I'm no electrical expert but it sounds like a bad ground or some sort of back feeding of the electrical system.

Haven't tried disconnecting the sender yet. Will give that a try and see what happens. I do have a ground strap on the head.

Try removing the switch from the dash and connecting a jumper cable from the body of the switch to battery negative. I'm not sure if that circuit gets grounded at the switch or the pump body.

could try adding a diode like from a battery isolator ( high current) on the pump motor positive and see if it stops.

Another thing to try would be to add a capacitor near the motor.
May filter out the noise.
Try a condenser from points or for voltage regulator.
Might need a larger one.

Sounds similar to typical alternator whine on your radio.
Do you hear it on radio when you operate the motor?

Or just leave the top down😁

Last edited by Nos681 (2/22/2020 11:27 PM)

50vert wrote:

I'm as good as AAW ...stumped. Does it do it if you disconnect the sender? Do you have a ground from a head to the firewall?

 
Ok, you are on to something here.

I double checked the ground for the top motor, worked the top switch and there was no change- the oil pressure gauge moved when operating the top.

I next disconnected the positive side of the Speedhut oil pressure sending unit (they use a sender that has two terminals) and worked the top switch and the problem stopped- no movement at all on the oil pressure gauge.

So, now I'm wondering if the ground for the oil pressure sender wasn't good (I have the negative post on the sending unit grounded into an aluminum head), so I disconnected the ground wire and ran a jumper straight from the negative terminal on the sending unit to the battery and worked the top switch and the problem returned- oil pressure gauge sweeps with the top switch, so it's not the ground at the pump or the ground at the sending unit.

That leaves only one potential ground and that is the ground for the oil pressure gauge itself, but that is grounded back through the harness  with every other gauge and the rest of the gauges are not affected by the top motor.

Couple other questions to through in the mix. Does the oil pressure act any different when the car is running, you have oil pressure, then run the top?
Does the key need to be in the on position when running the top?
If not, does the key need to be on to get 12 volt to the gauges?
Do you have voltage across the pressure switch when running the top? Is it DC or (AC maybe noise form the motor)
Do you have voltage across the sending unit?
 

As Barry suggested, I would also investigate the switch.
Start by just removing from dash board and leave it floating and operate motor.
See if your problem is still present.
If it disappears, clean switch or replace.

Not being familiar with pump circuit, does motor have two leads?
Is there a relay in the circuit to energize the pump motor?
Or is motor connected to switch?
How old is the switch?

Most toggle switches, even if it has metal exterior, isn’t used for the circuit.
It’s for mechanical reliability.
With age, use, and exposed to environment ( i can’t believe I just used that word 😁)  it is possible that when you’re energizing the motor with the switch, some of your electrons are going to ground ( metal housing).

As for the oil pressure switch ground, it should be fine as long as you have full voltage from battery positive to the oil pressure switch ground terminal lug. It’s an easy way to verify that it is at ground potential ( battery negative).

Definitely want to fix this before potentially...electrical joke😂...damage to gauge occurs.

Last edited by Nos681 (2/23/2020 2:09 PM)

I'd also ask the question to Speedhut. Helpful when I dealt with them.

You may want to use relays to control the top motor/pump.

Well, it's fixed. And I embarrassed to say that it was a ground problem. RV6 nailed it from the get go!

Don't know if I didn't tighten it all the way when I first installed it and it vibrated loose or I just never tightened it, but the bolt for the firewall ground strap was only finger tight in the firewall.  I took it out, cleaned up the ring terminal and the firewall, put it back in, snugged it down, then ran the fan and operated the top switch and the gauges didn't budge.  I will keep an eye on that ground screw and see if it loosens up again because I am pretty sure I tightened it last time. If it does, I'll run another ground strap somewhere else.

Thanks for the help!

I may do the same if I find it loosening up. The other thing I was thinking about was putting a bolt through the firewall from the inside and creating a stud to bolt the strap down onto.