Problem with 70 rallye gauges

[SlinktRR]

Hi all, trying to diagnose this problem with a 70 roadrunner. I just put in a new solid state voltage regulator IVR this weekend on the circuit board for the fuel/temp/oil gauges, they went out earlier this year but didnt peg...just died. I've been carrying the old 1 gallon gas can in the trunk with mechanical gauges for the rest, but Ive been wanting to get the stock setup working properly again.

I took it out for a test cruise today and the fuel gauge was rock steady at about 1/4 full indicated, but the tank was actually half full. I filled it up to the neck and the gauge is showing a bit over 1/2. The temp gauge is showing 125 when the motor is warmed up (around 185F actual). I havent installed the oil pressure sending unit back on the block yet so that gauge is still reading 0.

Then, when I hit the headlight switch all the gauges pegged. The fuel gauge immediately sweeps to F and the temp gauge steps like a clock from 120 to 160 to 200 to 240 to peg. So I promptly switched the headlights off and headed home.

I'm guessing the headlight deal is a ground issue but I'm not sure about the faulty readings, probably grounds too. All the dash screws are in, but I'm planning to put in a ground wire from the cluster..what are good locations for it? Circuit board mounting screw to steering column? Let me know if you have had similar problems. Thanks.

 

[bigmanjbmopar]

I have been hearing the new voltage limiters (assuming) that is what you are talking about and not the regulator in the engine bay? are cheap and do not work well with old boards. As much as it sux due to price try a new board and limiter. stock gauges use a ground and 5 volts DC to make them move basicly a rheostat like you fuel gauge when it moves it just varies the ohms and voltage the gauge gets and the needle is indicating at what point that position is I.E. how much gas is in the tank. New sending units are not coming out good either, usually you have to calibrate of test them prior to installing to make sure it is right.

I would first make sure all grounds are good especially at the cluster as the housing is metal it uses the frame of the dash chassis as a ground reference.

If grounds are good move to circuit board pull the cluster out check board for burnt solder traces or broken parts etc. if that's all good make sure your connector from the wiring harness is good and connectors are clean and solid not loose etc.

Let me know what you find after these few checks.

 

[nm9stheham]

The low fuel and temp guage readings indicate that there is below normal voltage going to both sensors. This can be due to a bad ground or a low voltage regulator. Do you have a voltmeter? If so, it would be an easy thing measure the voltage from the new regulator. It should be 5V as noted above and steady if this is a solid state regulator. You can do 1st check of this by removing the wire from the temp sensor; the 5V should be there with this wire disconnected. If this is good, then reconnect and check the 5v directly at the regulator output with the engine warmed up (so that the temp sensor is at lower resistance and loading the regulator); do this to make sure the regulator can handle the current load of the sensors.

The issue with the headlights might be a short or or wrong connection between the instument lamps circuit and the guage circuit in the cluster. Again, use a voltmeter and monitor the output of the voltage regulator before and after the headlight switch is turned on, and see if the voltage suddenly rises to above 5v when the headlights are on.

 

[SlinktRR]

Yes its the voltage limiter in the instrument cluster (12V to 5V limiter), I am going to run new grounds this week and see how it affects the readings. Thanks for the ideas, I have a meter and a wiring diagram so I'll be able to run the checks and report back. If the voltage regulator output is low (<5V), then the IVR is faulty and needs to be returned?

 

[nm9stheham]

Do you have link to the IVR that you purchased that you can share?

 

[SlinktRR]

Do you have link to the IVR that you purchased that you can share?

http://www.rosevillemoparts.com/pro...e#-2258413/cPath/347_136_380/products_id/6044

When the headlight switch was on, if I turned off the dimmer switch the fuel gauge didnt peg (dash lights off). I ran a single ground wire from the PCB fastener closest to the radio side of the cluster down to the steering column, I'll probably add another one to the firewall. Now I can turn the lights on with dimmer on and no peg. So that issue was bad grounds in the instrument cluster from the dash lighting circuit.

The fuel gauge reads 3/4 now with a full tank. I can't get a good reading off the IVR with the multimeter leads, I need to take the cluster completely out to do testing. Hope to have some time tomorrow.

So if I read 5V from the IVR, can I calibrate the fuel gauge using resistors to get the correct ohm reading for "full" and "empty"? How can I test the fuel sending unit.

 

[nm9stheham]

Sounds like you solved it; the dimmer 'on' sent juice to the instrument lights in the cluster and that current went to ground via the guage sensors rather than a direct cluster ground; good deal.

The fuel sending unit should read 10-13 ohms full and 70-75 ohms empty. Make sure you have a good ground from the sender to chassis also. Some used a strap from the sender fuel neck to the main fuel line, and then via the fuel pump and engine block; you can run a ground wire direct from the sender to chassis.

You can't make the guage read higher by adding resistors in series, only lower. If you have access to the back of the guage, look for some small adjusters on the back (dunno how to describe them exactly). Some of the Mopar guages had these adjusters on the guages themselves that can be used to calibrate them.

 

[SlinktRR]

Sounds like you solved it; the dimmer 'on' sent juice to the instrument lights in the cluster and that current went to ground via the guage sensors rather than a direct cluster ground; good deal.

The fuel sending unit should read 10-13 ohms full and 70-75 ohms empty. Make sure you have a good ground from the sender to chassis also. Some used a strap from the sender fuel neck to the main fuel line, and then via the fuel pump and engine block; you can run a ground wire direct from the sender to chassis.

You can't make the guage read higher by adding resistors in series, only lower. If you have access to the back of the guage, look for some small adjusters on the back (dunno how to describe them exactly). Some of the Mopar guages had these adjusters on the guages themselves that can be used to calibrate them.

Thanks NM9. The IVR is putting out 1.5V with key on and 1.6V after it warms up. Rock steady within +/- a mV, but not nearly enough voltage. I'm going to call up Roseville and see what they can do. If not, I may go the Allpar route and piece together a radio shacked voltage regulator.

The good news is that I at least get a reading, and I'm assuming the movement is linear. I took a 40 mile trip today and it dropped an 1/8th. Still keeping the gas can for now...

 

[nm9stheham]

OK, so are you measuring this right at the IVR output and with the temp sensor and fuel senders connected? If so, look at the IVR output voltage with both of those wire disconnected from their sensors. That will tell you if the IVR has just a faulty output or if the combined loads are too much for the new IVR. That could again indicate a faulty IVR and will give you added test info to take to Roseville.

Also, while doing the above, check the voltage INTO the new IVR under loaded and unloaded conditions to make sure it is in the spec'd 9 to 18V range. You could have a poor conenction into the IVR that has high resistance and you are not getting adequate IVR input voltage to make it work.

As I recall, the fuel sender is not very linear: 10-13 ohms full, low 20 ohm range at 1/2 tank, and low 70 ohm range when empty.