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#994445 - 02/09/14 03:41 AM Factory OIL/TEMP/FUEL/VOLTS Gauges Diagnosis Tips  
Joined: Oct 2003
Posts: 1,068
RoadWarrior Offline
15+ Year
RoadWarrior  Offline
15+ Year

Joined: Oct 2003
Posts: 1,068
Mechanicsburg, Pa
Hello All,

Here are some Tips to help in troubleshooting your factory accessory gauges.

First NOTE: is that the VOLTMETER gauge DOES NOT run out to the alternator, nor does it really monitor ALTERNATOR output.

to see what your alternator is really putting out verses what the dash says, start the car with everything off. then take a DVOM meter, and while being mindful of the test lead wires and the belts and fan, connect the red test lead to output post on the backside of the alternator, and the black test lead to the negative post/terminal on the battery. this is a base "no load" reading. should be somewhere around 14.0 to 14.5V, now compare this reading to the VOLTMETER on the dash, it'll prob be pretty close

now you can start turning things on, like the headlights, rear defroster, blower motor, a/c, and yes the voltage will drop some, but it should maintain greater than battery voltage, if your alternator is in good condition, and sized properly for all your electrical loads you may have added, even with everything on you should still be above 13V at the alternator ( some situations may be different, extreme aftermarket accessories/ audio systems can drag even a good alternator down below this at idle, if so, rev and hold the engine to about 1500 RPM, and see where you voltage reading is now at the ALT. If it is now in the safe range, just know that either you have exceeded the alt's output capabilities at idle, or if you haven't added extra electrical loads to your vehicle your ALT maybe be wearing out, however the factory 78 Amp alternator will only produce about 50 amps at idle, so this test may exceed a stock alternator's output even with only O.E. equipped components, and everything functioning as it should.)

through this test, you should be able to take both a live reading at the alt, and then a gauge reading from the VOLTMETER. It should become quite clear that the alternator is maintaining somewhere around where it should 13V and up, but the VOLTMETER could be as low 9V.

this is because:
The VOLTMETER monitors the voltage on the GAUGES FUSE circuit.

Here is the list for the Gauges Fuse:
All Gauges
Light driver Module
Brake Light switch
the rest of the instrument cluster
Vehicle Speed Sensor
Audio Alarm module
Defogger controller
Trunk release switch

ALSO the power from the IGNITION SWITCH that supplies the GAUGES Fuse, also supplies power to the TURN B/U fuse and the ECM fuse and the IGN/ECM fuse.

so this being said, really what the VOLTMETER is showing you is the load on these circuit

which you see it when you turn on the headlights or highbeams, or the turn signals or anything on these circuits, the VOLTMETER will tend to drop, or in the cause of the turn signals bounce in correlation with the turn signals. this is because the load on the circuit has increased, and possibly the available voltage on the circuit may infact be what the VOLTMETER is displaying, this does not mean that's what your alternator is producing and supplying to your battery, and to the rest of the car.

there are ways to improve/fix this problem.

increasing the gauge of the supply wires from the ignition switch to the fuse block and fuse block to the instrument cluster will help with the load on these circuits and your VOLTMETER readings.

Adding a larger gauge wire from either the fender battery ground location or the engine ground location to the ground locations at the base of the Dashboard

voltage drop due to high resistance in an old factory Ignition switch can cause low voltage readings( also can be the cause of low voltage readings in everything inside the vehicle and low voltage at the ECM. Personally my factory ignition switch was so poor that i lost 1.2v through the ignition switch!! so even without any large electrical load running or even on, i was already a volt low an all Ignition hot devices.)

another solution for those with extra add-on electronics or electrical devices (Rad fans, Audio systems, heated seats, etc...) would be to upgrade to a higher output alternator. This isn't really a fix for gauge performance, but by having more available power (amperage) and having an alternator that can more easily maintain voltage with extra load, the VOLTMETER will "seem" to read more correctly

The only true way to make the VOLTMETER read the actual output voltage of the alternator would to disconnect the VOLTMETER from it's pin on the Printer circuit board in the cluster, and through use of a fused relay attatch it to either the output post of the alternator. or from the battery.

I have yet to do this myself, but do plan on it.

the pinout for a relay would be as followed

Pin 85 of relay-ground
Pin 86 or relay-to an ignition hot signal wire
pin 30 of relay-to Power pin on VOLTMETER
pin 87 of relay-To either battery or alternator, fused close as possible to battery or alt, with no more than a 3 amp fuse.

ALSO it may be best to remove the ground pin from the printed circuit board and directly ground the gauge either at the base of the Dashboard, or to the firewall, but this may be unnecessary once the gauge is supplied with an "isolated" power supply for gauge readings.

Alright, now to move on to the easier gauges to fix;

NOTE: wiring color/pin locations is based off of the 87/88
Manual. The same should hold true for all 83-88 SS though.

Through-out this Post i will be basing my theory off of the stock style gauges/senders/wiring set-up. The factory senders and gauges and wiring will work with a replacement/modified engine so long as the factory senders(whether OE or equivalent replacements) and wiring from the factory Firewall connector are utilized.

here are some basics to start with:

The COOLANT TEMP Gauge sender wire is DARK GREEN and should be routed from the Driver's side firewall bulkhead connector (C100) located at pin D5 to the coolant temp sensor located in the Driver's side cylinder head coolant passage port between cylinders 1 and 3 exhaust ports.

