connect it to its own 12 volt battery so it is completely isolated. If problem goes away then it is something on the 12 volt rail. If you still have strange problems then it is probably the meter.


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## sailfish11 (Sep 15, 2007)

jwiger said:


> I'm curious how the sensor is hooked to the transmission. If it has a cable or plugged directly into the transmission in place of the cable.


 The speedo has a hall effect sensor mounted near the driveshaft.



jwiger said:


> You mentioned the speedo starts acting up at low RPMs,


It doesn't seem related to rpms at all. It seems triggered by high amp draw (or low pack voltage). But once it starts acting up, it often stays that way even after amp draw goes back down. It could be the HV system is shorting out somewhere (like the speedo cable) or it could be related to the DC-DC (the only place I can think of where the HV and LV systems interact). Or it could be something else (or a combination of things)....



jwiger said:


> I'm also curious on how you're driving the ampmeter. Is it from an actual shunt or an output from your controller? If it's an actual shunt what is it, where is it mounted (physically and in your HV path) and how is it isolated from the chassis?


The ammeter is driven by a 300A shunt. The conversion originally used a -150 to 300A meter and the 300A shunt was for that. The gauge worked great but didn't look like the rest of the dash, so I replaced it with the VDO +-300A meter and ran it off the same shunt. It seemed to work for a while, but then stopped. The old meter ran only off the sensor; the VDO runs off the sensor, but also requires 12V to run the gauge. I just hooked it into the same switched power 12V lead as is running the rest of the dash gauges.



dougingraham said:


> This is easy to test. Disconnect the Speedo from the 12 volts and connect it to its own 12 volt battery so it is completely isolated.


You mentioned this earlier and it's a good idea. I've ordered a small 12V battery. Once it gets here I'll give it a try and let you know what happens.


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## sailfish11 (Sep 15, 2007)

I finally had the time to rewire the dash and hook the speedo up to a separate 12v bat. All symptoms disappeared and it worked fine - no jumping around at any speed or amp draw.

Since I had the wiring out, I also tried hooking up the speedo to a different switched power (from the radio). It had the same problem (jumping around). 

So the problem is with my switched power. I hooked up a multimeter to switched power and drove around and watched the voltage jump all over as I drove. Under heavy or uphill acceleration, voltage would drop to just over 12v with the headlights off. With them on, the voltage dropped to just under 11.5v. (I was driving alone, so I couldn't get exact readings).

The 12v deep cycle marine bat is new (replaced this summer) and the symptoms were the same with the old bat. Last month I had tried disconnecting the DC-DC (and drove the car solely on the 12V bat.) to see if that might be the problem, and the symptoms still persisted, so the only thing I can think of is that something is drawing too much 12v power. 

But I don't understand why it seems to be related to amp draw (or pack voltage). If I keep amp draw/pack voltage droop within normal (i.e. run the car in econ mode), everything works fine - even with the headlights on. What in the HV system could cause the 12v system to droop so much - even when the DC-DC is disconnected?


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## sailfish11 (Sep 15, 2007)

By the way, when I had the dash out, I did a little diagnosing of the ammeter too. I'm not sure how it's supposed to act, but I tried hooking the small 12v bat to it directly and the gauge didn't do anything (the needle stayed in its "parked" position, which is all the way to the leftmost end of the gauge, past the negative 300A range). I would think it should move to the middle, or zero, range of the gauge once 12v power is applied.

I also hooked the multimeter up to the wires from the shunt (that would feed the ammeter) and it showed activity while driving (showed amp draw under load and negative flow during regen). So maybe the gauge is bad. It worked for a while after I first installed it about a year ago, then stopped. Maybe it couldn't handle the inconsistent 12v.....


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## dougingraham (Jul 26, 2011)

sailfish11 said:


> The 12v deep cycle marine bat is new (replaced this summer) and the symptoms were the same with the old bat. Last month I had tried disconnecting the DC-DC (and drove the car solely on the 12V bat.) to see if that might be the problem, and the symptoms still persisted, so the only thing I can think of is that something is drawing too much 12v power.


Since you can see the problem with your meter try watching the voltage directly at the battery or DC-DC converter. Someplace before the 12 volt switch. What I am guessing is that you have overloaded the 12 volt switch or you have a high resistance switch or connection in the switched 12 volt circuit.

If the problem is also at the battery then I would think you might have a bad connecton to the battery or a cracked plate in the battery. Under heavy loads when the traction pack sags you could have the input to the DC-DC go below its input voltage threshold and the output drops off. Then the bat battery or bad connection to the battery allows the voltage to drop off.

It may be that you can find the problem just by turning on the headlights and then checking the voltage before and after the ignition switch. Another thing to check is your frame ground connection to the battery.


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## sailfish11 (Sep 15, 2007)

Thanks - I'll give it a try. Not sure when I'll have time again to diagnose (I have military duty next weekend), but I'll look into it. I'm still confused by the symptoms' relationship to HV. As you say, an overloaded switch could cause my problems, but why do the symptoms only show up under load? With the headlights on, the symptoms show up even when the pack voltage is still well above the DC-DC input cutoff. So it would seem the "overload" or whatever it is only shows up under certain situations - but related to stuff going on in the HV system. What in the LV system would be affected by HV? Vexing.....


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