HELP -1988 GT V8 Swap - Oil Pressure, Water Temperature, and Gas Gauges Not Working (Page 1/5)
V8Steve DEC 28, 09:02 PM
I'm on the last steps of the resurrection of this car but really need some help with the gauges.

First, the history:

The V8 swap was done in 1999 with a ZZ4, TPI, and Painless harness. After roughly 4 years, I swapped the car over to a center mounted throttle body and BigStuff3 injection. The wiring mess was not visible to the casual observer but I continued to drive the car until the ZZ4 failed in 2007. During that time and 26,000 miles, despite the wiring mess, all the gauges worked. The gas gauge tank float was probably original. From 2007 until 2015 the car sat in my shop with a cracked block.

In summer 2015, I removed the ZZ4 and began the rebuild. Inside the engine bay were remnants of the OEM harness, remnants of the Painless TPI harness, wires too long, wires too short, burned wires, etc. I decided to clean up the entire engine bay and clean up the wiring. Before engine removal, every wire was labeled. The cleanup involved removing the C500 connector as well as the old power and ground posts. When I eliminated the C500 connector, I probed every wire going in and checked continuity to the mating wire leaving the connector. One by one, I cut and spliced them. The connectors were all marine grade with a heat sensitive adhesive.

The car is now running with a new 355 cubic inch motor and Holley multi-port injection.

Back to the gauges:
It seems odd that all three would fail and I suspect there is a common cause to this problem and it may not be that all three of them have failed.

Fuel Gauge - The fuel float sender is new and I'm aware that there are sometimes problems with these getting stuck inside the tank.

Oil Gauge - The oil sending unit is new. Right now it's not connected because I put in a mechanical gage for the dyno testing. When I turn on the key, the needle peaks to the right.

Water Temperature - Today I studied the 88 GM Manual and checked the ohms on the sensor. I couldn't get consistent readings to match the GM Manual but I noticed the ohms dropping while it warmed up in my hand. A new sensor is on the way. Meanwhile, I checked resistance on the female plug to ground and found both of them were grounded. That seems not right but my strong point is not in the electrical side.

I'm hoping someone might lead me toward a common cause that impacts all three gauges. Actually, I'd welcome any help with this last nagging problem.

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88GT 355 CI Sequential Holley Multiport , AFR milled 180, Dyno'd at 427 HP, 320 WHP
anderson@gdsconsulting.com

olejoedad DEC 28, 11:30 PM
Did you check the GAGE fuse?

Have you had the instrument cluster out of the car?

The water temp circuit is routed through the C500, but the oil pressure circuit goes through C203 and the fuel gage circuit doesnt pass through either connector.
V8Steve DEC 29, 12:09 AM
Thanks. I'll find and check that fuse tomorrow.

You make good points. The oil pressure tan wire should not be rocket science. The fuel sender should not have been impacted at all.

Instrument cluster - No, it was never removed since 1999 when I made a change to accommodate the V8 tach signal. The cluster is almost ready to come out now. I'm talking about the large cluster in front of the steering wheel. I've had the little one out lately ----> the one with oil pressure and voltage. BTW, the voltage charging gauge works fine.

[This message has been edited by V8Steve (edited 12-29-2019).]

fierofool DEC 29, 08:07 AM
I have had the same problem since converting my 87 3.4 to the 88 style pressure sending unit. As soon as I turn on the ignition, it blows the gauge fuse. With the fuse blown the gauge stays pegged to the bottom of the gauge. Disconnect the harness, the fuse doesn't blow and the gauge still stays pegged.
olejoedad DEC 29, 08:45 AM

quote
Originally posted by fierofool:

I have had the same problem since converting my 87 3.4 to the 88 style pressure sending unit. As soon as I turn on the ignition, it blows the gauge fuse. With the fuse blown the gauge stays pegged to the bottom of the gauge. Disconnect the harness, the fuse doesn't blow and the gauge still stays pegged.



It sounds like you dont have the new plug wired properly.
V8Steve DEC 29, 11:15 AM
I checked the fuse for the gauges and it's OK. The GM factory manual for the 88 shows all the routing so I think the next step is make sure all the wires go to the right places. The simplest is the single wire from the oil pressure gage so I'll trace it from the sensor through the console (which is still apart) right up to center mounted gage pod above the radio. The sensor is new and I'll double check the resistance. I had also checked the old one and it was OK. The ohms changed when run up against my air compressor nozzle.

The car is almost ready to drive after the resurrection began in June 2015, going on 5 years now but I don't want to drive it without monitoring oil pressure and water temperature on this new motor. A laptop connected to the Holley ECU tells me the coolant temperature and the fan goes on and off per the programmed setpoints.
V8Steve DEC 29, 11:58 AM
Here's the schematic. Can anyone tell me what the numbers mean like the 3 and .35 and 31 next to TAN?

I know this gauge set is getting power because the alternator charging voltage gauge right next to it is working.

[This message has been edited by V8Steve (edited 12-29-2019).]

FieroJimmy DEC 29, 02:13 PM
3 is the pin number in the Aux gauge pod connector. 31 is the circuit number. .35 is the metric wire size (0.35mm).

I didn't see if anyone else mentioned it yet, but if you don't have the sender for the oil pressure plugged in the gauge will peg high.
V8Steve DEC 29, 03:24 PM
FieroJimmy - Thanks. Right now the sensor is in the box. Solving the EFI starting problem with the old Bigstuff3 was such a huge problem all efforts went into just getting it running. I was so concerned about the oil pressure as related to the image below, changing hose lengths, etc. that I put a regular mechanical gage right on the motor with copper tubing. We made certain there was always oil pressure. There was a lot at stake. In the image below you can see the "A" engraved on both housings.

Now that it's running with good oil pressure, I can spend time on the dash gage.
V8Steve DEC 29, 04:53 PM
I ran a test for the oil pressure gauge.

First I turned on the key with the sensor disconnected. The pointer pinned right away to the right and the little oil can light was out.

Then I plugged in the connector to the sensor and made sure the housing was grounded. When I turned the key, the needle stayed at the low end and bumped a bit when key was turned. ALSO, the red oil can light went on.

So this tells me the front of the car is communicating with the sensor at the back of the car.

I've been talking about the TAN wire because that's what the manual shows. But look at the plug...it only has a bluish green and the tan had been cut off. Since my 1st V8 swap was almost 20 years ago, I can't remember why it ended up this way. Even so, it still worked OK for years.

Here's the plug: