voltage question (instrument cluster)

[scbill]

What is the voltage supposed to be @ the top connector blu/grn from ign switch, should start as 12v thru a resistor wire. Does the resistor wire drop voltage, or amps?  I am a volts/amps/elect challenged person.
Thanks  Bill

[Gary Lewis]

I can't really read that schematic due to the size, and if I make it larger it get blurry.  Where did you get it?  What is it for?

If it is for a Duraspark ignition can we use the schematic from Ford?  Documentation/Electrical/EVTM/1985 EVTM.  Here's the schematic for Duraspark.

But the answer to the question about resistance is that in this case it drops voltage and current.  The equation is E (voltage) = I (current) x R (resistance).  BUT, if you are trying to measure the voltage after a resistor you must have the load on pulling current or the voltage will not drop.

So, tell us more about what you are doing?

[scbill]

Gary, It's the blk/grn wire from the ignition...changes to blu/grn when it gets to the big plug on back of the main plug on the instrument cluster. I assume it's the power for the gauges. Page is from Haynes Ford truck/bronco manual. I'm trying to find the voltage input for the cluster.

[Gary Lewis]

Ok, I think I understand.  Here's the info from the '85 EVTM.  We'll start with the ignition switch.  Note the big blue triangle with an "N"  right in the middle of the page.  It says "To Instrument Cluster Page 98, 106".




And here's page 98.  Does that help?  Do you need to know what pin that wire is on?

[scbill]

Voltage at C208A at the cluster plug is what I was needing, I'm trying to chase down gauges not working. going to save these diagrams tho..may try the offboard voltage setup that you posted.  Thanks Gary.

[Gary Lewis]

You do know that those diagrams are on the site?  Documentation/Electrical/EVTM/1985 EVTM.  Every circuit on the truck is shown in there.

[ArdWrknTrk]

A lot of times it's easier to have a piece of paper in hand.

Greasy fingers and sun glare make working with a mobile device 'challenging' at best, and impossible often enough.

But I'll agree Chilton and Haynes schematics are next to useless when you have an EVTM in hand.

[PetesPonies]

Resistors in series create voltage drops. The drop is proportional to the resistors resistance in reference to the entire resistance of the circuit. If you had 12V and a 10ohm resistor and the load was also 10 ohms, you would find 6V at the resistor and 6V at the load. Most of the time, gauges work around 5-6V. They are feed through an IVR, Instrument Voltage Regulator.

[scbill]

Methinks it's the wire that is the culprit in this series of trucks, apparently most all of them has the same problem(s).

Thanks for all the info..Bill

[Gary Lewis]

What is the problem?  I haven't heard of a lot of wiring problems.  Yes, the ICVR is a problem, but not usually the wiring.

[scbill]

None of the gauges are working, except amp and tach, I plan to do the DE-SWADJ 3A Adjustable Voltage Regulator that you have posted on the ICVR page, looks like easiest simplest solution. I have tried different regulators but all have different results, none close to accurate.

[Gary Lewis]

You will like it.  I set my voltage to 5.4v, but you could tune it to your fuel level sender.  Fill a tank full and set the voltage to give Full on the gauge.

[PetesPonies]

If no gauges are working, it screams instrument regulator. Having an adjustable one would sure add some effectiveness to the readings, especially gas gauge as mentioned.

[scbill]

I've tried three, or more regulators, results from barely working gauges to not at all.

[ArdWrknTrk]

Well, if you go with the SWAD-Just you can cut out the resistance wire, no?
Have you tried testing for voltage at the ICVR connector?
Be sure to give it some kind of load, so you can tell if the resistor in the harness is actually pulling it down.

I think it was Matt Wood in England that used an infrared camera to find his ignition coil resistor in the harness behind the cluster.
But I don't know if the three senders for the gauges would be enough load to really heat that one up.

[Gary Lewis]

By the way, I need to point out that the gauges are actually thermometers and the power we feed them heats them up.  So that takes time to kick in.  In other words, if you turn the voltage up give it at least a minute, and maybe two, before deciding that wasn't enough.

[ArdWrknTrk]

Ship-night....
It's amazing to me that these tiny springs (filaments?) Can be so accurately calibrated for loss of tension when they heat up.
We all know that grounding the sender wire is a delicate situation where you can't leave it too long or you'll ruin the spring temper of the instrument.

[Gary Lewis]

They are essentially the same as the round wall thermostats that had an "anticipator" circuit in them to heat them up just a bit to cut the response time.  But in our case the anticipator is much bigger.

As for the tiny springs, just think about the shock those things endure when we bounce down the road.  How can they last?  But they seem to do so.

[ArdWrknTrk]

They're not really supporting anything but themselves, right?

So, do we know what voltage the ICVR should see?
I haven't gone through the troubleshooting in the EVTM because I don't have gauges like that or an ICVR.

[Gary Lewis]

Hard to say what voltage the ICVR sees because the current varies so much.  It takes ~.2a to take a gauge to full scale, so if all three are at full scale then there will be .6a and if the battery voltage is 14v then the drop will be about 5v, meaning the ICVR will see ~9v.  And if the gauges aren't at full scale the voltage will be closer to battery voltage.