Hello all,
My truck is a 1978 F150 . 351/400. C-6 . Had a problem with the oil gauge going to the top 1/3 of gauge , checked real oil pressure and was good also replaced old sending unit. That left gauge so had two salvaged ones , put in each and both read same as the one that was in . My question is , could both salvaged gauges be bad or could the Voltage stabilizer on the back of the circuit board be bad . What exactly does the voltage stabilizer do and control ? Does it effect the way the oil pressure gauge reads? Also may need to replace my voltage gauge since it always reads just into the neg. side of center , the charge system is working fine and checks out great.
Thanks everyone
Kelly

 

The voltage regulator controls power to the oil pressure and temperature gauges. If it fails, it commonly causes both gauges to read high or peg to the full sides. The point of the regulator is to (I believe) take the 12 volts coming into the dash cluster and turn it into 5 volts for the gauges to use. Is your temp gauge reading high also? Keep in mind these are 40 year old gauges and are merely a point of reference.

To test the oil pressure gauge, pull the wire off the sending unit under the dash. The gauge should be dead. Then ground the sender wire to the engine block and have a buddy watch the gauge. The gauge should go full positive.

As I said BOTH the temp and oil gauges run off the regulator. If it DOES fail, it normally causes one to read erratically, then both peg full.

The regulator is still a mechanical bimetalic spring that's lived for 40+ years behind your dashboard. Replacing this with a new unit really isn't a bad idea, especially if you're willing to spend either a few more $$ to buy a pre-made transistor unit, or spend about $10 and an hour of time and make your own regulator with RadioShack parts. (I'd recommend buying a pre-made one!)

As for the charging system....

The voltage regulator under the hood tells the alternator to turn on and off. It sense when the battery is fully charged while the truck is running, and turns the alternator off, until it sees the battery voltage drop again (whether due to running accessories, lights, or just the truck itself). When these fail, the common symptom that Ive seen is either the battery being overcharged by the alternator never turning off, or by the battery always going dead because the regulator never tells the alternator to turn on to charge it. If the charging system works fine, I'd leave it alone. It doesn't hurt to replace it with a modern transistorized one, but the old mechanical one will work til it fails.

The amp gauge reading I'm going to say is normal so long as it isn't way positive or way negative. If it were a volt gauge I'd be more concerned, but if it's more or less in the middle then you should be fine. Most amp gauges don't move much at all one way or the other.

 

There is a resistor mounted on the back of the cluster that will drive gauges crazy when it goes bad. Ran into this with my Fathers truck, took a long time to find the issue, swapped it out and it's been good ever since. It's worth a look if you have a spare one.

 

The only gauge that I'm having trouble with is the oil pressure gauge all the other gauges are reading fine, could it simply be the oil pressure gauge the ones that I scavenged are bad? Was planning on replacing the gauge resistor no matter what since it could be original

 

Three gauges that read exactly alike will not all be bad. If anything try replacing the resistor first, the the board regulator.

Oh yes and post your results when you test the oil pressure gauge.

 

D1AZ-10804-A .. ICVR = Instrument Cluster Voltage Regulator (Motorcraft GR-513) / Obsolete ~ Available NOS & from auto parts stores.

1971/86 F100/350 / 1971/91 Econoline / 1978/86 Bronco // 1971/72 Galaxie/LTD / 1972/76 Thunderbird/Mark IV/Pinto/Bobcat / 1974/78 Mustang II.

1979/82 LTD / 1979/86 Mustang/Capri / 1979/89 Grand Marquis / 1981/82 Granada / 1983/86 LTD/Marquis (Fox bodied mid-sized) / 1983/89 Crown Victoria.

 

Thanks for clearing that up ND, wasn't sure if it controlled the fuel gauge too or not.

 

hay Dave145
I understand the gauges are approximate readings , before the problem with the oil gauge it would read 1/4 way up at warm idle and maybe 70% at high speed now it reads 60% at warm idle and 90% at speed, to confirm wire off new sending unit and key on - gauge reads zero. wire grounded to block with key on , gauge pegs means gauge is good?
ND thanks for telling me the IVCR is obsolete lol, have seen it in LMC but will look for it in local parts store also what does NOS stand for? also how does the circuit board fasten to the IVCR looks like it is just snapped on and the IVCR is screwed into the back of the cluster housing .. if after all replacement and testing gauge , if bad gauge ideas who would have a new old stock oil gauge ? took the D-1AZ and the Motorcraft # anything else to know so get the correct version at parts store?

