Kamilo77

My 1981 auto 928 has more electrical gremlins. After some gauge cluster work my 928 is not starting. It cranks but I'm getting no fuel as the fuel pump is not priming and the fuse is blowing. I think the sequence of events here is really important so ill go through it.

1) Last year I was also blowing fuel pump fuses, thanks to some smart help on here a member had me try the auxiliary air valve, which was found to be defective and was since replaced. Everything went back to normal. Its still a rebuild project so its not driving but it started every time, no issues.

2) Since I bought the 928, I've had the charging system issue where the alternator does not charge the battery until about 3000rpms are first reached. With some more smart help I diagnosed the voltmeter bulb and replaced it. This over course involved removing the gauge cluster. Also did some minor repairs while the gauge cluster was off. See thread https://rennlist.com/forums/928-foru...e-cluster.html

3) I put the cluster together and in place, and hooked up everything short of the stalk combo and the steering wheel. I started the 928 and everything was fine, on top of that the charging system issue was fixed so you can imagine my excitement.

4) I then waited to get a new stalk combo as the old one had the copper plate missing that closes the circuit for the horn. I installed that, and tested the horns, the horn fuse blew so i thought ok, I'm sure the horns are old and broken. So i put the horns in the back of my head. I assembled everything including the steering wheel. I attempt to start the 928 again and this time it sputters and continues to crank. I try again and i didn't hear the fuel pump priming so i checked the fuse and it was blown.

Heres what i know from diagnostics.

1) I immediately remembered the auxiliary air valve being the culprit last time, i disconnected the valve and the fuse was still blowing, so I know its not the valve.
2) I disconnected the fuel pump connections and then attempted to start the 928, the fuse does not blow with the connections off at the fuel pump directly.
3) I checked my fuel pump, ran it directly to 12v normal and reverse polarity. Ran fine, no debris came out of the pump, I even swapped it for a spare pump. Still the same issue.
4) When i jump the relay the fuse blows instantly.

Ive seen in passing that the ignition switch can cause some odd problems is there a chance something was effected in taking the gauge cluster in and out? Im pretty lost right now, my electrical diagnostics skills are fairly novice. Its a major disappointment because after getting the gauge stuff done I was going to finish some paint prep and send it to paint and get this project close to done/ drivable. Now I'm at a stand still again. I would really really appreciate some insight and help.

Kamilo77

Anyone?

ltoolio

Could one of the wires from the three white electrical connectors rip out and possibly be making contact with the steering column?

Some of the wires & pins within the connectors were in poor shape, and I repaired a few of the wires on them while the pod was out.

Majestic Moose

The first thing I would try is disconnecting the positive from the fuel pump and test for continuity from fuel pump relay terminal 87 to ground. If there is, you will have to physically follow the wire out to find a short. You can further isolate the feed to the fuel pump from the feed to aux air valve by unplugging bottom connector Q and T and test Q1 (aux air valve) and T5 (fuel pump) for continuity to ground.

Mrmerlin

did you drop the header pipes? if so the O2 sensor wires could be damaged and shorting on the chassis or crossmember,
disconnect the o2 sensor plug under the CE panel and see if the fuse blows.
Otherwise the engine will run without any of the connections made to the pod gauges and should not cause a pump fuse to blow.
also check to make sure you have the AIRtemp sensor wires going to the AIRtemp sensor in the aircleaner,
and the AUX air valve connector going to the AAV.
NOTE AIR temp sensor runs on 5V,
the AAV runs on 12v,
so pull the plugs and verify what voltage you have with the key to run.

NOTE these two connectors look alike and can be swapped by accident

The Forgotten On

^^ an 81 with L jetronic wont have a air temp sensor in the air filter housing.

The early cars like ours also don't have heaters in the oxygen sensor so it doesn't get any power from the fuel pump fuse like in the later cars. The 02 sensor just slowly warms up until it can send a signal.

The only parts that get power from that fuse are the AAV and the fuel pump (and the WUR in the CIS cars). Both of which have been verified as good.

By any chance are you using a fuse with a lower current handling ability than 25 amps? If you stuck an 8 amp fuse in there by mistake it would easily keep blowing.

Petza914

Does the 81 also use a cold start injector? If so, there are 4 virtually identical connectors at the front of the engine that can be mixed up. I'm going through this myself right now and in swapping a couple around to try and diagnose a running issue ended up blowing my fuel pump fuse too.

Both the AAV and WUR get a two wire connector and those get 12v with the key switched on. I don't think the wire for the AAV will reach any of the other plugs. If you have a cold start valve, that connector is the same but has 3 wires going to it. It only gets voltage when the car is being cranked to start. The remaining connector goes to the thermo switch near the coolant hose and is what makes the ground to the cold start injector when the temperature is cold enough. If it's warm that's what keeps the cold start injector from firing.

928FIXER

I'm going to do something here I don't do very often.
Did you notice some extra wires back at the fuel pump?If memory serves me correctly,the 81 still has the wires for the intank pump,even when it does not have that pump.They are tied up back at the fuel pump mount and are still powered up,and can short out.I say this because when you unhooked your fuel pump,you may have moved the harness around just enough to open circuit the short that is blowing the fuse.

Kamilo77

Still nothing so far. I'm trying to find the ground in the rear wheel well to clean it up and verify nothing is shorting. Haven't found any obvious wiring issues.

What is the correlation between the ignition switch and the fuel pump circuit?

Is it possible that the relay is bad but not in the common way (87&30 points) according to that diagram the relay is related to ignition? Also this is way left field but.. I sprayed some WD40 into the ignition key hole as it was a little sticky. That's around the time everything went south.

Kamilo77

And yes, I'm using a 25a fuse.

Majestic Moose

The fuse and thus relay term. 30 is always hot. When the key is turned to ignition, term 15 is energized and closes the contact from 30 to 87 for about two seconds unless it receives a signal from the TSZ via term 31b.

Have you performed the checks stated earlier?

Kamilo77

You are the winner. I disconnected a pair of the connectors, the thinner of the two (they are the same colors) and it fired up and ran fine.

So there is my short. For you happen to know where the wiring leads to exactly. Looks like it ticks into the skid plate under the tank that is apart of the tank brackets.

Thank you for the help.