The OIL PRESSURE Gauge sender wire is TAN and should be routed from C100 also, located at pin A6. and is then routed to the Oil pressure sending unit at the driver's rear side of the engine block, above the oil Filter in an oil passage port about about 1 inch above the oil filter adapter flange in the block.

The FUEL LEVEL Gauage sender wire is PINK and is routed through connector C209 at Pin H. Connector C209 is the "Rear body harness" connector attached to the the LH side of the Fuse block under the dash. The Sender wire in this harness runs underneath the carpet into the trunk where it meets the rear light harness connector C210 and then exits the trunks and runs to connector 510 (or 511 for v-6 cars) where it connects to the sender wire from the Gas tank.

NOTE: the temp sender in the thermostat housing with two wires in the sealed connector is not for the gauge but is input DATA used by the ECM only. Does not send a signal to the Gauge

NOTE: the Oil pressure switch on upper rear of block below the Distributor is an on/off style switch for use for the choke, and also does not route/control the gauge in the dash.

Alright, step one:

Does only one gauge not work, or are all accessory (Tach, oil, Fuel, Coolant) inop?

IF all gauges are inop, then you'll want to start by checking:

The gauges fuse for being blown
The ground wire at the bottom left bracket of the dashboard (G118)
wiggle/push & pull/tap the "dogbox" (the dash-colored housing the instrument cluster sits in) the connector for everything in the gauges cluster is a press fit between this plastic housing and the actual dashboard.

if it is still indeterminable as to the cause for all gauges not working at this point, I would suggest removing the bezel, shadow box, and gauge cluster to check the pins at the printed circuit board.

once everything is removed from the "dogbox" to access the contact pins for the gauges, you should see three metal clips clustered behind the OIL/TEMP/FUEL gauges (only two will be behind the VOLTMETER) using either a test light or a DVOM, check each cluster of 3 clips, with the ignition key on, for 1 power and 2 grounds. (this is from memory, but i believe all the power clips are on the left had side of the clusters, the grounds on the RH clips, and then the sender wires are the center clips, but i'm not 100 on their location.)

If you're missing power here at all clips, or ground, then i would continue to remove the dogbox and check at the dashboard connector for the printed circuit board.

at this point this repair has be discussed prior, i will give you a quick pinout.

pin 16-pink/black wire-power (also at pin 11)
pin 2-Tan-Oil pressure sender
pin 4 Pink-Fuel Level
pin 3 Dark Green-Coolant temp
pin 9 Ground.

IF only one gauge is inop;

NOTE: INFO: Both OIL PRESSURE and FUEL LEVEL work off of a 90 to 0 ohm scale. 90 ohms at max travel, and 0 ohms at min travel.

the COOLANT TEMP gauge is reversed and on a different scale
260 degrees F-55 ohms
100 degrees F-1365 ohms.

NOTE: the FUEL LEVEL and OIL PRESSURE gauge will respond the same. THE COOLANT TEMP will respond reversely.

1:) Find the corresponding sender and wire and disconnect the wire and isolate it from ground.

now turn the key on.

Does the gauge read it it's max level of travel?

(60 PSI OIL, FULL for FUEL, and 100 Degrees for COOLANT)

if yes, continue

if no, gauge sender wire shorted/contacting ground between cluster connector and sender connector. Possible shorted gauge.

2:) Turn off key, Connect sender wire directly to a good ground. then turn ignition back on

Did gauge reach travel to it's lowest min limit?

( O psi OIL, EMPTY for FUEL, 260 degrees COOLANT)

if yes, continue

If no, Possible open circuit for gauge sender wire between sender and gauge connection in cluster. Possible open gauge.

3:) this test will require either a 45 OHM resistor and a 680 OHM resistor.

turn key back off

Connect 45 OHM resistor to either FUEL LEVEL or OIL PRESSURE sender wire (IF diag COOLANT TEMP, use 680 OHM resistor) then connect other end of resistor to ground.

now turn ignition key back on.

Gauge should now read in the center of it's travel


if gauge is not in the center if it's travel, most likely cause is either loose/poor or corroded connections at:

1-Pins of printed circuit board to gauge
2-Pins of dashboard connector to Printed circuit board.
3-pins of Dashboard harness to bulkhead connector C100
4-pins of C100 to underhood side of pin C100

IF the gauge passes all tests but still reads incorrectly then most likely the sender has failed.

(This is assuming that oil pressure was confirmed with a mechanical gauge, that fuel level was verified, and coolant temp was verified with a mechanical gauge)

Lastly, the gauge can be removed from the dashboard, connected to power and ground, and then connected to the sender under the hood (or back by the tank) to verify a faulty gauge, this way you eliminate the possibility of a poor connection somewhere in the system. IF the gauge works perfectly this way, but poorly in the vehicle, you may have missed a connection issue somewhere.

whew! so i'll read over this again once it's posted and add/correct anything.

and i'll let you all know how my VOLTMETER fix goes this year.

hope that helps someone.