 

NOS = New Old Stock, new parts that are obsolete that FoMoCo dealers and obsolete parts vendors have.

 

Someone who knows better can correct me - seems to me that I read all the subordinate gauges are the same, just different fascias - they simply show a swing between 70 and 20 ohms resistance. When you ground the wire for any sender it will peg at the top end because the resistance drops dramatically.

I am thinking you are correct in focusing on the sender - you have three gauges working right and a gauge reading off and a sender whose function changed. Senders are less than $10. If you can rig up a way to put pressure on your sender, you can test it by applying pressure and checking that the resistance across the sender changes. Given the gauges are the same and the high pressure reading is on the same side as the full on a gas gauge, I would expect that as you applied more pressure the resistance would drop.
I may be all wet on this, maybe someone who knows more can correct?

 

Three on the tree,

That to me indicates the gauge is good. The gauge reads off a resistance value taken from the sending unit, which is why grounding the unit makes it go to full pressure.

The ICVR is held in the the board by snaps. Why snaps? Well it's better than dodges idea of using spade connections I'll tell ya that. But anyway yes snaps hold it in place and are the connections for the 12V IN power and the 5V OUT power. The screw is the ground. It is anything but it uncommon to have these things come ungrounded or have shaky ground due to clusters aging, corrosion, yada yada yada. What I did in mine to make sure it is in fact getting a solid ground is I ran a 16ga jumper wire from the firewall support straight to the screw to be a secondary ground. Clean up the snap connections and use a bit of di electric grease on them to keep em clean.

On your sending unit, did you by chance install it wrapped in Teflon tape? I've seen that done enough times and people have similar symptoms as you describe, because the sending unit relies in the engine block ground. So if you isolate it with Teflon tape....well you get my drift. All in all it sounds like a faulty ground to me. If all the gauges read the same, you've tried a new sending unit, and it's still acting up, I'd look at the ICVR.

- Turn the key ON.
- Take a multimeter, ground the black lead to unpainted metal, then using the red lead probe both snap connections.
- One should read solid 12V. This is your input.
- The other will be a fluctuating 5V. This is your output. It is normal for the voltage to flicker but it should stay around 5V.

If all is well there, try a redundant ground as I described, and clean up the threads and the sending unit to ensure good electrical ground.

 

F100/350 & Econoline: Beginning in 1970, when printed circuit boards were introduced, the ICVR snaps onto it.

1969 and earlier F100/350/Econoline did not have printed circuit boards. The ICVR's had spade connectors and bolted to the back of the instrument cluster.

 

Number Dummy and Dave145;
thanks for the info on the ICVR and extra ground . My back was out when I had shop put in new oil sending unit , at that time they also separately checked engine oil pressure and it was well within specs. About a month ago was the time the gauge started to act up and since we had the old sending unit out simple to replace with new. Will call them and ask if they put tape or brushed on anything on threads. shop said no tape and nothing brushed on. .... Can't believe that I didn't know NOS was for old new stock since I always write it out lol BA-DAH guess I should check my own bulbs to see if one is out. lol

 

was wondering , if the ICVR is going out but not completely gone yet could it cause the oil pressure gauge to read higher and not affect the other gauges yet? or am I shooting in the dark. Will check the ICVR ground and input and output.

 

E4ZZ-9278-A (replaced C9ZZ-9278-A, C7ZZ-9278-A & B7T-9278-B) .. Oil Pressure Gauge Sending Unit (Motorcraft SW-1547-B) / Available from Ford & auto parts stores.

Myriad 1957/90's FoMoCo vehicles.

The ICVR affects all three gauges, oil pressure, temp and fuel. If no oil pressure gauge (optional), the ICVR only affects the fuel and temp gauges.

On my 1965 F100 352 with an oil warning lamp (no pressure gauge), when the ICVR began to fail, the fuel and temp gauges did "the hula" swinging rapidly back and forth on their own.