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Erratic Fuel level oil pressure and Tachometer

C

cubzy

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Hi. I’m looking for some insight if I have singular or multiple problems. Present status of the gages:

Fuel level-continuously cycles between 1/4 and 3/4 when tank is around 1/2. Car is stationary and not moving.

Oil pressure-cycles between 0-80psig when running. Freshly rebuilt motor with no knocking.

Temperature gage-not working at all.

Alternator-appears to be working normally.

Tachometer-reads normally most times but on occasion pins out to 7-8000rpm then returns to normal.

I checked the dash grounds to make sure the paint was removed at the two bolts down by the kick panels. Also sanded the ground at the battery that grounds to the radiator support.

The vehicle is a 1970 Dodge Super Bee. Fresher restoration with all new gages. If the instrument voltage regulator is on the fritz should it only affect the fuel level and temperature gage? Thanks
 
Did this condition happen suddenly, or has it been like this for a long time ? Can't speak confidently for a 1970 car, but in pre 1970 Mopar's, the gauge voltage regulator sources the fuel, oil pressure and temperature gauges. The tach is not related to these 3 gauges nor is the alt gauge. Tach has separate circuitry that drives it from the ignition coil. You may have a defective gauge regulator, or it's possible that the non functional gauge may be shorting dragging down the output of the regulator having an effect on the other gauges, or you may have a defective sender or wiring. You may want to try lifting the wire off the senders temporarily to open the circuit to the gauges and see if things change. Having a wiring diagram may be helpful.
 
I am not a auto electrician but I think that 1970 also has a voltage limiter under the dash to keep all that at aprox 5 volts.
Outside of the ground, that limiter may be it.
 
twecomm said:
Did this condition happen suddenly, or has it been like this for a long time ? Can't speak confidently for a 1970 car, but in pre 1970 Mopar's, the gauge voltage regulator sources the fuel, oil pressure and temperature gauges. The tach is not related to these 3 gauges nor is the alt gauge. Tach has separate circuitry that drives it from the ignition coil. You may have a defective gauge regulator, or it's possible that the non functional gauge may be shorting dragging down the output of the regulator having an effect on the other gauges, or you may have a defective sender or wiring. You may want to try lifting the wire off the senders temporarily to open the circuit to the gauges and see if things change. Having a wiring diagram may be helpful.
Click to expand...
twecomm said:
Did this condition happen suddenly, or has it been like this for a long time ? Can't speak confidently for a 1970 car, but in pre 1970 Mopar's, the gauge voltage regulator sources the fuel, oil pressure and temperature gauges. The tach is not related to these 3 gauges nor is the alt gauge. Tach has separate circuitry that drives it from the ignition coil. You may have a defective gauge regulator, or it's possible that the non functional gauge may be shorting dragging down the output of the regulator having an effect on the other gauges, or you may have a defective sender or wiring. You may want to try lifting the wire off the senders temporarily to open the circuit to the gauges and see if things change. Having a wiring diagram may be helpful.
Click to expand...
The car was purchased this way. I believe the tach is a separate thing. It operates with out problem 99% of the time, I’ll try and test the voltage regulator circuit by having one gage wired at a time. See if it makes a difference. In the end I’ll probably try another voltage regulator as well. Thank you for your insight.
 
I like the rt-eng limiter and always replace the mechanical limiters wether they need it or not. It's good insurance to protect your delicate gauges.

I wish you were closer I'd love to have this cluster on my bench and troubleshoot this gremlin.

I'd tackle this as follows
1) new rt-eng limiter
2) remove fuel sender wire at gas tank and ground through a 10ohm 10W resistor. The result we want is the fuel gauge at F and no fluctuations.
3) repeat above with the temp sender and the gauge should be H and no fluctuations.
4) the tach could probably use a upgrade and again I use the rt-eng tach module. I'll look for my tach videos and post a link.

I've rebuilt and or repaired many clusters by reinforcement of the circular pins with JBWeld and generous solder flow. This may be your issue.

Let us know how you do here.
 
Agree, If you have it apart, replace the mechanical limiter with the solid state RT Engineering unit. Good insurance, as a defective limiter can fry you gauges.
 
