69 Gauge problem

[classicdoug]

We putting a 69 fastback together and are having a gauge problem. It is an original set of gauges and we are using an aftermarket printed circuit. The gauges work, but they all read too high. I've already changed out the Instrument Voltage Regulator and nothing changed. I've also changed out the temp gauge and temp sender with NOS ones to rule out individual problems, but that didn't change anything either. It seems to me that too much voltage is getting through the IVR. Any ideas?

Doug

 

[Midlife]

In 1969, Ford moved to a resistor wire as input into the CVR. Why? I don't know.

If your resistor wire is bad, the resistance would be too high, and your voltage at the gauges too low, and the gauges would read low.

Do your gauges peg out? For that to happen, all of your gauge sending lines would have to grounded...probably not the case.

Is the circuit board correct for your application? They do differ between tach dash and standard dash.

 

[classicdoug]

Resistor wire is good. You rebuilt the harness!

Voltage at the resistor wire where it plugs into the harness jumps between about 6.5 and 10 volts.

I checked continuity between the signal terminals on the temp and fuel gauges and the gauge housing. When I plug in the IVR, there is continuity between the signal terminals and the housing. This doesn't seem right, but I have 2 IVRs and they do the same thing. So I opened up one of the IVRs and it looks like it works by shorting some of the power to ground. So maybe some continuity to ground is ok.

With the IVR unplugged, there is no short from the signal wires to the gauge housing.

This only happens with the fuel and temp. The oil pressure gauge doesn't have any continuity to the housing.

 

[Midlife]

I wouldn't check for continuity between the gauge signal line and the housing just yet.

Here's a sweet trick: swap the pin for the oil line (it shows a good signal when the engine is running, right?) with the fuel wire on the dash cluster connector. Oil is white/red; fuel is yellow/white. This only changes the gauge that reflects the signals (all three gauges are identical except for the face plates!). If your fuel now reads good but the oil doesn't, then the problem is in the sending units/wiring; if things stay the same, the problem has to be in the dash cluster area.

 

[classicdoug]

Thanks Midlife. Here's what we found.

The new oil sender we put on the new motor was no good. Another new sender fixed that.

I found out that we didn't ground the engine to the chassis. I always do this on all of my builds because a lot of the cars we build are fuel injected now so I don't know how this got skipped. That helped a little, but the fuel and temp still read just a little high. They seemed to work like they were supposed to, but just slightly high. So, I don't know if you're supposed to do this or not, but we used the little adjustment on the back of the IVR. By knowing where the fuel gauge should be reading we adjusted the IVR to bring the fuel gauge needle to where it should be and the temp needle ended up reading right in the middle at normal operating temp.

The owner picked up the car this afternoon and drove it 2 hours to Houston. He said the car was perfect all the way.

Thanks for the help,
Doug

 

[AzPete]

Many have adjusted the IVR to bring the gauges in line as long as they all need the same change, Sounds like a good fix all around. And you are not the first to miss that engine ground.