69 Temp and Oil Pressure gauges weird behavior

[Babaso]

Hi Everyone!

First post here. I've been lurking around for years and have learned quite a bit. I am in the process of restoring a 69 mustang fastback with a 302.

I ran into a problem that I cannot figure out and I have searched like crazy on this forum. I have tried many of the tests that were suggested and while most tests go as was stated they would, some do not.

I should first note that everything was working fine. Meaning both gauges were working correctly. However, recently I took apart the cluster to replace the lenses and those blue bulb covers. I also relocated the wiring in the engine compartment. then the problems started. So two variables to deal with here. Ok, here goes:

Here is the issue: When I turn the ignition to ACC, the temp gauge immediately pegs to the H. If I continue and turn on the engine, the oil pressure gauge will peg as well.

Gauge tests: I disconnected the sender, turned ignition to ACC, applied ground to the wire in the engine compartment and the gauge pegged. Another quick test I used that I got from here is I got a AA battery and attached it to both posts. The gauge went to about a quarter as predicted.

I did the same tests for Oil and Temp, and both tests on both gauges were fine. From what I have read, this shows that the gauges are good and that the wiring between the sender and the gauge is good.

I also tried a test suggested by egnorant where he wrote the following:

...Second test is to leave it plugged in (key on) and test light both posts. One will do nothing (sender) and the other will be bright (bad regulator) or flicker O.K. regulator. (volt meter will show 5.5 volt +/-. ...

This is the test where I didn't get the results I expected. On the post that goes to the sender I got a dim light. I expected no light as explained. On the post that goes to the CVR (or IVR) I got a somewhat bright light. Based on this I would think the CVR is bad but why would I get a dim light on the post that goes to the sender?

NOTE: I did this test with only the wire harness plugged into the cluster. Meaning, it wasn't mounted to the body. Not sure if this matters, but thought I'd throw it out there.

I also tried another test suggested by egnorant as well which states:

... Now use the continuity tester to check if any of the studs are grounded to the mounting case, If it lights up, look at the mounting. It should have...nut, washer, circuit, insulation pad (one per gauge), mounting case. ...

I did this test with the test light hooked up to a 12v source and the tester did light up when I touched any of the posts on the gauges. This would indicate that I am grounding to the mounting. So I took out the cluster and put it on the bench. I disconnected the circuit board and verified that everything is there. It was. Starting from the outside I have nut, washer, circuit board, insulation pad, mounting case. I also verified that the posts are centered on the holes and not touching the case.

With the cluster still on the bench, I did a quick test where I left the circuit board disconnected from the oil pressure gauge. I applied a ground to the housing and then with a hot test light I touched the posts and nothing, no light. So it's not grounding out. Good. I then reconnected the circuit board and did the same test and I got a light. So when I reconnect the circuit board, the posts on the oil pressure gauge are grounding out. But I never took apart the center part of the gauge cluster so I am not sure where the ground is coming from.

The only weird thing that happened was that when I was reconnecting the gauge cluster one time I got a little spark from one of the connectors as I was plugging it in. I checked the 10A fuse for the cluster and it was blown. Replaced the fuse and everything has been fine since. Could this have fried the CVR? Or maybe the sending unit?

I am at my wits end because I don't know what else to try. I have tried to describe everything I have done. I know I have done more but I could be here writing another couple of paragraphs.

Any help or guidance would be GREATLY appreciated.

Thank you all in advance.

[Babaso]

I also forgot to mention one more test that I did that was suggested by egnorant:

... Check the voltage regulator and mount. Plug in the plug, ground the case,turn key to run and test for voltage at the 2 studs (one per gauge) nearest the regulator. It should be about 5 volts and may flash,as the regulator reduces voltage by an on/off. ...

When I do this test I am getting 7.7 volts at each stud. I also checked the voltage on the studs furthest away from the CVR (the ones that go to the sender) and I am getting 6.5 volts there.

Could this also point to a bad CVR?

Thanks!

[Franks68coupe]

I would try a new CVR... you should only be getting 3.5 volts after it in the circut. Might have locked in the closed position putting your whole current through.

[Babaso]

Originally Posted by Franks68coupe

I would try a new CVR... you should only be getting 3.5 volts after it in the circut. Might have locked in the closed position putting your whole current through.

Thanks for the reply. My gut feeling is telling me the same thing so I just ordered one. Thanks for the suggestion!

I just don't understand why I'm seeing the results I got from those tests. We'll see. I'll update once I install the new CVR.

[Babaso]

Alright, I finally got around to solving this problem. It was the CVR.

What I forgot to mention previously was that we had relocated some wiring in the engine bay. During that process, two wires got crossed. If I remember correctly, the tach wire was crossed with the temp sensor wire. So instead of ground, a nice dose of electricity was sent back through the temp sensor wire to the cluster since it was hooked up to the negative post on the coil. This is what fried my CVR.

Fixed the wiring; installed a new CVR; everything is working perfectly now.

Hopefully this will help someone out in the future. Thanks for the help!

[Asm109]

Glad you found the problem. And a big THANK YOU for coming back and providing closure!

[Babaso]

Originally Posted by Asm109

Glad you found the problem. And a big THANK YOU for coming back and providing closure!

Thanks and no problem! I really hate when threads are left hanging, so I'm trying not to do that.