How did you test the alternator? I had a similar issue with my tach. I discovered I had no regulator and the alternator was overcharging. That caused all kind of ignition issues until I put a one-wire alternator in it.
 
cubzy said:
Hi. I’m looking for some insight if I have singular or multiple problems. Present status of the gages:

Fuel level-continuously cycles between 1/4 and 3/4 when tank is around 1/2. Car is stationary and not moving.

Oil pressure-cycles between 0-80psig when running. Freshly rebuilt motor with no knocking.

Temperature gage-not working at all.

Alternator-appears to be working normally.

Tachometer-reads normally most times but on occasion pins out to 7-8000rpm then returns to normal.

I checked the dash grounds to make sure the paint was removed at the two bolts down by the kick panels. Also sanded the ground at the battery that grounds to the radiator support.

The vehicle is a 1970 Dodge Super Bee. Fresher restoration with all new gages. If the instrument voltage regulator is on the fritz should it only affect the fuel level and temperature gage? Thanks
Click to expand...
An update. I ran a shunt wire bypass from the alternator to the starter relay with a fusible link. Prior to this I was only getting 3.5V on the Alternator post prior. Testing found I had a very large resistance and voltage drop from the time in went into the bulkhead and back out to the alternator. The bypass corrected this. The alternator now charges the battery.

Now onto the large voltage drop under the dash. I removed the 1970 rally cluster(real pain in the butt) and found the red terminal on the back of the ammeter was shorting to the cluster as the insulator was broken.

My next question is I’d like to abandon the ammeter and run aftermarket gages below dash. Can I just butt connect the red/black ammeter wires together and not loose power to anything else? Aka Fuse panel or other gages. I still haven’t found the designated power feed to the fuse box. I see some people put both red and black wires together on one of the ammeter posts. I don’t want to do this as the insulator to will eventually breakdown and short out.

I’m just looking for clarification if I will still have power to all necessary items by connecting them together before reinstalling the cluster.

Thank you
 
I'm kinda opposed to bypassing the ammeter but if that's what you'd like to do:

You can simply bolt the two wires together and heat shrink or tape heavily the setup.
It should never be an issue at that spot afterwards.
 
I prefer to add a voltmeter. As Don recommended use a screw and nut to jump the ring terminals. Add a voltmeter connection here while you are at it. I use large heat shrink, but lots of electrical tape works as well.

You can also pull the cluster and convert the ammeter to a voltmeter. Search around here as many have performed this with great results.
 
cubzy said:
Hi. I’m looking for some insight if I have singular or multiple problems. Present status of the gages:

Fuel level-continuously cycles between 1/4 and 3/4 when tank is around 1/2. Car is stationary and not moving.

Oil pressure-cycles between 0-80psig when running. Freshly rebuilt motor with no knocking.

Temperature gage-not working at all.

Alternator-appears to be working normally.

Tachometer-reads normally most times but on occasion pins out to 7-8000rpm then returns to normal.

I checked the dash grounds to make sure the paint was removed at the two bolts down by the kick panels. Also sanded the ground at the battery that grounds to the radiator support.

The vehicle is a 1970 Dodge Super Bee. Fresher restoration with all new gages. If the instrument voltage regulator is on the fritz should it only affect the fuel level and temperature gage? Thanks
Click to expand...
As you suspect, a bad ground is the only thing common to all three issues. I would run a wire from the instrument cluster to a known good ground for starters.
If the gauges still swing up/down then the IVR might have issues. Because the Temp gauge is not working, I would check all the gauges at the dash, and also make sure none of them are shorting to the dash by using an ohm meter between the gauge connections and dash frame (with the IVR and wiring harness removed.)
The Tachometer would be a different issue unless related to a bad ground.
 
So I look to have the swinging gages corrected now. I had to remove all gages and scuff the printed circuit board pretty much wherever there was a contact. I couldn't get a ground on the board to the IVR at all before.

Bypassing the Ammeter corrected the voltage drop. After removing the ammeter you could see the peg got hot and melted the insulator.
IMG_3725.jpeg
IMG_3726.jpeg
 
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