Code 36 and 35 revisited

[olbooker]

'95 Suburban had injection pump replaced at dealer several years ago. About 2 years ago started the usual dying bit. Since it is one of our good customers I replaced the PMD with the seperate cooler. Recently, it has started dying again and starts back up with some black smoke. And an occasional hickup while cruising, though I haven't seen that yet. It has a code 36 and 35 in history with 35 coming back instantly.

I do have a factory manual as well as the wonderful Mitchell. The diags leave a lot to be desired.

The feedback wire from the PMD to the ECM (red wire) does have approx 5 volts on it, at the ECM. Mitchell shows 5.6v key on and 4v running. So I can assume that the PMD is outputting a feedback signal.

What is this signal on a properly operating engine? It doesn't apppear to have much amplitude on my scope, if it is in fact a frequency signal, which I would think that it should be. Surely it is not just voltage. Any help will be appreciated.

Is it possible that this new PMD has already failed?

Should I have installed the remote/extension harness?

BTW, there are no other codes stored, leading to a bad pump.

I will inspect the grounds, as in a previous message, tomorrow.

Thanks for any help!

Oscar

[moondoggie]

Good Day!

My factory (Helms) manual says,

[ucdavis]

The GM standard VOMeter is a Fluke 87 mdel which has Hz capacity; you can check the circuit for Hz w/that or equivalent VOM.
The only truly reliable check on FSD quality is to replace with a new or known good one.
The 94/95 programming has a tighter tolerance on the pulse width than later years, so you get the 35, 36 error code for a smaller out-of-spec signal. Don't know if that info is useful in any way tho.
If 35 is instant or nearly so after clearing, it is probably the IP if it is not grounds; obviously ohm-check all grounds especially on top of IP, block to battery, & PCM to batt (probably have some sheet metal link in this last one).
Also, check the lift pump operation. There is no warning light/code etc. for failure to deliver adequate lubrication to IP, so a dead OPS or lift pump could be killing the IP.
You asked about remote harness- IMO it will lengthen life of the heat-sensitive fsd by relocating out of the heat-soak valley area (probably drops operating temp to ambient + 30 degrees; in vee of engine, ambient could be 170-210), but the fsd's on remote coolers also die on occasion so it isn't bulletproof.

[olbooker]

Thanks so much for the replies...all good info. Still haven't had a chance to get back to the vehicle, but should soon.