HELP -1988 GT V8 Swap - Oil Pressure, Water Temperature, and Gas Gauges Not Working (Page 2/5)
olejoedad DEC 29, 05:44 PM
The correct plug will have three wires.
The two outside wires (C & D, farthest apart) are for the oil pump pressure switch that backs up the relay, the center wire (A)is the resistance circuit for the gage.
(A) is the wire closest to the clip on the plug.
Are you using a three pin sending unit for Oil Pressure?

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

V8Steve DEC 29, 06:17 PM
Both the old and the new sending units are identical three wire units. From Day 1 of the 1999 swap I'm certain we used only the center wire. The two side wires are orange and tan but both are snipped off. The 1st generation had a Painless harness with own relay that started the fuel pump. Same thing with Bigstuff3 injection after that. The new Holley also turns on the fuel pump with own relay.

The factory manual for the 1988 shows only one wire from the sending unit and that's what I have and it's from the center.

Will put it back into the motor and try tomorrow.
olejoedad DEC 29, 07:18 PM
That is the correct location for the gage signal.

The other two wires acted as a backup power circuit for the fuel pump, in case the relay failed.
The circuit is in the ECU wiring section of the FSM.
V8Steve JAN 03, 01:26 PM
Oil pressure now working! I removed the gauge pod and separated the plug underneath. Then I checked the ohms from the sending unit plug all the way to the first plug and finally through the second plug under the pod. All was OK. I started the car and gauge works perfectly. I don't know why it didn't work before but at least I know now there is continuity from the sender plug all the way to the pod and it works.

Now, on to the water temperature and gas gage plus problems with the serpentine walking off the tensioner pully (different thread).
olejoedad JAN 03, 03:07 PM
Cool! On to the water temp then.....

What temperature sending unit are you using?
V8Steve JAN 03, 11:31 PM
ACDelco 213-928. It's exactly like the one I took out.

Next step is to trace all the wires from the sender plug up to the cluster. Meanwhile, the serpentine tensioner belt walking must be solved so that I can run the motor long enough to produce temperature.
olejoedad JAN 04, 07:15 AM
That sensor will not run the gage, it is the sensor that communicates with the PCM.
It can be replaced with the 3 wire sensor from certain GM models (sorry, dont have P/N right now).

Your tensioner pulley axis of rotation is not parallel with the rest of the pulleys, either from improper mounting, bad bearings or a worn out tensioner assembly.
V8Steve JAN 04, 09:07 AM
The water temperature sensor has always been a two wire sensor and the new one is identical to the old one. I believe the old one still works. More work to do on my end.

I agree 100% on the belt problem and pulley alignment. See other thread, page 2 : Serpentine

I borrowed the Gates laser tool again and rechecked all the pulleys one more time. They all checked out but there's still a walking problem. I think the new tensioner meets parallel and angularity requirements when unloaded but is deflecting out of position when loaded with the belt. Will check again with laser, loaded and unloaded on Tuesday when I borrow the tool again. Meanwhile, I swapped the tensioner pulley to a smooth non-ribbed Dayco. It seems to have higher flanges and it has a crown in the middle. This is going to take more time as well as shimming the pulley in/out as well as possibly shimming/tilting the tensioner itself. Dayco tells me it's OK to run the ribbed belt on a smooth tensioner pulley.

[This message has been edited by V8Steve (edited 01-04-2020).]

olejoedad JAN 04, 10:23 AM
Are you using stock Fiero gages?

The sensor you linked to is hooked up to the PCM via a yellow wire and a black wire.
It does not work with the stock Fiero temperature gage.

To run the gage and the PCM, use AC Delco P/N 12551708.
It is a three pin sensor - two wires to communicate with the PCM and 1 wire to run the gage.
V8Steve JAN 04, 11:06 AM
Gages are stock Fiero. Ever since the 1st V8 swap in 1999 the car has always run with two temperature sensors, one for the ECU and another for the gage, both with just two wires.

I plan to connect the tach next to see if it still works. The white wire simply plugs into the MSD 6+ box. Since the tach, temperature, and fuel are all in the main cluster I might learn more since both the temperature and fuel gages don't work. If the tach doesn't work it might point to a problem within the cluster itself.