88 SS, 355 owned since '03
#1062701 - 07/12/19 10:42 AM Re: Factory OIL/TEMP/FUEL/VOLTS Gauges Diagnosis Tips [Re: RoadWarrior]  
Joined: Jun 2001
Posts: 1,462
markg Offline
15+ Year
markg  Offline
15+ Year

Joined: Jun 2001
Posts: 1,462
I put a gage cluster in my 84 where there were just lights.i got a used cluster from an ss .I had many connection problems with contact points and broken ribbon but finally got everything working.the problem im having is with the oil pressure gage.when I start the car it goes up to 60 and after running it drops down very low.if I give it gas it should go up but it really just stays where it is in the using the same connector that was on eng sensor for the idiot lite behind the has 2 posts and one does nothing with the gage and the other will make the gage I missing something?i bought a 60 psi sender from autozone.

86 ss 86 cl,330hp GM vortec cratemotor-700-r trans worked,ford 9.5 inch rear with 3-50s,custom 3 inch exhaust with flowmasters.
#1062703 - 07/12/19 06:24 PM Re: Factory OIL/TEMP/FUEL/VOLTS Gauges Diagnosis Tips [Re: RoadWarrior]  
Joined: Feb 2008
Posts: 895
AkronAero Offline
10+ Year
AkronAero  Offline
10+ Year

Joined: Feb 2008
Posts: 895
Akron OH

There are TWO oil pressure senders, at least on my 87SS. The 84 may not be similar and thus create confusion.

One is a variable resistance sender for the gauge and one is an on/off switch for the choke (with idiot light) .
I think that the sender for the gauge was in the block on the lower left side of the engine and the sender for the switch was on the left and just below the distributor.?
Sounds like your gauge may be seeing the switch signal? Or if this is the 60 psi autozone gauge, it may be the wrong one (not understood at Autozone) and they may have given you one for the wrong car/application (as you have changed gauges).

For your gauge; you should have 90 ohms at max pressure, and 0 ohms at no pressure. Confirm with Autozone that this is what you have.
For your sender, the switched is closed below 4psi, during which time it puts heat to the choke heater. Once oil pressure builds, the idiot idiot light turn off and the choke heater turns off....

When I put in my 383 SBC, I used the upper port (ex-switch) for the gauge and just dropped the switch.

Does this help???

#1062710 - 07/13/19 07:05 PM Re: Factory OIL/TEMP/FUEL/VOLTS Gauges Diagnosis Tips [Re: RoadWarrior]  
Joined: Jun 2001
Posts: 1,462
markg Offline
15+ Year
markg  Offline
15+ Year

Joined: Jun 2001
Posts: 1,462
thanks Gordon,my other 86 monte has the sender in the back by the dist.this is also where the original 2 prong idiot lite switch was on the 84 im working on.i just put the new pressure switch with just one connector where the idiot lite 2 prong switch was.from there im just running one of the wires from the 2 prong plug to the 1 wire wire does nothing and I get movement from the other but it doesn't go up if I rev the engine like my 86 and after a while it drops down into the red.

86 ss 86 cl,330hp GM vortec cratemotor-700-r trans worked,ford 9.5 inch rear with 3-50s,custom 3 inch exhaust with flowmasters.
#1062888 - 07/25/19 03:30 AM Re: Factory OIL/TEMP/FUEL/VOLTS Gauges Diagnosis Tips [Re: RoadWarrior]  
Joined: Jan 2000
Posts: 5,391
mmc427ss Online content
20+ Year
mmc427ss  Online Content
20+ Year

Joined: Jan 2000
Posts: 5,391
Pottstown, Pa
Mark, in case you don't have the pinout for the SS cluster.

From looking at the cluster wiring schematic for both the SS and non-gauge clusters the TAN wire on an SS runs from the stock sender above the oil filter to the #2 pin on the cluster.
The non-gauge cluster runs a TAN wire from the #2 pin on the cluster to the oil pressure switch which only turns off the idiot light and also turns on power to the choke. That was the switch located next to the dist. There would be three wires on the switch, BROWN, TAN and LT BLUE. The TAN would be the one to use for the gauge. The BROWN is 12+ with the key on, The BROWN may have 2 wires on the switch. The LT BLUE wire runs to the choke coil.

Make sure you have the correct sender for the 60 lb gauge, there is also an 80 psi gauge and sender.

Would think a 60 sender placed next to the dist and the TAN wire connected to it and you should have a working gauge.

#1062908 - 07/26/19 03:07 PM Re: Factory OIL/TEMP/FUEL/VOLTS Gauges Diagnosis Tips [Re: RoadWarrior]  
Joined: Jun 2001
Posts: 1,462
markg Offline
15+ Year
markg  Offline
15+ Year

Joined: Jun 2001
Posts: 1,462
thank you all so much! ill look into it soon as it comes back from the trans shop.

86 ss 86 cl,330hp GM vortec cratemotor-700-r trans worked,ford 9.5 inch rear with 3-50s,custom 3 inch exhaust with flowmasters.

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